参考链接

• FFmpeg源代码简单分析：结构体成员管理系统-AVClass_雷霄骅的博客-CSDN博客
• FFmpeg源代码简单分析：结构体成员管理系统-AVOption_雷霄骅的博客-CSDN博客

概述

• AVOption用于在FFmpeg中描述结构体中的成员变量。它最主要的作用可以概括为两个字：“赋值”。一个AVOption结构体包含了变量名称，简短的帮助，取值等等信息。
• 所有和AVOption有关的数据都存储在AVClass结构体中。如果一个结构体（例如AVFormatContext或者AVCodecContext）想要支持AVOption的话，它的第一个成员变量必须是一个指向AVClass结构体的指针。该AVClass中的成员变量option必须指向一个AVOption类型的静态数组。

AVOption

•  AVOption是用来设置FFmpeg中变量的值的结构体。可能说到这个作用有的人会奇怪：设置系统中变量的值，直接使用等于号“=”就可以，为什么还要专门定义一个结构体呢？其实AVOption的特点就在于它赋值时候的灵活性。AVOption可以使用字符串为任何类型的变量赋值。
• 传统意义上，如果变量类型为int，则需要使用整数来赋值；如果变量为double，则需要使用小数来赋值；如果变量类型为char *，才需要使用字符串来赋值。而AVOption将这些赋值“归一化”了，统一使用字符串赋值。例如给int型变量qp设定值为20，通过AVOption需要传递进去一个内容为“20”的字符串。
• 此外，AVOption中变量的名称也使用字符串来表示。结合上面提到的使用字符串赋值的特性，我们可以发现使用AVOption之后，传递两个字符串（一个是变量的名称，一个是变量的值）就可以改变系统中变量的值。
• 上文提到的这种方法的意义在哪里？我个人感觉对于直接使用C语言进行开发的人来说，作用不是很明显：完全可以使用等于号“=”就可以进行各种变量的赋值。但是对于从外部系统中调用FFmpeg的人来说，作用就很大了：从外部系统中只可以传递字符串给内部系统。比如说对于直接调用ffmpeg.exe的人来说，他们是无法修改FFmpeg内部各个变量的数值的，这种情况下只能通过输入“名称”和“值”这样的字符串，通过AVOption改变FFmpeg内部变量的值。由此可见，使用AVOption可以使FFmpeg更加适应多种多样的外部系统。
•  可以对FFmpeg常用结构体AVFormatContext，AVCodecContext等进行赋值之外，还可以对它们的私有数据priv_data进行赋值。这个字段里通常存储了各种编码器特有的结构体。而这些结构体的定义在FFmpeg的SDK中是找不到的。例如使用libx264进行编码的时候，通过AVCodecContext的priv_data字段可以对X264Context结构体中的变量进行赋值，设置preset，profile等。使用libx265进行编码的时候，通过AVCodecContext的priv_data字段可以对libx265Context结构体中的变量进行赋值，设置preset，tune等。

结构体定义

/**
 * AVOption
 */
typedef struct AVOption {
    const char *name;
 
    /**
     * short English help text
     * @todo What about other languages?
     */
    const char *help;
 
    /**
     * The offset relative to the context structure where the option
     * value is stored. It should be 0 for named constants.
     */
    int offset;
    enum AVOptionType type;
 
    /**
     * the default value for scalar options
     */
    union {
        int64_t i64;
        double dbl;
        const char *str;
        /* TODO those are unused now */
        AVRational q;
    } default_val;
    double min;                 ///< minimum valid value for the option
    double max;                 ///< maximum valid value for the option
 
    int flags;
#define AV_OPT_FLAG_ENCODING_PARAM  1   ///< a generic parameter which can be set by the user for muxing or encoding
#define AV_OPT_FLAG_DECODING_PARAM  2   ///< a generic parameter which can be set by the user for demuxing or decoding
#define AV_OPT_FLAG_AUDIO_PARAM     8
#define AV_OPT_FLAG_VIDEO_PARAM     16
#define AV_OPT_FLAG_SUBTITLE_PARAM  32
/**
 * The option is intended for exporting values to the caller.
 */
#define AV_OPT_FLAG_EXPORT          64
/**
 * The option may not be set through the AVOptions API, only read.
 * This flag only makes sense when AV_OPT_FLAG_EXPORT is also set.
 */
#define AV_OPT_FLAG_READONLY        128
#define AV_OPT_FLAG_BSF_PARAM       (1<<8) ///< a generic parameter which can be set by the user for bit stream filtering
#define AV_OPT_FLAG_RUNTIME_PARAM   (1<<15) ///< a generic parameter which can be set by the user at runtime
#define AV_OPT_FLAG_FILTERING_PARAM (1<<16) ///< a generic parameter which can be set by the user for filtering
#define AV_OPT_FLAG_DEPRECATED      (1<<17) ///< set if option is deprecated, users should refer to AVOption.help text for more information
#define AV_OPT_FLAG_CHILD_CONSTS    (1<<18) ///< set if option constants can also reside in child objects
//FIXME think about enc-audio, ... style flags
 
    /**
     * The logical unit to which the option belongs. Non-constant
     * options and corresponding named constants share the same
     * unit. May be NULL.
     */
    const char *unit;
} AVOption;

• 下面简单解释一下AVOption的几个成员变量：
  • name：名称。
  • help：简短的帮助。
  • offset：选项相对结构体首部地址的偏移量（这个很重要）。
  • type：选项的类型。
  • default_val：选项的默认值。
  • min：选项的最小值。
  • max：选项的最大值。
  • flags：一些标记。
  • unit：该选项所属的逻辑单元，可以为空。
•  其中，default_val是一个union类型的变量，可以根据选项数据类型的不同，取int，double，char*，AVRational（表示分数）几种类型。
• type是一个AVOptionType类型的变量。
• AVOptionType是一个枚举类型，定义如下。 

/**
 * @defgroup avoptions AVOptions
 * @ingroup lavu_data
 * @{
 * AVOptions provide a generic system to declare options on arbitrary structs
 * ("objects"). An option can have a help text, a type and a range of possible
 * values. Options may then be enumerated, read and written to.
 *
 * @section avoptions_implement Implementing AVOptions
 * This section describes how to add AVOptions capabilities to a struct.
 *
 * All AVOptions-related information is stored in an AVClass. Therefore
 * the first member of the struct should be a pointer to an AVClass describing it.
 * The option field of the AVClass must be set to a NULL-terminated static array
 * of AVOptions. Each AVOption must have a non-empty name, a type, a default
 * value and for number-type AVOptions also a range of allowed values. It must
 * also declare an offset in bytes from the start of the struct, where the field
 * associated with this AVOption is located. Other fields in the AVOption struct
 * should also be set when applicable, but are not required.
 *
 * The following example illustrates an AVOptions-enabled struct:
 * @code
 * typedef struct test_struct {
 *     const AVClass *class;
 *     int      int_opt;
 *     char    *str_opt;
 *     uint8_t *bin_opt;
 *     int      bin_len;
 * } test_struct;
 *
 * static const AVOption test_options[] = {
 *   { "test_int", "This is a test option of int type.", offsetof(test_struct, int_opt),
 *     AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX },
 *   { "test_str", "This is a test option of string type.", offsetof(test_struct, str_opt),
 *     AV_OPT_TYPE_STRING },
 *   { "test_bin", "This is a test option of binary type.", offsetof(test_struct, bin_opt),
 *     AV_OPT_TYPE_BINARY },
 *   { NULL },
 * };
 *
 * static const AVClass test_class = {
 *     .class_name = "test class",
 *     .item_name  = av_default_item_name,
 *     .option     = test_options,
 *     .version    = LIBAVUTIL_VERSION_INT,
 * };
 * @endcode
 *
 * Next, when allocating your struct, you must ensure that the AVClass pointer
 * is set to the correct value. Then, av_opt_set_defaults() can be called to
 * initialize defaults. After that the struct is ready to be used with the
 * AVOptions API.
 *
 * When cleaning up, you may use the av_opt_free() function to automatically
 * free all the allocated string and binary options.
 *
 * Continuing with the above example:
 *
 * @code
 * test_struct *alloc_test_struct(void)
 * {
 *     test_struct *ret = av_mallocz(sizeof(*ret));
 *     ret->class = &test_class;
 *     av_opt_set_defaults(ret);
 *     return ret;
 * }
 * void free_test_struct(test_struct **foo)
 * {
 *     av_opt_free(*foo);
 *     av_freep(foo);
 * }
 * @endcode
 *
 * @subsection avoptions_implement_nesting Nesting
 *      It may happen that an AVOptions-enabled struct contains another
 *      AVOptions-enabled struct as a member (e.g. AVCodecContext in
 *      libavcodec exports generic options, while its priv_data field exports
 *      codec-specific options). In such a case, it is possible to set up the
 *      parent struct to export a child's options. To do that, simply
 *      implement AVClass.child_next() and AVClass.child_class_iterate() in the
 *      parent struct's AVClass.
 *      Assuming that the test_struct from above now also contains a
 *      child_struct field:
 *
 *      @code
 *      typedef struct child_struct {
 *          AVClass *class;
 *          int flags_opt;
 *      } child_struct;
 *      static const AVOption child_opts[] = {
 *          { "test_flags", "This is a test option of flags type.",
 *            offsetof(child_struct, flags_opt), AV_OPT_TYPE_FLAGS, { .i64 = 0 }, INT_MIN, INT_MAX },
 *          { NULL },
 *      };
 *      static const AVClass child_class = {
 *          .class_name = "child class",
 *          .item_name  = av_default_item_name,
 *          .option     = child_opts,
 *          .version    = LIBAVUTIL_VERSION_INT,
 *      };
 *
 *      void *child_next(void *obj, void *prev)
 *      {
 *          test_struct *t = obj;
 *          if (!prev && t->child_struct)
 *              return t->child_struct;
 *          return NULL
 *      }
 *      const AVClass child_class_iterate(void **iter)
 *      {
 *          const AVClass *c = *iter ? NULL : &child_class;
 *          *iter = (void*)(uintptr_t)c;
 *          return c;
 *      }
 *      @endcode
 *      Putting child_next() and child_class_iterate() as defined above into
 *      test_class will now make child_struct's options accessible through
 *      test_struct (again, proper setup as described above needs to be done on
 *      child_struct right after it is created).
 *
 *      From the above example it might not be clear why both child_next()
 *      and child_class_iterate() are needed. The distinction is that child_next()
 *      iterates over actually existing objects, while child_class_iterate()
 *      iterates over all possible child classes. E.g. if an AVCodecContext
 *      was initialized to use a codec which has private options, then its
 *      child_next() will return AVCodecContext.priv_data and finish
 *      iterating. OTOH child_class_iterate() on AVCodecContext.av_class will
 *      iterate over all available codecs with private options.
 *
 * @subsection avoptions_implement_named_constants Named constants
 *      It is possible to create named constants for options. Simply set the unit
 *      field of the option the constants should apply to a string and
 *      create the constants themselves as options of type AV_OPT_TYPE_CONST
 *      with their unit field set to the same string.
 *      Their default_val field should contain the value of the named
 *      constant.
 *      For example, to add some named constants for the test_flags option
 *      above, put the following into the child_opts array:
 *      @code
 *      { "test_flags", "This is a test option of flags type.",
 *        offsetof(child_struct, flags_opt), AV_OPT_TYPE_FLAGS, { .i64 = 0 }, INT_MIN, INT_MAX, "test_unit" },
 *      { "flag1", "This is a flag with value 16", 0, AV_OPT_TYPE_CONST, { .i64 = 16 }, 0, 0, "test_unit" },
 *      @endcode
 *
 * @section avoptions_use Using AVOptions
 * This section deals with accessing options in an AVOptions-enabled struct.
 * Such structs in FFmpeg are e.g. AVCodecContext in libavcodec or
 * AVFormatContext in libavformat.
 *
 * @subsection avoptions_use_examine Examining AVOptions
 * The basic functions for examining options are av_opt_next(), which iterates
 * over all options defined for one object, and av_opt_find(), which searches
 * for an option with the given name.
 *
 * The situation is more complicated with nesting. An AVOptions-enabled struct
 * may have AVOptions-enabled children. Passing the AV_OPT_SEARCH_CHILDREN flag
 * to av_opt_find() will make the function search children recursively.
 *
 * For enumerating there are basically two cases. The first is when you want to
 * get all options that may potentially exist on the struct and its children
 * (e.g.  when constructing documentation). In that case you should call
 * av_opt_child_class_iterate() recursively on the parent struct's AVClass.  The
 * second case is when you have an already initialized struct with all its
 * children and you want to get all options that can be actually written or read
 * from it. In that case you should call av_opt_child_next() recursively (and
 * av_opt_next() on each result).
 *
 * @subsection avoptions_use_get_set Reading and writing AVOptions
 * When setting options, you often have a string read directly from the
 * user. In such a case, simply passing it to av_opt_set() is enough. For
 * non-string type options, av_opt_set() will parse the string according to the
 * option type.
 *
 * Similarly av_opt_get() will read any option type and convert it to a string
 * which will be returned. Do not forget that the string is allocated, so you
 * have to free it with av_free().
 *
 * In some cases it may be more convenient to put all options into an
 * AVDictionary and call av_opt_set_dict() on it. A specific case of this
 * are the format/codec open functions in lavf/lavc which take a dictionary
 * filled with option as a parameter. This makes it possible to set some options
 * that cannot be set otherwise, since e.g. the input file format is not known
 * before the file is actually opened.
 */
 
enum AVOptionType{
    AV_OPT_TYPE_FLAGS,
    AV_OPT_TYPE_INT,
    AV_OPT_TYPE_INT64,
    AV_OPT_TYPE_DOUBLE,
    AV_OPT_TYPE_FLOAT,
    AV_OPT_TYPE_STRING,
    AV_OPT_TYPE_RATIONAL,
    AV_OPT_TYPE_BINARY,  ///< offset must point to a pointer immediately followed by an int for the length
    AV_OPT_TYPE_DICT,
    AV_OPT_TYPE_UINT64,
    AV_OPT_TYPE_CONST,
    AV_OPT_TYPE_IMAGE_SIZE, ///< offset must point to two consecutive integers
    AV_OPT_TYPE_PIXEL_FMT,
    AV_OPT_TYPE_SAMPLE_FMT,
    AV_OPT_TYPE_VIDEO_RATE, ///< offset must point to AVRational
    AV_OPT_TYPE_DURATION,
    AV_OPT_TYPE_COLOR,
#if FF_API_OLD_CHANNEL_LAYOUT
    AV_OPT_TYPE_CHANNEL_LAYOUT,
#endif
    AV_OPT_TYPE_BOOL,
    AV_OPT_TYPE_CHLAYOUT,
};

补充

•  突然想到了JavaEE开发中也有这种类似的机制。互联网上只可以传输字符串，即是没有方法传输整形、浮点型这种的数据。而Java系统中却包含整形、浮点型等各种数据类型。因此开发JSP中的Servlet的时候经常需要将整数字符串手工转化成一个整型的变量。使用最多的一个函数就是Integer.parseInt()方法。例如下面代码可以将字符串“123”转化成整数123。
• int a=Integer.parseInt("123");
•  而在使用JavaEE中的Struts2进行开发的时候，就不需要进行手动转换处理了。Struts2中包含了类似AVOption的这种数据类型自动转换机制，可以将互联网上收到的字符串“名称”和“值”的组合自动赋值给相应名称的变量。由此发现了一个结论：编程语言之间真的是相通的！ 

AVOption有关的API

•  AVOption常用的API可以分成两类：用于设置参数的API和用于读取参数的API。
• 其中最有代表性的用于设置参数的API就是av_opt_set()；
• 而最有代表性的用于读取参数的API就是av_opt_get()。
• 除了记录以上两个函数之外，本文再记录一个在FFmpeg的结构体初始化代码中最常用的用于设置默认值的函数av_opt_set_defaults()。 

av_opt_set()

•  通过AVOption设置参数最常用的函数就是av_opt_set()了。
• 该函数通过字符串的方式（传入的参数是变量名称的字符串和变量值的字符串）设置一个AVOption的值。此外，还包含了它的一系列“兄弟”函数av_opt_set_XXX()，其中“XXX”代表了int，double这些数据类型。使用这些函数的时候，可以指定int，double这些类型的变量（而不是字符串）作为输入，设定相应的AVOption的值。

/**
 * @defgroup opt_set_funcs Option setting functions
 * @{
 * Those functions set the field of obj with the given name to value.
 *
 * @param[in] obj A struct whose first element is a pointer to an AVClass.
 * @param[in] name the name of the field to set
 * @param[in] val The value to set. In case of av_opt_set() if the field is not
 * of a string type, then the given string is parsed.
 * SI postfixes and some named scalars are supported.
 * If the field is of a numeric type, it has to be a numeric or named
 * scalar. Behavior with more than one scalar and +- infix operators
 * is undefined.
 * If the field is of a flags type, it has to be a sequence of numeric
 * scalars or named flags separated by '+' or '-'. Prefixing a flag
 * with '+' causes it to be set without affecting the other flags;
 * similarly, '-' unsets a flag.
 * If the field is of a dictionary type, it has to be a ':' separated list of
 * key=value parameters. Values containing ':' special characters must be
 * escaped.
 * @param search_flags flags passed to av_opt_find2. I.e. if AV_OPT_SEARCH_CHILDREN
 * is passed here, then the option may be set on a child of obj.
 *
 * @return 0 if the value has been set, or an AVERROR code in case of
 * error:
 * AVERROR_OPTION_NOT_FOUND if no matching option exists
 * AVERROR(ERANGE) if the value is out of range
 * AVERROR(EINVAL) if the value is not valid
 */
int av_opt_set         (void *obj, const char *name, const char *val, int search_flags);
int av_opt_set_int     (void *obj, const char *name, int64_t     val, int search_flags);
int av_opt_set_double  (void *obj, const char *name, double      val, int search_flags);
int av_opt_set_q       (void *obj, const char *name, AVRational  val, int search_flags);
int av_opt_set_bin     (void *obj, const char *name, const uint8_t *val, int size, int search_flags);
int av_opt_set_image_size(void *obj, const char *name, int w, int h, int search_flags);
int av_opt_set_pixel_fmt (void *obj, const char *name, enum AVPixelFormat fmt, int search_flags);
int av_opt_set_sample_fmt(void *obj, const char *name, enum AVSampleFormat fmt, int search_flags);
int av_opt_set_video_rate(void *obj, const char *name, AVRational val, int search_flags);
#if FF_API_OLD_CHANNEL_LAYOUT
attribute_deprecated
int av_opt_set_channel_layout(void *obj, const char *name, int64_t ch_layout, int search_flags);
#endif
int av_opt_set_chlayout(void *obj, const char *name, const AVChannelLayout *layout, int search_flags);
/**
 * @note Any old dictionary present is discarded and replaced with a copy of the new one. The
 * caller still owns val is and responsible for freeing it.
 */
int av_opt_set_dict_val(void *obj, const char *name, const AVDictionary *val, int search_flags);
 
/**
 * Set a binary option to an integer list.
 *
 * @param obj    AVClass object to set options on
 * @param name   name of the binary option
 * @param val    pointer to an integer list (must have the correct type with
 *               regard to the contents of the list)
 * @param term   list terminator (usually 0 or -1)
 * @param flags  search flags
 */
#define av_opt_set_int_list(obj, name, val, term, flags) \
    (av_int_list_length(val, term) > INT_MAX / sizeof(*(val)) ? \
     AVERROR(EINVAL) : \
     av_opt_set_bin(obj, name, (const uint8_t *)(val), \
                    av_int_list_length(val, term) * sizeof(*(val)), flags))

• 函数调用关系图如下所示。

• av_opt_set()的定义位于libavutil\opt.c，如下所示。 

int av_opt_set(void *obj, const char *name, const char *val, int search_flags)
{
    int ret = 0;
    void *dst, *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
FF_DISABLE_DEPRECATION_WARNINGS
    if (!val && (o->type != AV_OPT_TYPE_STRING &&
                 o->type != AV_OPT_TYPE_PIXEL_FMT && o->type != AV_OPT_TYPE_SAMPLE_FMT &&
                 o->type != AV_OPT_TYPE_IMAGE_SIZE &&
                 o->type != AV_OPT_TYPE_DURATION && o->type != AV_OPT_TYPE_COLOR &&
#if FF_API_OLD_CHANNEL_LAYOUT
                 o->type != AV_OPT_TYPE_CHANNEL_LAYOUT &&
#endif
                 o->type != AV_OPT_TYPE_BOOL))
        return AVERROR(EINVAL);
FF_ENABLE_DEPRECATION_WARNINGS
 
    if (o->flags & AV_OPT_FLAG_READONLY)
        return AVERROR(EINVAL);
 
    if (o->flags & AV_OPT_FLAG_DEPRECATED)
        av_log(obj, AV_LOG_WARNING, "The \"%s\" option is deprecated: %s\n", name, o->help);
 
    dst = ((uint8_t *)target_obj) + o->offset;
    switch (o->type) {
    case AV_OPT_TYPE_BOOL:
        return set_string_bool(obj, o, val, dst);
    case AV_OPT_TYPE_STRING:
        return set_string(obj, o, val, dst);
    case AV_OPT_TYPE_BINARY:
        return set_string_binary(obj, o, val, dst);
    case AV_OPT_TYPE_FLAGS:
    case AV_OPT_TYPE_INT:
    case AV_OPT_TYPE_INT64:
    case AV_OPT_TYPE_UINT64:
    case AV_OPT_TYPE_FLOAT:
    case AV_OPT_TYPE_DOUBLE:
    case AV_OPT_TYPE_RATIONAL:
        return set_string_number(obj, target_obj, o, val, dst);
    case AV_OPT_TYPE_IMAGE_SIZE:
        return set_string_image_size(obj, o, val, dst);
    case AV_OPT_TYPE_VIDEO_RATE: {
        AVRational tmp;
        ret = set_string_video_rate(obj, o, val, &tmp);
        if (ret < 0)
            return ret;
        return write_number(obj, o, dst, 1, tmp.den, tmp.num);
    }
    case AV_OPT_TYPE_PIXEL_FMT:
        return set_string_pixel_fmt(obj, o, val, dst);
    case AV_OPT_TYPE_SAMPLE_FMT:
        return set_string_sample_fmt(obj, o, val, dst);
    case AV_OPT_TYPE_DURATION:
        {
            int64_t usecs = 0;
            if (val) {
                if ((ret = av_parse_time(&usecs, val, 1)) < 0) {
                    av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as duration\n", val);
                    return ret;
                }
            }
            if (usecs < o->min || usecs > o->max) {
                av_log(obj, AV_LOG_ERROR, "Value %f for parameter '%s' out of range [%g - %g]\n",
                       usecs / 1000000.0, o->name, o->min / 1000000.0, o->max / 1000000.0);
                return AVERROR(ERANGE);
            }
            *(int64_t *)dst = usecs;
            return 0;
        }
    case AV_OPT_TYPE_COLOR:
        return set_string_color(obj, o, val, dst);
#if FF_API_OLD_CHANNEL_LAYOUT
FF_DISABLE_DEPRECATION_WARNINGS
    case AV_OPT_TYPE_CHANNEL_LAYOUT:
        if (!val || !strcmp(val, "none")) {
            *(int64_t *)dst = 0;
        } else {
            int64_t cl = av_get_channel_layout(val);
            if (!cl) {
                av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as channel layout\n", val);
                ret = AVERROR(EINVAL);
            }
            *(int64_t *)dst = cl;
            return ret;
        }
        break;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    case AV_OPT_TYPE_CHLAYOUT:
        ret = set_string_channel_layout(obj, o, val, dst);
        if (ret < 0) {
            av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as channel layout\n", val);
            ret = AVERROR(EINVAL);
        }
        return ret;
    case AV_OPT_TYPE_DICT:
        return set_string_dict(obj, o, val, dst);
    }
 
    av_log(obj, AV_LOG_ERROR, "Invalid option type.\n");
    return AVERROR(EINVAL);
}

•  从源代码可以看出，av_opt_set()首先调用av_opt_find2()查找AVOption。
• 如果找到了，则根据AVOption的type，调用不同的函数（set_string()，set_string_number()，set_string_image_size()等等）将输入的字符串转化为相应type的数据并对该AVOption进行赋值。
• 如果没有找到，则立即返回“没有找到AVOption”的错误。 
•  现在再回到刚才的av_opt_set()函数。该函数有一个void型的变量dst用于确定需要设定的AVOption对应的变量的位置。具体的方法就是将输入的AVClass结构体的首地址加上该AVOption的偏移量offset。确定了AVOption对应的变量的位置之后，就可以根据该AVOption的类型type的不同调用不同的字符串转换函数设置相应的值了。

av_opt_find2() / av_opt_find()

• av_opt_find2()本身也是一个API函数，用于查找AVOption。
• 它的声明位于libavutil\opt.h中，如下所示。

/**
 * Look for an option in an object. Consider only options which
 * have all the specified flags set.
 *
 * @param[in] obj A pointer to a struct whose first element is a
 *                pointer to an AVClass.
 *                Alternatively a double pointer to an AVClass, if
 *                AV_OPT_SEARCH_FAKE_OBJ search flag is set.
 * @param[in] name The name of the option to look for.
 * @param[in] unit When searching for named constants, name of the unit
 *                 it belongs to.
 * @param opt_flags Find only options with all the specified flags set (AV_OPT_FLAG).
 * @param search_flags A combination of AV_OPT_SEARCH_*.
 * @param[out] target_obj if non-NULL, an object to which the option belongs will be
 * written here. It may be different from obj if AV_OPT_SEARCH_CHILDREN is present
 * in search_flags. This parameter is ignored if search_flags contain
 * AV_OPT_SEARCH_FAKE_OBJ.
 *
 * @return A pointer to the option found, or NULL if no option
 *         was found.
 */
const AVOption *av_opt_find2(void *obj, const char *name, const char *unit,
                             int opt_flags, int search_flags, void **target_obj);

• 此外还有一个和av_opt_find2()“长得很像”的API函数av_opt_find()，功能与av_opt_find2()基本类似，与av_opt_find2()相比少了最后一个参数。
• 从源代码中可以看出它只是简单调用了av_opt_find2()并把所有的输入参数原封不动的传递过去，并把最后一个参数设置成NULL。 

const AVOption *av_opt_find2(void *obj, const char *name, const char *unit,
                             int opt_flags, int search_flags, void **target_obj)
{
    const AVClass  *c;
    const AVOption *o = NULL;
 
    if(!obj)
        return NULL;
 
    c= *(AVClass**)obj;
 
    if (!c)
        return NULL;
 
    if (search_flags & AV_OPT_SEARCH_CHILDREN) {
        if (search_flags & AV_OPT_SEARCH_FAKE_OBJ) {
            void *iter = NULL;
            const AVClass *child;
            while (child = av_opt_child_class_iterate(c, &iter))
                if (o = av_opt_find2(&child, name, unit, opt_flags, search_flags, NULL))
                    return o;
        } else {
            void *child = NULL;
            while (child = av_opt_child_next(obj, child))
                if (o = av_opt_find2(child, name, unit, opt_flags, search_flags, target_obj))
                    return o;
        }
    }
 
    while (o = av_opt_next(obj, o)) {
        if (!strcmp(o->name, name) && (o->flags & opt_flags) == opt_flags &&
            ((!unit && o->type != AV_OPT_TYPE_CONST) ||
             (unit  && o->type == AV_OPT_TYPE_CONST && o->unit && !strcmp(o->unit, unit)))) {
            if (target_obj) {
                if (!(search_flags & AV_OPT_SEARCH_FAKE_OBJ))
                    *target_obj = obj;
                else
                    *target_obj = NULL;
            }
            return o;
        }
    }
    return NULL;
}

const AVOption *av_opt_find(void *obj, const char *name, const char *unit,
                            int opt_flags, int search_flags)
{
    return av_opt_find2(obj, name, unit, opt_flags, search_flags, NULL);
}

•  这段代码的前半部分暂时不关注，前半部分的if()语句中的内容只有在search_flags指定为AV_OPT_SEARCH_CHILDREN的时候才会执行。
• 后半部分代码是重点。后半部分代码是一个while()循环，该循环的条件是一个函数av_opt_next()。
•  现在再回到av_opt_find2()函数。我们发现在while()循环中有一个strcmp()函数，正是这个函数比较输入的AVOption的name和AVClass的option数组中每个元素的name，当上述两个name相等的时候，就代表查找到了AVOption，接着就可以返回获得的AVOption。

av_opt_next()

• av_opt_next()也是一个FFmpeg的API函数。使用它可以循环遍历目标结构体的所有AVOption，它的声明如下。

/**
 * Iterate over all AVOptions belonging to obj.
 *
 * @param obj an AVOptions-enabled struct or a double pointer to an
 *            AVClass describing it.
 * @param prev result of the previous call to av_opt_next() on this object
 *             or NULL
 * @return next AVOption or NULL
 */
const AVOption *av_opt_next(const void *obj, const AVOption *prev);

• av_opt_next()的定义如下所示。

const AVOption *av_opt_next(const void *obj, const AVOption *last)
{
    const AVClass *class;
    if (!obj)
        return NULL;
    class = *(const AVClass**)obj;
    if (!last && class && class->option && class->option[0].name)
        return class->option;
    if (last && last[1].name)
        return ++last;
    return NULL;
}

•  从av_opt_next()的代码可以看出，输入的AVOption类型的last变量为空的时候，会返回该AVClass的option数组的第一个元素，否则会返回数组的下一个元素。

我们可以看几个设置值的的简单例子： 

1. AV_OPT_TYPE_STRING

• 当AVOption的type为AV_OPT_TYPE_STRING的时候，调用set_string()方法设置相应的值。
• set_string()的定义如下：

static int set_string(void *obj, const AVOption *o, const char *val, uint8_t **dst)
{
    av_freep(dst);
    *dst = av_strdup(val);
    return *dst ? 0 : AVERROR(ENOMEM);
}

• 其中又调用了一个函数av_strdup()，这是一个FFmpeg的API函数，用于拷贝字符串。
• 它的代码如下所示，其中调用了memcpy()。

char *av_strdup(const char *s)
{
    char *ptr = NULL;
    if (s) {
        size_t len = strlen(s) + 1;
        ptr = av_realloc(NULL, len);
        if (ptr)
            memcpy(ptr, s, len);
    }
    return ptr;
}

 2. AV_OPT_TYPE_IMAGE_SIZE

• 当AVOption的type为AV_OPT_TYPE_IMAGE_SIZE的时候，调用set_string_image_size ()方法设置相应的值。
• set_string_image_size()的定义如下。

static int set_string_image_size(void *obj, const AVOption *o, const char *val, int *dst)
{
    int ret;
 
    if (!val || !strcmp(val, "none")) {
        dst[0] =
        dst[1] = 0;
        return 0;
    }
    ret = av_parse_video_size(dst, dst + 1, val);
    if (ret < 0)
        av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as image size\n", val);
    return ret;
}

•  可见其中调用了另一个函数av_parse_video_size()。

av_parse_video_size()

•  av_parse_video_size()是一个FFmpeg的API函数，用于解析出输入的分辨率字符串的宽高信息。例如，输入的字符串为“1920x1080”或者“1920*1080”，经过av_parse_video_size()的处理之后，可以得到宽度为1920，高度为1080；
• 此外，输入一个“特定分辨率”字符串例如“vga”，也可以得到宽度为640，高度为480。
• 该函数不属于AVOption这部分的内容，而是整个FFmpeg通用的一个字符串解析函数。
• 声明位于libavutil\parseutils.h中，如下所示。

/**
 * Parse str and put in width_ptr and height_ptr the detected values.
 *
 * @param[in,out] width_ptr pointer to the variable which will contain the detected
 * width value
 * @param[in,out] height_ptr pointer to the variable which will contain the detected
 * height value
 * @param[in] str the string to parse: it has to be a string in the format
 * width x height or a valid video size abbreviation.
 * @return >= 0 on success, a negative error code otherwise
 */
int av_parse_video_size(int *width_ptr, int *height_ptr, const char *str);

• av_parse_video_size()定义位于libavutil\parseutils.c中，代码如下。 

//解析分辨率
int av_parse_video_size(int *width_ptr, int *height_ptr, const char *str)
{
    int i;
    int n = FF_ARRAY_ELEMS(video_size_abbrs);
    const char *p;
    int width = 0, height = 0;
    //先看看有没有“分辨率简称”相同的（例如vga，qcif等）
    for (i = 0; i < n; i++) {
        if (!strcmp(video_size_abbrs[i].abbr, str)) {
            width  = video_size_abbrs[i].width;
            height = video_size_abbrs[i].height;
            break;
        }
    }
    //如果没有使用“分辨率简称”，而是使用具体的数值（例如“1920x1080”），则执行下面的步骤
    if (i == n) {
    	//strtol()：字符串转换成整型，遇到非数字则停止
        width = strtol(str, (void*)&p, 10);
        if (*p)
            p++;
        height = strtol(p, (void*)&p, 10);
 
        /* trailing extraneous data detected, like in 123x345foobar */
        if (*p)
            return AVERROR(EINVAL);
    }
    //检查一下正确性
    if (width <= 0 || height <= 0)
        return AVERROR(EINVAL);
    *width_ptr  = width;
    *height_ptr = height;
    return 0;
}

• 从声明中可以看出，该函数输入一个字符串str，输出结果保存在width_ptr和height_ptr所指向的内存中。
• 上述代码中包含了FFmpeg中两种解析视频分辨率的方法。FFmpeg中包含两种设定视频分辨率的方法：通过已经定义好的“分辨率简称”，或者通过具体的数值。代码中首先遍历“特定分辨率”的数组video_size_abbrs。
• 该数组定义如下所示。

static const VideoSizeAbbr video_size_abbrs[] = {
    { "ntsc",      720, 480 },
    { "pal",       720, 576 },
    { "qntsc",     352, 240 }, /* VCD compliant NTSC */
    { "qpal",      352, 288 }, /* VCD compliant PAL */
    { "sntsc",     640, 480 }, /* square pixel NTSC */
    { "spal",      768, 576 }, /* square pixel PAL */
    { "film",      352, 240 },
    { "ntsc-film", 352, 240 },
    { "sqcif",     128,  96 },
    { "qcif",      176, 144 },
    { "cif",       352, 288 },
    { "4cif",      704, 576 },
    { "16cif",    1408,1152 },
    { "qqvga",     160, 120 },
    { "qvga",      320, 240 },
    { "vga",       640, 480 },
    { "svga",      800, 600 },
    { "xga",      1024, 768 },
    { "uxga",     1600,1200 },
    { "qxga",     2048,1536 },
    { "sxga",     1280,1024 },
    { "qsxga",    2560,2048 },
    { "hsxga",    5120,4096 },
    { "wvga",      852, 480 },
    { "wxga",     1366, 768 },
    { "wsxga",    1600,1024 },
    { "wuxga",    1920,1200 },
    { "woxga",    2560,1600 },
    { "wqhd",     2560,1440 },
    { "wqsxga",   3200,2048 },
    { "wquxga",   3840,2400 },
    { "whsxga",   6400,4096 },
    { "whuxga",   7680,4800 },
    { "cga",       320, 200 },
    { "ega",       640, 350 },
    { "hd480",     852, 480 },
    { "hd720",    1280, 720 },
    { "hd1080",   1920,1080 },
    { "quadhd",   2560,1440 },
    { "2k",       2048,1080 }, /* Digital Cinema System Specification */
    { "2kdci",    2048,1080 },
    { "2kflat",   1998,1080 },
    { "2kscope",  2048, 858 },
    { "4k",       4096,2160 }, /* Digital Cinema System Specification */
    { "4kdci",    4096,2160 },
    { "4kflat",   3996,2160 },
    { "4kscope",  4096,1716 },
    { "nhd",       640,360  },
    { "hqvga",     240,160  },
    { "wqvga",     400,240  },
    { "fwqvga",    432,240  },
    { "hvga",      480,320  },
    { "qhd",       960,540  },
    { "uhd2160",  3840,2160 },
    { "uhd4320",  7680,4320 },
};

• 通过调用strcmp()方法比对输入字符串的值与video_size_abbrs数组中每个VideoSizeAbbr元素的abbr字段的值，判断输入的字符串是否指定了这些标准的分辨率。如果指定了的话，则返回该分辨率的宽和高。
•  如果从上述列表中没有找到相应的“特定分辨率”，则说明输入的字符串应该是一个具体的分辨率的值，形如“1920*1020”，“1280x720”这样的字符串。这个时候就需要对这个字符串进行解析，并从中提取出数字信息。通过两次调用strtol()方法，从字符串中提取出宽高信息（第一次提取出宽，第二次提取出高）。
• PS1：strtol()用于将字符串转换成整型，遇到非数字则停止。
• PS2：从这种解析方法可以得到一个信息——FFmpeg并不管“宽{X}高”中间的那个{X}是什么字符，也就是说中间那个字符不一定非得是“*”或者“x”。后来试了一下，中间那个字符使用其他字母也是可以的。 

av_opt_get()

• 函数调用关系图如下所示 

• av_opt_get()用于获取一个AVOption变量的值。
• 需要注意的是，不论是何种类型的变量，通过av_opt_get()取出来的值都是字符串类型的。
• 此外，还包含了它的一系列“兄弟”函数av_opt_get_XXX()（其中“XXX”代表了int，double这些数据类型）。通过这些“兄弟”函数可以直接取出int，double类型的数值。
• av_opt_get()的声明如下所示。 

/**
 * @defgroup opt_get_funcs Option getting functions
 * @{
 * Those functions get a value of the option with the given name from an object.
 *
 * @param[in] obj a struct whose first element is a pointer to an AVClass.
 * @param[in] name name of the option to get.
 * @param[in] search_flags flags passed to av_opt_find2. I.e. if AV_OPT_SEARCH_CHILDREN
 * is passed here, then the option may be found in a child of obj.
 * @param[out] out_val value of the option will be written here
 * @return >=0 on success, a negative error code otherwise
 */
/**
 * @note the returned string will be av_malloc()ed and must be av_free()ed by the caller
 *
 * @note if AV_OPT_ALLOW_NULL is set in search_flags in av_opt_get, and the
 * option is of type AV_OPT_TYPE_STRING, AV_OPT_TYPE_BINARY or AV_OPT_TYPE_DICT
 * and is set to NULL, *out_val will be set to NULL instead of an allocated
 * empty string.
 */
int av_opt_get         (void *obj, const char *name, int search_flags, uint8_t   **out_val);
int av_opt_get_int     (void *obj, const char *name, int search_flags, int64_t    *out_val);
int av_opt_get_double  (void *obj, const char *name, int search_flags, double     *out_val);
int av_opt_get_q       (void *obj, const char *name, int search_flags, AVRational *out_val);
int av_opt_get_image_size(void *obj, const char *name, int search_flags, int *w_out, int *h_out);
int av_opt_get_pixel_fmt (void *obj, const char *name, int search_flags, enum AVPixelFormat *out_fmt);
int av_opt_get_sample_fmt(void *obj, const char *name, int search_flags, enum AVSampleFormat *out_fmt);
int av_opt_get_video_rate(void *obj, const char *name, int search_flags, AVRational *out_val);
#if FF_API_OLD_CHANNEL_LAYOUT
attribute_deprecated
int av_opt_get_channel_layout(void *obj, const char *name, int search_flags, int64_t *ch_layout);
#endif
int av_opt_get_chlayout(void *obj, const char *name, int search_flags, AVChannelLayout *layout);
/**
 * @param[out] out_val The returned dictionary is a copy of the actual value and must
 * be freed with av_dict_free() by the caller
 */
int av_opt_get_dict_val(void *obj, const char *name, int search_flags, AVDictionary **out_val);

• 下面我们看一下av_opt_get()的定义，如下所示。
• 从av_opt_get()的定义可以看出，该函数首先通过av_opt_find2()查相应的AVOption，然后取出该变量的值，最后通过snprintf()将变量的值转化为字符串（各种各样类型的变量都这样处理）并且输出出来。
• 至此FFmpeg中和AVOption相关的源代码基本上就分析完毕了。

int av_opt_get(void *obj, const char *name, int search_flags, uint8_t **out_val)
{
    void *dst, *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
    uint8_t *bin, buf[128];
    int len, i, ret;
    int64_t i64;
 
    if (!o || !target_obj || (o->offset<=0 && o->type != AV_OPT_TYPE_CONST))
        return AVERROR_OPTION_NOT_FOUND;
 
    if (o->flags & AV_OPT_FLAG_DEPRECATED)
        av_log(obj, AV_LOG_WARNING, "The \"%s\" option is deprecated: %s\n", name, o->help);
 
    dst = (uint8_t *)target_obj + o->offset;
 
    buf[0] = 0;
    switch (o->type) {
    case AV_OPT_TYPE_BOOL:
        ret = snprintf(buf, sizeof(buf), "%s", (char *)av_x_if_null(get_bool_name(*(int *)dst), "invalid"));
        break;
    case AV_OPT_TYPE_FLAGS:
        ret = snprintf(buf, sizeof(buf), "0x%08X", *(int *)dst);
        break;
    case AV_OPT_TYPE_INT:
        ret = snprintf(buf, sizeof(buf), "%d", *(int *)dst);
        break;
    case AV_OPT_TYPE_INT64:
        ret = snprintf(buf, sizeof(buf), "%"PRId64, *(int64_t *)dst);
        break;
    case AV_OPT_TYPE_UINT64:
        ret = snprintf(buf, sizeof(buf), "%"PRIu64, *(uint64_t *)dst);
        break;
    case AV_OPT_TYPE_FLOAT:
        ret = snprintf(buf, sizeof(buf), "%f", *(float *)dst);
        break;
    case AV_OPT_TYPE_DOUBLE:
        ret = snprintf(buf, sizeof(buf), "%f", *(double *)dst);
        break;
    case AV_OPT_TYPE_VIDEO_RATE:
    case AV_OPT_TYPE_RATIONAL:
        ret = snprintf(buf, sizeof(buf), "%d/%d", ((AVRational *)dst)->num, ((AVRational *)dst)->den);
        break;
    case AV_OPT_TYPE_CONST:
        ret = snprintf(buf, sizeof(buf), "%f", o->default_val.dbl);
        break;
    case AV_OPT_TYPE_STRING:
        if (*(uint8_t **)dst) {
            *out_val = av_strdup(*(uint8_t **)dst);
        } else if (search_flags & AV_OPT_ALLOW_NULL) {
            *out_val = NULL;
            return 0;
        } else {
            *out_val = av_strdup("");
        }
        return *out_val ? 0 : AVERROR(ENOMEM);
    case AV_OPT_TYPE_BINARY:
        if (!*(uint8_t **)dst && (search_flags & AV_OPT_ALLOW_NULL)) {
            *out_val = NULL;
            return 0;
        }
        len = *(int *)(((uint8_t *)dst) + sizeof(uint8_t *));
        if ((uint64_t)len * 2 + 1 > INT_MAX)
            return AVERROR(EINVAL);
        if (!(*out_val = av_malloc(len * 2 + 1)))
            return AVERROR(ENOMEM);
        if (!len) {
            *out_val[0] = '\0';
            return 0;
        }
        bin = *(uint8_t **)dst;
        for (i = 0; i < len; i++)
            snprintf(*out_val + i * 2, 3, "%02X", bin[i]);
        return 0;
    case AV_OPT_TYPE_IMAGE_SIZE:
        ret = snprintf(buf, sizeof(buf), "%dx%d", ((int *)dst)[0], ((int *)dst)[1]);
        break;
    case AV_OPT_TYPE_PIXEL_FMT:
        ret = snprintf(buf, sizeof(buf), "%s", (char *)av_x_if_null(av_get_pix_fmt_name(*(enum AVPixelFormat *)dst), "none"));
        break;
    case AV_OPT_TYPE_SAMPLE_FMT:
        ret = snprintf(buf, sizeof(buf), "%s", (char *)av_x_if_null(av_get_sample_fmt_name(*(enum AVSampleFormat *)dst), "none"));
        break;
    case AV_OPT_TYPE_DURATION:
        i64 = *(int64_t *)dst;
        format_duration(buf, sizeof(buf), i64);
        ret = strlen(buf); // no overflow possible, checked by an assert
        break;
    case AV_OPT_TYPE_COLOR:
        ret = snprintf(buf, sizeof(buf), "0x%02x%02x%02x%02x",
                       (int)((uint8_t *)dst)[0], (int)((uint8_t *)dst)[1],
                       (int)((uint8_t *)dst)[2], (int)((uint8_t *)dst)[3]);
        break;
#if FF_API_OLD_CHANNEL_LAYOUT
FF_DISABLE_DEPRECATION_WARNINGS
    case AV_OPT_TYPE_CHANNEL_LAYOUT:
 
        i64 = *(int64_t *)dst;
        ret = snprintf(buf, sizeof(buf), "0x%"PRIx64, i64);
        break;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    case AV_OPT_TYPE_CHLAYOUT:
        ret = av_channel_layout_describe(dst, buf, sizeof(buf));
        break;
    case AV_OPT_TYPE_DICT:
        if (!*(AVDictionary **)dst && (search_flags & AV_OPT_ALLOW_NULL)) {
            *out_val = NULL;
            return 0;
        }
        return av_dict_get_string(*(AVDictionary **)dst, (char **)out_val, '=', ':');
    default:
        return AVERROR(EINVAL);
    }
 
    if (ret >= sizeof(buf))
        return AVERROR(EINVAL);
    *out_val = av_strdup(buf);
    return *out_val ? 0 : AVERROR(ENOMEM);
}

 av_opt_set_defaults()

• 函数调用关系图如下所示。

• av_opt_set_defaults()是一个FFmpeg的API，作用是给一个结构体的成员变量设定默认值。
• 在FFmpeg初始化其各种结构体（AVFormatContext，AVCodecContext等）的时候，通常会调用该函数设置结构体中的默认值。
• av_opt_set_defaults()的声明如下所示。

/**
 * Set the values of all AVOption fields to their default values.
 *
 * @param s an AVOption-enabled struct (its first member must be a pointer to AVClass)
 */
void av_opt_set_defaults(void *s);

• 可见只需要把包含AVOption功能的结构体（第一个变量是一个AVClass类型的指针）的指针提供给av_opt_set_defaults()，就可以初始化该结构体的默认值了。
• 下面看一下av_opt_set_defaults()的源代码，位于libavutil\opt.c，如下所示。

void av_opt_set_defaults(void *s)
{
    av_opt_set_defaults2(s, 0, 0);
}
 
void av_opt_set_defaults2(void *s, int mask, int flags)
{
    const AVOption *opt = NULL;
    while ((opt = av_opt_next(s, opt))) {
        void *dst = ((uint8_t*)s) + opt->offset;
 
        if ((opt->flags & mask) != flags)
            continue;
 
        if (opt->flags & AV_OPT_FLAG_READONLY)
            continue;
 
        switch (opt->type) {
            case AV_OPT_TYPE_CONST:
                /* Nothing to be done here */
                break;
            case AV_OPT_TYPE_BOOL:
            case AV_OPT_TYPE_FLAGS:
            case AV_OPT_TYPE_INT:
            case AV_OPT_TYPE_INT64:
            case AV_OPT_TYPE_UINT64:
            case AV_OPT_TYPE_DURATION:
#if FF_API_OLD_CHANNEL_LAYOUT
FF_DISABLE_DEPRECATION_WARNINGS
            case AV_OPT_TYPE_CHANNEL_LAYOUT:
FF_ENABLE_DEPRECATION_WARNINGS
#endif
            case AV_OPT_TYPE_PIXEL_FMT:
            case AV_OPT_TYPE_SAMPLE_FMT:
                write_number(s, opt, dst, 1, 1, opt->default_val.i64);
                break;
            case AV_OPT_TYPE_DOUBLE:
            case AV_OPT_TYPE_FLOAT: {
                double val;
                val = opt->default_val.dbl;
                write_number(s, opt, dst, val, 1, 1);
            }
            break;
            case AV_OPT_TYPE_RATIONAL: {
                AVRational val;
                val = av_d2q(opt->default_val.dbl, INT_MAX);
                write_number(s, opt, dst, 1, val.den, val.num);
            }
            break;
            case AV_OPT_TYPE_COLOR:
                set_string_color(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_STRING:
                set_string(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_IMAGE_SIZE:
                set_string_image_size(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_VIDEO_RATE:
                set_string_video_rate(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_BINARY:
                set_string_binary(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_CHLAYOUT:
                set_string_channel_layout(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_DICT:
                set_string_dict(s, opt, opt->default_val.str, dst);
                break;
        default:
            av_log(s, AV_LOG_DEBUG, "AVOption type %d of option %s not implemented yet\n",
                   opt->type, opt->name);
        }
    }
}

•  av_opt_set_defaults()主体部分是一个while()循环。该循环的判断条件是一个av_opt_next()，其作用是获得下一个AVOption。该函数的定义在前文中已经做过分析，这里不再重复。
• 下面简单解读一下av_opt_set_defaults()中while()循环语句里面的内容。有一个void类型的指针dst用于确定当前AVOption代表的变量的位置。该指针的位置有结构体的首地址和变量的偏移量offset确定。然后根据AVOption代表的变量的类型type，调用不同的函数设定相应的值。例如type为AV_OPT_TYPE_INT的话，则会调用write_number()；type为AV_OPT_TYPE_STRING的时候，则会调用set_string()；type为AV_OPT_TYPE_IMAGE_SIZE的时候，则会调用set_string_image_size()。有关这些设置值的函数在前文中已经有所叙述，不再重复。需要注意的是，该函数中设置的值都是AVOption中的default_val变量的值。

结构体成员管理系统-AVClass

•  AVClass最主要的作用就是给结构体（例如AVFormatContext等）增加AVOption功能的支持。换句话说AVClass就是AVOption和目标结构体之间的“桥梁”。AVClass要求必须声明为目标结构体的第一个变量。
•  AVClass中有一个option数组用于存储目标结构体的所有的AVOption。
• 举个例子，AVFormatContext结构体，AVClass和AVOption之间的关系如下图所示。

•  图中AVFormatContext结构体的第一个变量为AVClass类型的指针av_class，它在AVFormatContext结构体初始化的时候，被赋值指向了全局静态变量av_format_context_class结构体（定义位于libavformat\options.c）。而AVClass类型的av_format_context_class结构体中的option变量指向了全局静态数组avformat_options（定义位于libavformat\options_table.h）
•  AVClass中存储了AVOption类型的数组option，用于存储选项信息。AVClass有一个特点就是它必须位于其支持的结构体的第一个位置。例如，AVFormatContext和AVCodecContext都支持AVClass，观察它们结构体的定义可以发现他们结构体的第一个变量都是AVClass。截取一小段AVFormatContext的定义的开头部分，如下所示。

typedef struct AVFormatContext {
    /**
     * A class for logging and @ref avoptions. Set by avformat_alloc_context().
     * Exports (de)muxer private options if they exist.
     */
    const AVClass *av_class;
 
    /**
     * The input container format.
     *
     * Demuxing only, set by avformat_open_input().
     */
    struct AVInputFormat *iformat;
 
    /**
     * The output container format.
     *
     * Muxing only, must be set by the caller before avformat_write_header().
     */
struct AVOutputFormat *oformat;
//后文略

• 截取一小段AVCodecContext的定义的开头部分，如下所示。

typedef struct AVCodecContext {
    /**
     * information on struct for av_log
     * - set by avcodec_alloc_context3
     */
    const AVClass *av_class;
    int log_level_offset;
 
    enum AVMediaType codec_type; /* see AVMEDIA_TYPE_xxx */
    const struct AVCodec  *codec;
	//后文略

• 下面来看一下AVClass的定义，如下所示。

/**
 * Describe the class of an AVClass context structure. That is an
 * arbitrary struct of which the first field is a pointer to an
 * AVClass struct (e.g. AVCodecContext, AVFormatContext etc.).
 */
typedef struct AVClass {
    /**
     * The name of the class; usually it is the same name as the
     * context structure type to which the AVClass is associated.
     */
    const char* class_name;
 
    /**
     * A pointer to a function which returns the name of a context
     * instance ctx associated with the class.
     */
    const char* (*item_name)(void* ctx);
 
    /**
     * a pointer to the first option specified in the class if any or NULL
     *
     * @see av_set_default_options()
     */
    const struct AVOption *option;
 
    /**
     * LIBAVUTIL_VERSION with which this structure was created.
     * This is used to allow fields to be added without requiring major
     * version bumps everywhere.
     */
 
    int version;
 
    /**
     * Offset in the structure where log_level_offset is stored.
     * 0 means there is no such variable
     */
    int log_level_offset_offset;
 
    /**
     * Offset in the structure where a pointer to the parent context for
     * logging is stored. For example a decoder could pass its AVCodecContext
     * to eval as such a parent context, which an av_log() implementation
     * could then leverage to display the parent context.
     * The offset can be NULL.
     */
    int parent_log_context_offset;
 
    /**
     * Category used for visualization (like color)
     * This is only set if the category is equal for all objects using this class.
     * available since version (51 << 16 | 56 << 8 | 100)
     */
    AVClassCategory category;
 
    /**
     * Callback to return the category.
     * available since version (51 << 16 | 59 << 8 | 100)
     */
    AVClassCategory (*get_category)(void* ctx);
 
    /**
     * Callback to return the supported/allowed ranges.
     * available since version (52.12)
     */
    int (*query_ranges)(struct AVOptionRanges **, void *obj, const char *key, int flags);
 
    /**
     * Return next AVOptions-enabled child or NULL
     */
    void* (*child_next)(void *obj, void *prev);
 
    /**
     * Iterate over the AVClasses corresponding to potential AVOptions-enabled
     * children.
     *
     * @param iter pointer to opaque iteration state. The caller must initialize
     *             *iter to NULL before the first call.
     * @return AVClass for the next AVOptions-enabled child or NULL if there are
     *         no more such children.
     *
     * @note The difference between child_next and this is that child_next
     *       iterates over _already existing_ objects, while child_class_iterate
     *       iterates over _all possible_ children.
     */
    const struct AVClass* (*child_class_iterate)(void **iter);
} AVClass;

• 下面简单解释一下AVClass的几个已经理解的成员变量：
• class_name：AVClass名称。
• item_name：函数，获取与AVClass相关联的结构体实例的名称。
• option：AVOption类型的数组（最重要）。
• version：完成该AVClass的时候的LIBAVUTIL_VERSION。
• category：AVClass的类型，是一个类型为AVClassCategory的枚举型变量。

• 其中AVClassCategory定义如下。

typedef enum {
    AV_CLASS_CATEGORY_NA = 0,
    AV_CLASS_CATEGORY_INPUT,
    AV_CLASS_CATEGORY_OUTPUT,
    AV_CLASS_CATEGORY_MUXER,
    AV_CLASS_CATEGORY_DEMUXER,
    AV_CLASS_CATEGORY_ENCODER,
    AV_CLASS_CATEGORY_DECODER,
    AV_CLASS_CATEGORY_FILTER,
    AV_CLASS_CATEGORY_BITSTREAM_FILTER,
    AV_CLASS_CATEGORY_SWSCALER,
    AV_CLASS_CATEGORY_SWRESAMPLER,
    AV_CLASS_CATEGORY_DEVICE_VIDEO_OUTPUT = 40,
    AV_CLASS_CATEGORY_DEVICE_VIDEO_INPUT,
    AV_CLASS_CATEGORY_DEVICE_AUDIO_OUTPUT,
    AV_CLASS_CATEGORY_DEVICE_AUDIO_INPUT,
    AV_CLASS_CATEGORY_DEVICE_OUTPUT,
    AV_CLASS_CATEGORY_DEVICE_INPUT,
    AV_CLASS_CATEGORY_NB  ///< not part of ABI/API
}AVClassCategory;

•  上面解释字段还是比较抽象的，下面通过具体的例子看一下AVClass这个结构体。我们看几个具体的例子：
  • AVFormatContext中的AVClass
  • AVCodecContext中的AVClass
  • AVFrame中的AVClass
  • 各种组件（libRTMP，libx264，libx265）里面特有的AVClass。

AVFormatContext中的AVClass

• AVFormatContext 中的AVClass定义位于libavformat\options.c中，是一个名称为av_format_context_class的静态结构体。
• 如下所示。

static const AVClass av_format_context_class = {
    .class_name     = "AVFormatContext",
    .item_name      = format_to_name,
    .option         = avformat_options,
    .version        = LIBAVUTIL_VERSION_INT,
    .child_next     = format_child_next,
    .child_class_iterate = format_child_class_iterate,
    .category       = AV_CLASS_CATEGORY_MUXER,
    .get_category   = get_category,
};

• 从源代码可以看出以下几点
  • （1）class_name
• 该AVClass名称是“AVFormatContext”。
  • （2）item_name
• item_name指向一个函数format_to_name()，该函数定义如下所示。

static const char* format_to_name(void* ptr)
{
    AVFormatContext* fc = (AVFormatContext*) ptr;
    if(fc->iformat) return fc->iformat->name;
    else if(fc->oformat) return fc->oformat->name;
    else return "NULL";
}

• 从函数的定义可以看出，如果AVFormatContext结构体中的AVInputFormat结构体不为空，则返回AVInputFormat的name，然后尝试返回AVOutputFormat的name，如果AVOutputFormat也为空，则返回“NULL”。
• （3）option
• option字段则指向一个元素个数很多的静态数组avformat_options。该数组单独定义于libavformat\options_table.h中。其中包含了AVFormatContext支持的所有的AVOption，如下所示。

/*
 * Copyright (c) 2000, 2001, 2002 Fabrice Bellard
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */
 
#ifndef AVFORMAT_OPTIONS_TABLE_H
#define AVFORMAT_OPTIONS_TABLE_H
 
#include <limits.h>
 
#include "libavutil/opt.h"
#include "avformat.h"
#include "internal.h"
 
#define OFFSET(x) offsetof(AVFormatContext,x)
#define DEFAULT 0 //should be NAN but it does not work as it is not a constant in glibc as required by ANSI/ISO C
//these names are too long to be readable
#define E AV_OPT_FLAG_ENCODING_PARAM
#define D AV_OPT_FLAG_DECODING_PARAM
 
static const AVOption avformat_options[] = {
{"avioflags", NULL, OFFSET(avio_flags), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "avioflags"},
{"direct", "reduce buffering", 0, AV_OPT_TYPE_CONST, {.i64 = AVIO_FLAG_DIRECT }, INT_MIN, INT_MAX, D|E, "avioflags"},
{"probesize", "set probing size", OFFSET(probesize), AV_OPT_TYPE_INT64, {.i64 = 5000000 }, 32, INT64_MAX, D},
{"formatprobesize", "number of bytes to probe file format", OFFSET(format_probesize), AV_OPT_TYPE_INT, {.i64 = PROBE_BUF_MAX}, 0, INT_MAX-1, D},
{"packetsize", "set packet size", OFFSET(packet_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, E},
{"fflags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = AVFMT_FLAG_AUTO_BSF }, INT_MIN, INT_MAX, D|E, "fflags"},
{"flush_packets", "reduce the latency by flushing out packets immediately", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_FLUSH_PACKETS }, INT_MIN, INT_MAX, E, "fflags"},
{"ignidx", "ignore index", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNIDX }, INT_MIN, INT_MAX, D, "fflags"},
{"genpts", "generate pts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_GENPTS }, INT_MIN, INT_MAX, D, "fflags"},
{"nofillin", "do not fill in missing values that can be exactly calculated", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOFILLIN }, INT_MIN, INT_MAX, D, "fflags"},
{"noparse", "disable AVParsers, this needs nofillin too", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOPARSE }, INT_MIN, INT_MAX, D, "fflags"},
{"igndts", "ignore dts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNDTS }, INT_MIN, INT_MAX, D, "fflags"},
{"discardcorrupt", "discard corrupted frames", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_DISCARD_CORRUPT }, INT_MIN, INT_MAX, D, "fflags"},
{"sortdts", "try to interleave outputted packets by dts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_SORT_DTS }, INT_MIN, INT_MAX, D, "fflags"},
{"fastseek", "fast but inaccurate seeks", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_FAST_SEEK }, INT_MIN, INT_MAX, D, "fflags"},
{"nobuffer", "reduce the latency introduced by optional buffering", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOBUFFER }, 0, INT_MAX, D, "fflags"},
{"bitexact", "do not write random/volatile data", 0, AV_OPT_TYPE_CONST, { .i64 = AVFMT_FLAG_BITEXACT }, 0, 0, E, "fflags" },
{"shortest", "stop muxing with the shortest stream", 0, AV_OPT_TYPE_CONST, { .i64 = AVFMT_FLAG_SHORTEST }, 0, 0, E, "fflags" },
{"autobsf", "add needed bsfs automatically", 0, AV_OPT_TYPE_CONST, { .i64 = AVFMT_FLAG_AUTO_BSF }, 0, 0, E, "fflags" },
{"seek2any", "allow seeking to non-keyframes on demuxer level when supported", OFFSET(seek2any), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0, 1, D},
{"analyzeduration", "specify how many microseconds are analyzed to probe the input", OFFSET(max_analyze_duration), AV_OPT_TYPE_INT64, {.i64 = 0 }, 0, INT64_MAX, D},
{"cryptokey", "decryption key", OFFSET(key), AV_OPT_TYPE_BINARY, {.dbl = 0}, 0, 0, D},
{"indexmem", "max memory used for timestamp index (per stream)", OFFSET(max_index_size), AV_OPT_TYPE_INT, {.i64 = 1<<20 }, 0, INT_MAX, D},
{"rtbufsize", "max memory used for buffering real-time frames", OFFSET(max_picture_buffer), AV_OPT_TYPE_INT, {.i64 = 3041280 }, 0, INT_MAX, D}, /* defaults to 1s of 15fps 352x288 YUYV422 video */
{"fdebug", "print specific debug info", OFFSET(debug), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, INT_MAX, E|D, "fdebug"},
{"ts", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_FDEBUG_TS }, INT_MIN, INT_MAX, E|D, "fdebug"},
{"max_delay", "maximum muxing or demuxing delay in microseconds", OFFSET(max_delay), AV_OPT_TYPE_INT, {.i64 = -1 }, -1, INT_MAX, E|D},
{"start_time_realtime", "wall-clock time when stream begins (PTS==0)", OFFSET(start_time_realtime), AV_OPT_TYPE_INT64, {.i64 = AV_NOPTS_VALUE}, INT64_MIN, INT64_MAX, E},
{"fpsprobesize", "number of frames used to probe fps", OFFSET(fps_probe_size), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX-1, D},
{"audio_preload", "microseconds by which audio packets should be interleaved earlier", OFFSET(audio_preload), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
{"chunk_duration", "microseconds for each chunk", OFFSET(max_chunk_duration), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
{"chunk_size", "size in bytes for each chunk", OFFSET(max_chunk_size), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
/* this is a crutch for avconv, since it cannot deal with identically named options in different contexts.
 * to be removed when avconv is fixed */
{"f_err_detect", "set error detection flags (deprecated; use err_detect, save via avconv)", OFFSET(error_recognition), AV_OPT_TYPE_FLAGS, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"err_detect", "set error detection flags", OFFSET(error_recognition), AV_OPT_TYPE_FLAGS, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"crccheck", "verify embedded CRCs", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"bitstream", "detect bitstream specification deviations", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BITSTREAM }, INT_MIN, INT_MAX, D, "err_detect"},
{"buffer", "detect improper bitstream length", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BUFFER }, INT_MIN, INT_MAX, D, "err_detect"},
{"explode", "abort decoding on minor error detection", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_EXPLODE }, INT_MIN, INT_MAX, D, "err_detect"},
{"ignore_err", "ignore errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_IGNORE_ERR }, INT_MIN, INT_MAX, D, "err_detect"},
{"careful",    "consider things that violate the spec, are fast to check and have not been seen in the wild as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CAREFUL }, INT_MIN, INT_MAX, D, "err_detect"},
{"compliant",  "consider all spec non compliancies as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_COMPLIANT | AV_EF_CAREFUL }, INT_MIN, INT_MAX, D, "err_detect"},
{"aggressive", "consider things that a sane encoder shouldn't do as an error", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_AGGRESSIVE | AV_EF_COMPLIANT | AV_EF_CAREFUL}, INT_MIN, INT_MAX, D, "err_detect"},
{"use_wallclock_as_timestamps", "use wallclock as timestamps", OFFSET(use_wallclock_as_timestamps), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, D},
{"skip_initial_bytes", "set number of bytes to skip before reading header and frames", OFFSET(skip_initial_bytes), AV_OPT_TYPE_INT64, {.i64 = 0}, 0, INT64_MAX-1, D},
{"correct_ts_overflow", "correct single timestamp overflows", OFFSET(correct_ts_overflow), AV_OPT_TYPE_BOOL, {.i64 = 1}, 0, 1, D},
{"flush_packets", "enable flushing of the I/O context after each packet", OFFSET(flush_packets), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 1, E},
{"metadata_header_padding", "set number of bytes to be written as padding in a metadata header", OFFSET(metadata_header_padding), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX, E},
{"output_ts_offset", "set output timestamp offset", OFFSET(output_ts_offset), AV_OPT_TYPE_DURATION, {.i64 = 0}, -INT64_MAX, INT64_MAX, E},
{"max_interleave_delta", "maximum buffering duration for interleaving", OFFSET(max_interleave_delta), AV_OPT_TYPE_INT64, { .i64 = 10000000 }, 0, INT64_MAX, E },
{"f_strict", "how strictly to follow the standards (deprecated; use strict, save via avconv)", OFFSET(strict_std_compliance), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "strict"},
{"strict", "how strictly to follow the standards", OFFSET(strict_std_compliance), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "strict"},
{"very", "strictly conform to a older more strict version of the spec or reference software", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_VERY_STRICT }, INT_MIN, INT_MAX, D|E, "strict"},
{"strict", "strictly conform to all the things in the spec no matter what the consequences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_STRICT }, INT_MIN, INT_MAX, D|E, "strict"},
{"normal", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_NORMAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"unofficial", "allow unofficial extensions", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_UNOFFICIAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"experimental", "allow non-standardized experimental variants", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_EXPERIMENTAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"max_ts_probe", "maximum number of packets to read while waiting for the first timestamp", OFFSET(max_ts_probe), AV_OPT_TYPE_INT, { .i64 = 50 }, 0, INT_MAX, D },
{"avoid_negative_ts", "shift timestamps so they start at 0", OFFSET(avoid_negative_ts), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 2, E, "avoid_negative_ts"},
{"auto",              "enabled when required by target format",    0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_AVOID_NEG_TS_AUTO },              INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"disabled",          "do not change timestamps",                  0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_AVOID_NEG_TS_DISABLED },          INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"make_non_negative", "shift timestamps so they are non negative", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"make_zero",         "shift timestamps so they start at 0",       0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_AVOID_NEG_TS_MAKE_ZERO },         INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"dump_separator", "set information dump field separator", OFFSET(dump_separator), AV_OPT_TYPE_STRING, {.str = ", "}, 0, 0, D|E},
{"codec_whitelist", "List of decoders that are allowed to be used", OFFSET(codec_whitelist), AV_OPT_TYPE_STRING, { .str = NULL },  0, 0, D },
{"format_whitelist", "List of demuxers that are allowed to be used", OFFSET(format_whitelist), AV_OPT_TYPE_STRING, { .str = NULL },  0, 0, D },
{"protocol_whitelist", "List of protocols that are allowed to be used", OFFSET(protocol_whitelist), AV_OPT_TYPE_STRING, { .str = NULL },  0, 0, D },
{"protocol_blacklist", "List of protocols that are not allowed to be used", OFFSET(protocol_blacklist), AV_OPT_TYPE_STRING, { .str = NULL },  0, 0, D },
{"max_streams", "maximum number of streams", OFFSET(max_streams), AV_OPT_TYPE_INT, { .i64 = 1000 }, 0, INT_MAX, D },
{"skip_estimate_duration_from_pts", "skip duration calculation in estimate_timings_from_pts", OFFSET(skip_estimate_duration_from_pts), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, D},
{"max_probe_packets", "Maximum number of packets to probe a codec", OFFSET(max_probe_packets), AV_OPT_TYPE_INT, { .i64 = 2500 }, 0, INT_MAX, D },
{NULL},
};
 
#undef E
#undef D
#undef DEFAULT
#undef OFFSET
 
#endif /* AVFORMAT_OPTIONS_TABLE_H */

AVCodecContext中的AVClass

• AVFormatContext 中的AVClass定义位于libavcodec\options.c中，是一个名称为av_codec_context_class的静态结构体。如下所示。

static const AVClass av_codec_context_class = {
    .class_name              = "AVCodecContext",
    .item_name               = context_to_name,
    .option                  = avcodec_options,
    .version                 = LIBAVUTIL_VERSION_INT,
    .log_level_offset_offset = offsetof(AVCodecContext, log_level_offset),
    .child_next              = codec_child_next,
    .child_class_iterate     = codec_child_class_iterate,
    .category                = AV_CLASS_CATEGORY_ENCODER,
    .get_category            = get_category,
};

• 从源代码可以看出：
• （1）class_name
  • 该AVClass名称是“AVCodecContext”。
• （2）item_name
  • item_name指向一个函数context_to_name ()，该函数定义如下所示。

static const char* context_to_name(void* ptr) {
    AVCodecContext *avc= ptr;
 
    if (avc && avc->codec)
        return avc->codec->name;
    else
        return "NULL";
}

•  从函数的定义可以看出，如果AVCodecContext中的Codec结构体不为空，则返回Codec的name，否则返回“NULL”。
• （3）category
• option字段则指向一个元素个数极多的静态数组avcodec_options。该数组单独定义于libavcodec\options_table.h中。其中包含了AVCodecContext支持的所有的AVOption。由于该数组定义实在是太多了，在这里仅贴出它前面的一小部分。

/*
 * Copyright (c) 2001 Fabrice Bellard
 * Copyright (c) 2002-2004 Michael Niedermayer <michaelni@gmx.at>
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */
 
#ifndef AVCODEC_OPTIONS_TABLE_H
#define AVCODEC_OPTIONS_TABLE_H
 
#include "config_components.h"
 
#include <float.h>
#include <limits.h>
#include <stdint.h>
 
#include "libavutil/opt.h"
#include "avcodec.h"
#include "version_major.h"
 
#define OFFSET(x) offsetof(AVCodecContext,x)
#define DEFAULT 0 //should be NAN but it does not work as it is not a constant in glibc as required by ANSI/ISO C
//these names are too long to be readable
#define V AV_OPT_FLAG_VIDEO_PARAM
#define A AV_OPT_FLAG_AUDIO_PARAM
#define S AV_OPT_FLAG_SUBTITLE_PARAM
#define E AV_OPT_FLAG_ENCODING_PARAM
#define D AV_OPT_FLAG_DECODING_PARAM
#define CC AV_OPT_FLAG_CHILD_CONSTS
 
#define AV_CODEC_DEFAULT_BITRATE 200*1000
 
static const AVOption avcodec_options[] = {
{"b", "set bitrate (in bits/s)", OFFSET(bit_rate), AV_OPT_TYPE_INT64, {.i64 = AV_CODEC_DEFAULT_BITRATE }, 0, INT64_MAX, A|V|E},
{"ab", "set bitrate (in bits/s)", OFFSET(bit_rate), AV_OPT_TYPE_INT64, {.i64 = 128*1000 }, 0, INT_MAX, A|E},
{"bt", "Set video bitrate tolerance (in bits/s). In 1-pass mode, bitrate tolerance specifies how far "
       "ratecontrol is willing to deviate from the target average bitrate value. This is not related "
       "to minimum/maximum bitrate. Lowering tolerance too much has an adverse effect on quality.",
       OFFSET(bit_rate_tolerance), AV_OPT_TYPE_INT, {.i64 = AV_CODEC_DEFAULT_BITRATE*20 }, 1, INT_MAX, V|E},
{"flags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, UINT_MAX, V|A|S|E|D, "flags"},
{"unaligned", "allow decoders to produce unaligned output", 0, AV_OPT_TYPE_CONST, { .i64 = AV_CODEC_FLAG_UNALIGNED }, INT_MIN, INT_MAX, V | D, "flags" },
{"mv4", "use four motion vectors per macroblock (MPEG-4)", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_4MV }, INT_MIN, INT_MAX, V|E, "flags"},
{"qpel", "use 1/4-pel motion compensation", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_QPEL }, INT_MIN, INT_MAX, V|E, "flags"},
{"loop", "use loop filter", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_LOOP_FILTER }, INT_MIN, INT_MAX, V|E, "flags"},
{"qscale", "use fixed qscale", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_QSCALE }, INT_MIN, INT_MAX, 0, "flags"},
{"pass1", "use internal 2-pass ratecontrol in first  pass mode", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_PASS1 }, INT_MIN, INT_MAX, 0, "flags"},
{"pass2", "use internal 2-pass ratecontrol in second pass mode", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_PASS2 }, INT_MIN, INT_MAX, 0, "flags"},
{"gray", "only decode/encode grayscale", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_GRAY }, INT_MIN, INT_MAX, V|E|D, "flags"},
{"psnr", "error[?] variables will be set during encoding", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_PSNR }, INT_MIN, INT_MAX, V|E, "flags"},
#if FF_API_FLAG_TRUNCATED
{"truncated", "(Deprecated, use parsers instead.) Input bitstream might be randomly truncated", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_TRUNCATED }, INT_MIN, INT_MAX, V|D | AV_OPT_FLAG_DEPRECATED, "flags"},
#endif
{"ildct", "use interlaced DCT", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_INTERLACED_DCT }, INT_MIN, INT_MAX, V|E, "flags"},
{"low_delay", "force low delay", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_LOW_DELAY }, INT_MIN, INT_MAX, V|D|E, "flags"},
{"global_header", "place global headers in extradata instead of every keyframe", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_GLOBAL_HEADER }, INT_MIN, INT_MAX, V|A|E, "flags"},
{"bitexact", "use only bitexact functions (except (I)DCT)", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_BITEXACT }, INT_MIN, INT_MAX, A|V|S|D|E, "flags"},
{"aic", "H.263 advanced intra coding / MPEG-4 AC prediction", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_AC_PRED }, INT_MIN, INT_MAX, V|E, "flags"},
{"ilme", "interlaced motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_INTERLACED_ME }, INT_MIN, INT_MAX, V|E, "flags"},
{"cgop", "closed GOP", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_CLOSED_GOP }, INT_MIN, INT_MAX, V|E, "flags"},
{"output_corrupt", "Output even potentially corrupted frames", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_OUTPUT_CORRUPT }, INT_MIN, INT_MAX, V|D, "flags"},
{"drop_changed", "Drop frames whose parameters differ from first decoded frame", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_DROPCHANGED }, INT_MIN, INT_MAX, A|V|D, "flags"},
{"flags2", NULL, OFFSET(flags2), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT}, 0, UINT_MAX, V|A|E|D|S, "flags2"},
{"fast", "allow non-spec-compliant speedup tricks", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG2_FAST }, INT_MIN, INT_MAX, V|E, "flags2"},
{"noout", "skip bitstream encoding", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG2_NO_OUTPUT }, INT_MIN, INT_MAX, V|E, "flags2"},
{"ignorecrop", "ignore cropping information from sps", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG2_IGNORE_CROP }, INT_MIN, INT_MAX, V|D, "flags2"},
{"local_header", "place global headers at every keyframe instead of in extradata", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG2_LOCAL_HEADER }, INT_MIN, INT_MAX, V|E, "flags2"},
{"chunks", "Frame data might be split into multiple chunks", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG2_CHUNKS }, INT_MIN, INT_MAX, V|D, "flags2"},
{"showall", "Show all frames before the first keyframe", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG2_SHOW_ALL }, INT_MIN, INT_MAX, V|D, "flags2"},
{"export_mvs", "export motion vectors through frame side data", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG2_EXPORT_MVS}, INT_MIN, INT_MAX, V|D, "flags2"},
{"skip_manual", "do not skip samples and export skip information as frame side data", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG2_SKIP_MANUAL}, INT_MIN, INT_MAX, A|D, "flags2"},
{"ass_ro_flush_noop", "do not reset ASS ReadOrder field on flush", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG2_RO_FLUSH_NOOP}, INT_MIN, INT_MAX, S|D, "flags2"},
{"export_side_data", "Export metadata as side data", OFFSET(export_side_data), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT}, 0, UINT_MAX, A|V|S|D|E, "export_side_data"},
{"mvs", "export motion vectors through frame side data", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_EXPORT_DATA_MVS}, INT_MIN, INT_MAX, V|D, "export_side_data"},
{"prft", "export Producer Reference Time through packet side data", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_EXPORT_DATA_PRFT}, INT_MIN, INT_MAX, A|V|S|E, "export_side_data"},
{"venc_params", "export video encoding parameters through frame side data", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_EXPORT_DATA_VIDEO_ENC_PARAMS}, INT_MIN, INT_MAX, V|D, "export_side_data"},
{"film_grain", "export film grain parameters through frame side data", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_EXPORT_DATA_FILM_GRAIN}, INT_MIN, INT_MAX, V|D, "export_side_data"},
{"time_base", NULL, OFFSET(time_base), AV_OPT_TYPE_RATIONAL, {.dbl = 0}, 0, INT_MAX},
{"g", "set the group of picture (GOP) size", OFFSET(gop_size), AV_OPT_TYPE_INT, {.i64 = 12 }, INT_MIN, INT_MAX, V|E},
{"ar", "set audio sampling rate (in Hz)", OFFSET(sample_rate), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, A|D|E},
#if FF_API_OLD_CHANNEL_LAYOUT
{"ac", "set number of audio channels", OFFSET(channels), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, A|D|E},
#endif
{"cutoff", "set cutoff bandwidth", OFFSET(cutoff), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, A|E},
{"frame_size", NULL, OFFSET(frame_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, A|E},
{"frame_number", NULL, OFFSET(frame_number), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX},
{"delay", NULL, OFFSET(delay), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX},
{"qcomp", "video quantizer scale compression (VBR). Constant of ratecontrol equation. "
          "Recommended range for default rc_eq: 0.0-1.0",
          OFFSET(qcompress), AV_OPT_TYPE_FLOAT, {.dbl = 0.5 }, -FLT_MAX, FLT_MAX, V|E},
{"qblur", "video quantizer scale blur (VBR)", OFFSET(qblur), AV_OPT_TYPE_FLOAT, {.dbl = 0.5 }, -1, FLT_MAX, V|E},
{"qmin", "minimum video quantizer scale (VBR)", OFFSET(qmin), AV_OPT_TYPE_INT, {.i64 = 2 }, -1, 69, V|E},
{"qmax", "maximum video quantizer scale (VBR)", OFFSET(qmax), AV_OPT_TYPE_INT, {.i64 = 31 }, -1, 1024, V|E},
{"qdiff", "maximum difference between the quantizer scales (VBR)", OFFSET(max_qdiff), AV_OPT_TYPE_INT, {.i64 = 3 }, INT_MIN, INT_MAX, V|E},
{"bf", "set maximum number of B-frames between non-B-frames", OFFSET(max_b_frames), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, -1, INT_MAX, V|E},
{"b_qfactor", "QP factor between P- and B-frames", OFFSET(b_quant_factor), AV_OPT_TYPE_FLOAT, {.dbl = 1.25 }, -FLT_MAX, FLT_MAX, V|E},
{"codec_tag", NULL, OFFSET(codec_tag), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX},
{"bug", "work around not autodetected encoder bugs", OFFSET(workaround_bugs), AV_OPT_TYPE_FLAGS, {.i64 = FF_BUG_AUTODETECT }, INT_MIN, INT_MAX, V|D, "bug"},
{"autodetect", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_AUTODETECT }, INT_MIN, INT_MAX, V|D, "bug"},
{"xvid_ilace", "Xvid interlacing bug (autodetected if FOURCC == XVIX)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_XVID_ILACE }, INT_MIN, INT_MAX, V|D, "bug"},
{"ump4", "(autodetected if FOURCC == UMP4)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_UMP4 }, INT_MIN, INT_MAX, V|D, "bug"},
{"no_padding", "padding bug (autodetected)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_NO_PADDING }, INT_MIN, INT_MAX, V|D, "bug"},
{"amv", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_AMV }, INT_MIN, INT_MAX, V|D, "bug"},
{"qpel_chroma", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_QPEL_CHROMA }, INT_MIN, INT_MAX, V|D, "bug"},
{"std_qpel", "old standard qpel (autodetected per FOURCC/version)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_STD_QPEL }, INT_MIN, INT_MAX, V|D, "bug"},
{"qpel_chroma2", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_QPEL_CHROMA2 }, INT_MIN, INT_MAX, V|D, "bug"},
{"direct_blocksize", "direct-qpel-blocksize bug (autodetected per FOURCC/version)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_DIRECT_BLOCKSIZE }, INT_MIN, INT_MAX, V|D, "bug"},
{"edge", "edge padding bug (autodetected per FOURCC/version)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_EDGE }, INT_MIN, INT_MAX, V|D, "bug"},
{"hpel_chroma", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_HPEL_CHROMA }, INT_MIN, INT_MAX, V|D, "bug"},
{"dc_clip", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_DC_CLIP }, INT_MIN, INT_MAX, V|D, "bug"},
{"ms", "work around various bugs in Microsoft's broken decoders", 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_MS }, INT_MIN, INT_MAX, V|D, "bug"},
{"trunc", "truncated frames", 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_TRUNCATED}, INT_MIN, INT_MAX, V|D, "bug"},
{"iedge", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_BUG_IEDGE }, INT_MIN, INT_MAX, V|D, "bug"},
{"strict", "how strictly to follow the standards", OFFSET(strict_std_compliance), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, A|V|D|E, "strict"},
{"very", "strictly conform to a older more strict version of the spec or reference software", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_VERY_STRICT }, INT_MIN, INT_MAX, A|V|D|E, "strict"},
{"strict", "strictly conform to all the things in the spec no matter what the consequences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_STRICT }, INT_MIN, INT_MAX, A|V|D|E, "strict"},
{"normal", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_NORMAL }, INT_MIN, INT_MAX, A|V|D|E, "strict"},
{"unofficial", "allow unofficial extensions", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_UNOFFICIAL }, INT_MIN, INT_MAX, A|V|D|E, "strict"},
{"experimental", "allow non-standardized experimental things", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_EXPERIMENTAL }, INT_MIN, INT_MAX, A|V|D|E, "strict"},
{"b_qoffset", "QP offset between P- and B-frames", OFFSET(b_quant_offset), AV_OPT_TYPE_FLOAT, {.dbl = 1.25 }, -FLT_MAX, FLT_MAX, V|E},
{"err_detect", "set error detection flags", OFFSET(err_recognition), AV_OPT_TYPE_FLAGS, {.i64 = 0 }, INT_MIN, INT_MAX, A|V|S|D|E, "err_detect"},
{"crccheck", "verify embedded CRCs", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, A|V|S|D|E, "err_detect"},
{"bitstream", "detect bitstream specification deviations", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BITSTREAM }, INT_MIN, INT_MAX, A|V|S|D|E, "err_detect"},
{"buffer", "detect improper bitstream length", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BUFFER }, INT_MIN, INT_MAX, A|V|S|D|E, "err_detect"},
{"explode", "abort decoding on minor error detection", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_EXPLODE }, INT_MIN, INT_MAX, A|V|S|D|E, "err_detect"},
{"ignore_err", "ignore errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_IGNORE_ERR }, INT_MIN, INT_MAX, A|V|S|D|E, "err_detect"},
{"careful",    "consider things that violate the spec, are fast to check and have not been seen in the wild as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CAREFUL }, INT_MIN, INT_MAX, A|V|S|D|E, "err_detect"},
{"compliant",  "consider all spec non compliancies as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_COMPLIANT | AV_EF_CAREFUL }, INT_MIN, INT_MAX, A|V|S|D|E, "err_detect"},
{"aggressive", "consider things that a sane encoder should not do as an error", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_AGGRESSIVE | AV_EF_COMPLIANT | AV_EF_CAREFUL}, INT_MIN, INT_MAX, A|V|S|D|E, "err_detect"},
{"has_b_frames", NULL, OFFSET(has_b_frames), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX},
{"block_align", NULL, OFFSET(block_align), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX},
{"rc_override_count", NULL, OFFSET(rc_override_count), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX},
{"maxrate", "maximum bitrate (in bits/s). Used for VBV together with bufsize.", OFFSET(rc_max_rate), AV_OPT_TYPE_INT64, {.i64 = DEFAULT }, 0, INT_MAX, V|A|E},
{"minrate", "minimum bitrate (in bits/s). Most useful in setting up a CBR encode. It is of little use otherwise.",
            OFFSET(rc_min_rate), AV_OPT_TYPE_INT64, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|A|E},
{"bufsize", "set ratecontrol buffer size (in bits)", OFFSET(rc_buffer_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, A|V|E},
{"i_qfactor", "QP factor between P- and I-frames", OFFSET(i_quant_factor), AV_OPT_TYPE_FLOAT, {.dbl = -0.8 }, -FLT_MAX, FLT_MAX, V|E},
{"i_qoffset", "QP offset between P- and I-frames", OFFSET(i_quant_offset), AV_OPT_TYPE_FLOAT, {.dbl = 0.0 }, -FLT_MAX, FLT_MAX, V|E},
{"dct", "DCT algorithm", OFFSET(dct_algo), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, V|E, "dct"},
{"auto", "autoselect a good one", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DCT_AUTO }, INT_MIN, INT_MAX, V|E, "dct"},
{"fastint", "fast integer", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DCT_FASTINT }, INT_MIN, INT_MAX, V|E, "dct"},
{"int", "accurate integer", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DCT_INT }, INT_MIN, INT_MAX, V|E, "dct"},
{"mmx", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_DCT_MMX }, INT_MIN, INT_MAX, V|E, "dct"},
{"altivec", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_DCT_ALTIVEC }, INT_MIN, INT_MAX, V|E, "dct"},
{"faan", "floating point AAN DCT", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DCT_FAAN }, INT_MIN, INT_MAX, V|E, "dct"},
{"lumi_mask", "compresses bright areas stronger than medium ones", OFFSET(lumi_masking), AV_OPT_TYPE_FLOAT, {.dbl = 0 }, -FLT_MAX, FLT_MAX, V|E},
{"tcplx_mask", "temporal complexity masking", OFFSET(temporal_cplx_masking), AV_OPT_TYPE_FLOAT, {.dbl = 0 }, -FLT_MAX, FLT_MAX, V|E},
{"scplx_mask", "spatial complexity masking", OFFSET(spatial_cplx_masking), AV_OPT_TYPE_FLOAT, {.dbl = 0 }, -FLT_MAX, FLT_MAX, V|E},
{"p_mask", "inter masking", OFFSET(p_masking), AV_OPT_TYPE_FLOAT, {.dbl = 0 }, -FLT_MAX, FLT_MAX, V|E},
{"dark_mask", "compresses dark areas stronger than medium ones", OFFSET(dark_masking), AV_OPT_TYPE_FLOAT, {.dbl = 0 }, -FLT_MAX, FLT_MAX, V|E},
{"idct", "select IDCT implementation", OFFSET(idct_algo), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, V|E|D, "idct"},
{"auto", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_AUTO }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"int", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_INT }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"simple", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_SIMPLE }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"simplemmx", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_SIMPLEMMX }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"arm", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_ARM }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"altivec", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_ALTIVEC }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"simplearm", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_SIMPLEARM }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"simplearmv5te", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_SIMPLEARMV5TE }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"simplearmv6", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_SIMPLEARMV6 }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"simpleneon", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_SIMPLENEON }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"xvid", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_XVID }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"xvidmmx", "deprecated, for compatibility only", 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_XVID }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"faani", "floating point AAN IDCT", 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_FAAN }, INT_MIN, INT_MAX, V|D|E, "idct"},
{"simpleauto", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_IDCT_SIMPLEAUTO }, INT_MIN, INT_MAX, V|E|D, "idct"},
{"slice_count", NULL, OFFSET(slice_count), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX},
{"ec", "set error concealment strategy", OFFSET(error_concealment), AV_OPT_TYPE_FLAGS, {.i64 = 3 }, INT_MIN, INT_MAX, V|D, "ec"},
{"guess_mvs", "iterative motion vector (MV) search (slow)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_EC_GUESS_MVS }, INT_MIN, INT_MAX, V|D, "ec"},
{"deblock", "use strong deblock filter for damaged MBs", 0, AV_OPT_TYPE_CONST, {.i64 = FF_EC_DEBLOCK }, INT_MIN, INT_MAX, V|D, "ec"},
{"favor_inter", "favor predicting from the previous frame", 0, AV_OPT_TYPE_CONST, {.i64 = FF_EC_FAVOR_INTER }, INT_MIN, INT_MAX, V|D, "ec"},
{"bits_per_coded_sample", NULL, OFFSET(bits_per_coded_sample), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX},
{"aspect", "sample aspect ratio", OFFSET(sample_aspect_ratio), AV_OPT_TYPE_RATIONAL, {.dbl = 0}, 0, 10, V|E},
{"sar",    "sample aspect ratio", OFFSET(sample_aspect_ratio), AV_OPT_TYPE_RATIONAL, {.dbl = 0}, 0, 10, V|E},
{"debug", "print specific debug info", OFFSET(debug), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, INT_MAX, V|A|S|E|D, "debug"},
{"pict", "picture info", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_PICT_INFO }, INT_MIN, INT_MAX, V|D, "debug"},
{"rc", "rate control", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_RC }, INT_MIN, INT_MAX, V|E, "debug"},
{"bitstream", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_BITSTREAM }, INT_MIN, INT_MAX, V|D, "debug"},
{"mb_type", "macroblock (MB) type", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_MB_TYPE }, INT_MIN, INT_MAX, V|D, "debug"},
{"qp", "per-block quantization parameter (QP)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_QP }, INT_MIN, INT_MAX, V|D, "debug"},
{"dct_coeff", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_DCT_COEFF }, INT_MIN, INT_MAX, V|D, "debug"},
{"green_metadata", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_GREEN_MD }, INT_MIN, INT_MAX, V|D, "debug"},
{"skip", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_SKIP }, INT_MIN, INT_MAX, V|D, "debug"},
{"startcode", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_STARTCODE }, INT_MIN, INT_MAX, V|D, "debug"},
{"er", "error recognition", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_ER }, INT_MIN, INT_MAX, V|D, "debug"},
{"mmco", "memory management control operations (H.264)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_MMCO }, INT_MIN, INT_MAX, V|D, "debug"},
{"bugs", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_BUGS }, INT_MIN, INT_MAX, V|D, "debug"},
{"buffers", "picture buffer allocations", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_BUFFERS }, INT_MIN, INT_MAX, V|D, "debug"},
{"thread_ops", "threading operations", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_THREADS }, INT_MIN, INT_MAX, V|A|D, "debug"},
{"nomc", "skip motion compensation", 0, AV_OPT_TYPE_CONST, {.i64 = FF_DEBUG_NOMC }, INT_MIN, INT_MAX, V|A|D, "debug"},
{"dia_size", "diamond type & size for motion estimation", OFFSET(dia_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E},
{"last_pred", "amount of motion predictors from the previous frame", OFFSET(last_predictor_count), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E},
{"pre_dia_size", "diamond type & size for motion estimation pre-pass", OFFSET(pre_dia_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E},
{"subq", "sub-pel motion estimation quality", OFFSET(me_subpel_quality), AV_OPT_TYPE_INT, {.i64 = 8 }, INT_MIN, INT_MAX, V|E},
{"me_range", "limit motion vectors range (1023 for DivX player)", OFFSET(me_range), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E},
{"global_quality", NULL, OFFSET(global_quality), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|A|E},
{"slice_flags", NULL, OFFSET(slice_flags), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX},
{"mbd", "macroblock decision algorithm (high quality mode)", OFFSET(mb_decision), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, 2, V|E, "mbd"},
{"simple", "use mbcmp", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MB_DECISION_SIMPLE }, INT_MIN, INT_MAX, V|E, "mbd"},
{"bits", "use fewest bits", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MB_DECISION_BITS }, INT_MIN, INT_MAX, V|E, "mbd"},
{"rd", "use best rate distortion", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MB_DECISION_RD }, INT_MIN, INT_MAX, V|E, "mbd"},
{"rc_init_occupancy", "number of bits which should be loaded into the rc buffer before decoding starts", OFFSET(rc_initial_buffer_occupancy), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E},
{"threads", "set the number of threads", OFFSET(thread_count), AV_OPT_TYPE_INT, {.i64 = 1 }, 0, INT_MAX, V|A|E|D, "threads"},
{"auto", "autodetect a suitable number of threads to use", 0, AV_OPT_TYPE_CONST, {.i64 = 0 }, INT_MIN, INT_MAX, V|E|D, "threads"},
{"dc", "intra_dc_precision", OFFSET(intra_dc_precision), AV_OPT_TYPE_INT, {.i64 = 0 }, -8, 16, V|E},
{"nssew", "nsse weight", OFFSET(nsse_weight), AV_OPT_TYPE_INT, {.i64 = 8 }, INT_MIN, INT_MAX, V|E},
{"skip_top", "number of macroblock rows at the top which are skipped", OFFSET(skip_top), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|D},
{"skip_bottom", "number of macroblock rows at the bottom which are skipped", OFFSET(skip_bottom), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|D},
{"profile", NULL, OFFSET(profile), AV_OPT_TYPE_INT, {.i64 = FF_PROFILE_UNKNOWN }, INT_MIN, INT_MAX, V|A|E|CC, "avctx.profile"},
{"unknown", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_PROFILE_UNKNOWN }, INT_MIN, INT_MAX, V|A|E, "avctx.profile"},
{"main10",  NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_PROFILE_HEVC_MAIN_10 }, INT_MIN, INT_MAX, V|E, "avctx.profile"},
{"level", NULL, OFFSET(level), AV_OPT_TYPE_INT, {.i64 = FF_LEVEL_UNKNOWN }, INT_MIN, INT_MAX, V|A|E|CC, "avctx.level"},
{"unknown", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_LEVEL_UNKNOWN }, INT_MIN, INT_MAX, V|A|E, "avctx.level"},
{"lowres", "decode at 1= 1/2, 2=1/4, 3=1/8 resolutions", OFFSET(lowres), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, V|A|D},
{"cmp", "full-pel ME compare function", OFFSET(me_cmp), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"subcmp", "sub-pel ME compare function", OFFSET(me_sub_cmp), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"mbcmp", "macroblock compare function", OFFSET(mb_cmp), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"ildctcmp", "interlaced DCT compare function", OFFSET(ildct_cmp), AV_OPT_TYPE_INT, {.i64 = FF_CMP_VSAD }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"precmp", "pre motion estimation compare function", OFFSET(me_pre_cmp), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"sad", "sum of absolute differences, fast", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_SAD }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"sse", "sum of squared errors", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_SSE }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"satd", "sum of absolute Hadamard transformed differences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_SATD }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"dct", "sum of absolute DCT transformed differences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_DCT }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"psnr", "sum of squared quantization errors (avoid, low quality)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_PSNR }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"bit", "number of bits needed for the block", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_BIT }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"rd", "rate distortion optimal, slow", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_RD }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"zero", "0", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_ZERO }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"vsad", "sum of absolute vertical differences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_VSAD }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"vsse", "sum of squared vertical differences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_VSSE }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"nsse", "noise preserving sum of squared differences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_NSSE }, INT_MIN, INT_MAX, V|E, "cmp_func"},
#if CONFIG_SNOW_ENCODER
{"w53", "5/3 wavelet, only used in snow", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_W53 }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"w97", "9/7 wavelet, only used in snow", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_W97 }, INT_MIN, INT_MAX, V|E, "cmp_func"},
#endif
{"dctmax", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_DCTMAX }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"chroma", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_CHROMA }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"msad", "sum of absolute differences, median predicted", 0, AV_OPT_TYPE_CONST, {.i64 = FF_CMP_MEDIAN_SAD }, INT_MIN, INT_MAX, V|E, "cmp_func"},
{"mblmin", "minimum macroblock Lagrange factor (VBR)", OFFSET(mb_lmin), AV_OPT_TYPE_INT, {.i64 = FF_QP2LAMBDA * 2 }, 1, FF_LAMBDA_MAX, V|E},
{"mblmax", "maximum macroblock Lagrange factor (VBR)", OFFSET(mb_lmax), AV_OPT_TYPE_INT, {.i64 = FF_QP2LAMBDA * 31 }, 1, FF_LAMBDA_MAX, V|E},
{"skip_loop_filter", "skip loop filtering process for the selected frames", OFFSET(skip_loop_filter), AV_OPT_TYPE_INT, {.i64 = AVDISCARD_DEFAULT }, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"skip_idct"       , "skip IDCT/dequantization for the selected frames",    OFFSET(skip_idct),        AV_OPT_TYPE_INT, {.i64 = AVDISCARD_DEFAULT }, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"skip_frame"      , "skip decoding for the selected frames",               OFFSET(skip_frame),       AV_OPT_TYPE_INT, {.i64 = AVDISCARD_DEFAULT }, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"none"            , "discard no frame",                    0, AV_OPT_TYPE_CONST, {.i64 = AVDISCARD_NONE    }, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"default"         , "discard useless frames",              0, AV_OPT_TYPE_CONST, {.i64 = AVDISCARD_DEFAULT }, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"noref"           , "discard all non-reference frames",    0, AV_OPT_TYPE_CONST, {.i64 = AVDISCARD_NONREF  }, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"bidir"           , "discard all bidirectional frames",    0, AV_OPT_TYPE_CONST, {.i64 = AVDISCARD_BIDIR   }, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"nokey"           , "discard all frames except keyframes", 0, AV_OPT_TYPE_CONST, {.i64 = AVDISCARD_NONKEY  }, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"nointra"         , "discard all frames except I frames",  0, AV_OPT_TYPE_CONST, {.i64 = AVDISCARD_NONINTRA}, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"all"             , "discard all frames",                  0, AV_OPT_TYPE_CONST, {.i64 = AVDISCARD_ALL     }, INT_MIN, INT_MAX, V|D, "avdiscard"},
{"bidir_refine", "refine the two motion vectors used in bidirectional macroblocks", OFFSET(bidir_refine), AV_OPT_TYPE_INT, {.i64 = 1 }, 0, 4, V|E},
{"keyint_min", "minimum interval between IDR-frames", OFFSET(keyint_min), AV_OPT_TYPE_INT, {.i64 = 25 }, INT_MIN, INT_MAX, V|E},
{"refs", "reference frames to consider for motion compensation", OFFSET(refs), AV_OPT_TYPE_INT, {.i64 = 1 }, INT_MIN, INT_MAX, V|E},
{"trellis", "rate-distortion optimal quantization", OFFSET(trellis), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|A|E},
{"mv0_threshold", NULL, OFFSET(mv0_threshold), AV_OPT_TYPE_INT, {.i64 = 256 }, 0, INT_MAX, V|E},
{"compression_level", NULL, OFFSET(compression_level), AV_OPT_TYPE_INT, {.i64 = FF_COMPRESSION_DEFAULT }, INT_MIN, INT_MAX, V|A|E},
{"bits_per_raw_sample", NULL, OFFSET(bits_per_raw_sample), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX},
{"ch_layout", NULL, OFFSET(ch_layout), AV_OPT_TYPE_CHLAYOUT, {.str = NULL }, 0, 0, A|E|D, "ch_layout"},
#if FF_API_OLD_CHANNEL_LAYOUT
{"channel_layout", NULL, OFFSET(channel_layout), AV_OPT_TYPE_CHANNEL_LAYOUT, {.i64 = DEFAULT }, 0, UINT64_MAX, A|E|D, "channel_layout"},
{"request_channel_layout", NULL, OFFSET(request_channel_layout), AV_OPT_TYPE_CHANNEL_LAYOUT, {.i64 = DEFAULT }, 0, UINT64_MAX, A|D, "request_channel_layout"},
#endif
{"rc_max_vbv_use", NULL, OFFSET(rc_max_available_vbv_use), AV_OPT_TYPE_FLOAT, {.dbl = 0 }, 0.0, FLT_MAX, V|E},
{"rc_min_vbv_use", NULL, OFFSET(rc_min_vbv_overflow_use),  AV_OPT_TYPE_FLOAT, {.dbl = 3 },     0.0, FLT_MAX, V|E},
{"ticks_per_frame", NULL, OFFSET(ticks_per_frame), AV_OPT_TYPE_INT, {.i64 = 1 }, 1, INT_MAX, A|V|E|D},
{"color_primaries", "color primaries", OFFSET(color_primaries), AV_OPT_TYPE_INT, {.i64 = AVCOL_PRI_UNSPECIFIED }, 1, INT_MAX, V|E|D, "color_primaries_type"},
{"bt709",       "BT.709",         0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_BT709 },        INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"unknown",     "Unspecified",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_UNSPECIFIED },  INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"bt470m",      "BT.470 M",       0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_BT470M },       INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"bt470bg",     "BT.470 BG",      0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_BT470BG },      INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"smpte170m",   "SMPTE 170 M",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_SMPTE170M },    INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"smpte240m",   "SMPTE 240 M",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_SMPTE240M },    INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"film",        "Film",           0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_FILM },         INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"bt2020",      "BT.2020",        0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_BT2020 },       INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"smpte428",    "SMPTE 428-1",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_SMPTE428 },     INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"smpte428_1",  "SMPTE 428-1",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_SMPTE428 },     INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"smpte431",    "SMPTE 431-2",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_SMPTE431 },     INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"smpte432",    "SMPTE 422-1",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_SMPTE432 },     INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"jedec-p22",   "JEDEC P22",      0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_JEDEC_P22 },    INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"ebu3213",     "EBU 3213-E",     0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_EBU3213 },      INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"unspecified", "Unspecified",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_PRI_UNSPECIFIED },  INT_MIN, INT_MAX, V|E|D, "color_primaries_type"},
{"color_trc", "color transfer characteristics", OFFSET(color_trc), AV_OPT_TYPE_INT, {.i64 = AVCOL_TRC_UNSPECIFIED }, 1, INT_MAX, V|E|D, "color_trc_type"},
{"bt709",        "BT.709",           0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_BT709 },        INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"unknown",      "Unspecified",      0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_UNSPECIFIED },  INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"gamma22",      "BT.470 M",         0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_GAMMA22 },      INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"gamma28",      "BT.470 BG",        0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_GAMMA28 },      INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"smpte170m",    "SMPTE 170 M",      0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_SMPTE170M },    INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"smpte240m",    "SMPTE 240 M",      0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_SMPTE240M },    INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"linear",       "Linear",           0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_LINEAR },       INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"log100",       "Log",              0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_LOG },          INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"log316",       "Log square root",  0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_LOG_SQRT },     INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"iec61966-2-4", "IEC 61966-2-4",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_IEC61966_2_4 }, INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"bt1361e",      "BT.1361",          0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_BT1361_ECG },   INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"iec61966-2-1", "IEC 61966-2-1",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_IEC61966_2_1 }, INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"bt2020-10",    "BT.2020 - 10 bit", 0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_BT2020_10 },    INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"bt2020-12",    "BT.2020 - 12 bit", 0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_BT2020_12 },    INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"smpte2084",    "SMPTE 2084",       0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_SMPTE2084 },    INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"smpte428",     "SMPTE 428-1",      0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_SMPTE428 },     INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"arib-std-b67", "ARIB STD-B67",     0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_ARIB_STD_B67 }, INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"unspecified",  "Unspecified",      0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_UNSPECIFIED },  INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"log",          "Log",              0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_LOG },          INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"log_sqrt",     "Log square root",  0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_LOG_SQRT },     INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"iec61966_2_4", "IEC 61966-2-4",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_IEC61966_2_4 }, INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"bt1361",       "BT.1361",          0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_BT1361_ECG },   INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"iec61966_2_1", "IEC 61966-2-1",    0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_IEC61966_2_1 }, INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"bt2020_10bit", "BT.2020 - 10 bit", 0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_BT2020_10 },    INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"bt2020_12bit", "BT.2020 - 12 bit", 0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_BT2020_12 },    INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"smpte428_1",   "SMPTE 428-1",      0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_TRC_SMPTE428 },     INT_MIN, INT_MAX, V|E|D, "color_trc_type"},
{"colorspace", "color space", OFFSET(colorspace), AV_OPT_TYPE_INT, {.i64 = AVCOL_SPC_UNSPECIFIED }, 0, INT_MAX, V|E|D, "colorspace_type"},
{"rgb",               "RGB",                0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_RGB },                INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"bt709",             "BT.709",             0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_BT709 },              INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"unknown",           "Unspecified",        0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_UNSPECIFIED },        INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"fcc",               "FCC",                0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_FCC },                INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"bt470bg",           "BT.470 BG",          0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_BT470BG },            INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"smpte170m",         "SMPTE 170 M",        0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_SMPTE170M },          INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"smpte240m",         "SMPTE 240 M",        0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_SMPTE240M },          INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"ycgco",             "YCGCO",              0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_YCGCO },              INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"bt2020nc",          "BT.2020 NCL",        0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_BT2020_NCL },         INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"bt2020c",           "BT.2020 CL",         0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_BT2020_CL },          INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"smpte2085",         "SMPTE 2085",         0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_SMPTE2085 },          INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"chroma-derived-nc", "Chroma-derived NCL", 0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_CHROMA_DERIVED_NCL }, INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"chroma-derived-c",  "Chroma-derived CL",  0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_CHROMA_DERIVED_CL },  INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"ictcp",             "ICtCp",              0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_ICTCP },              INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"unspecified",       "Unspecified",        0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_UNSPECIFIED },        INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"ycocg",             "YCGCO",              0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_YCGCO },              INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"bt2020_ncl",        "BT.2020 NCL",        0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_BT2020_NCL },         INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"bt2020_cl",         "BT.2020 CL",         0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_SPC_BT2020_CL },          INT_MIN, INT_MAX, V|E|D, "colorspace_type"},
{"color_range", "color range", OFFSET(color_range), AV_OPT_TYPE_INT, {.i64 = AVCOL_RANGE_UNSPECIFIED }, 0, INT_MAX, V|E|D, "color_range_type"},
{"unknown", "Unspecified",     0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_RANGE_UNSPECIFIED }, INT_MIN, INT_MAX, V|E|D, "color_range_type"},
{"tv", "MPEG (219*2^(n-8))",   0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_RANGE_MPEG },        INT_MIN, INT_MAX, V|E|D, "color_range_type"},
{"pc", "JPEG (2^n-1)",         0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_RANGE_JPEG },        INT_MIN, INT_MAX, V|E|D, "color_range_type"},
{"unspecified", "Unspecified", 0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_RANGE_UNSPECIFIED }, INT_MIN, INT_MAX, V|E|D, "color_range_type"},
{"mpeg", "MPEG (219*2^(n-8))", 0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_RANGE_MPEG },        INT_MIN, INT_MAX, V|E|D, "color_range_type"},
{"jpeg", "JPEG (2^n-1)",       0, AV_OPT_TYPE_CONST, {.i64 = AVCOL_RANGE_JPEG },        INT_MIN, INT_MAX, V|E|D, "color_range_type"},
{"chroma_sample_location", "chroma sample location", OFFSET(chroma_sample_location), AV_OPT_TYPE_INT, {.i64 = AVCHROMA_LOC_UNSPECIFIED }, 0, INT_MAX, V|E|D, "chroma_sample_location_type"},
{"unknown",     "Unspecified", 0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_UNSPECIFIED }, INT_MIN, INT_MAX, V|E|D, "chroma_sample_location_type"},
{"left",        "Left",        0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_LEFT },        INT_MIN, INT_MAX, V|E|D, "chroma_sample_location_type"},
{"center",      "Center",      0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_CENTER },      INT_MIN, INT_MAX, V|E|D, "chroma_sample_location_type"},
{"topleft",     "Top-left",    0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_TOPLEFT },     INT_MIN, INT_MAX, V|E|D, "chroma_sample_location_type"},
{"top",         "Top",         0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_TOP },         INT_MIN, INT_MAX, V|E|D, "chroma_sample_location_type"},
{"bottomleft",  "Bottom-left", 0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_BOTTOMLEFT },  INT_MIN, INT_MAX, V|E|D, "chroma_sample_location_type"},
{"bottom",      "Bottom",      0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_BOTTOM },      INT_MIN, INT_MAX, V|E|D, "chroma_sample_location_type"},
{"unspecified", "Unspecified", 0, AV_OPT_TYPE_CONST, {.i64 = AVCHROMA_LOC_UNSPECIFIED }, INT_MIN, INT_MAX, V|E|D, "chroma_sample_location_type"},
{"log_level_offset", "set the log level offset", OFFSET(log_level_offset), AV_OPT_TYPE_INT, {.i64 = 0 }, INT_MIN, INT_MAX },
{"slices", "set the number of slices, used in parallelized encoding", OFFSET(slices), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, V|E},
{"thread_type", "select multithreading type", OFFSET(thread_type), AV_OPT_TYPE_FLAGS, {.i64 = FF_THREAD_SLICE|FF_THREAD_FRAME }, 0, INT_MAX, V|A|E|D, "thread_type"},
{"slice", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_THREAD_SLICE }, INT_MIN, INT_MAX, V|E|D, "thread_type"},
{"frame", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_THREAD_FRAME }, INT_MIN, INT_MAX, V|E|D, "thread_type"},
{"audio_service_type", "audio service type", OFFSET(audio_service_type), AV_OPT_TYPE_INT, {.i64 = AV_AUDIO_SERVICE_TYPE_MAIN }, 0, AV_AUDIO_SERVICE_TYPE_NB-1, A|E, "audio_service_type"},
{"ma", "Main Audio Service", 0, AV_OPT_TYPE_CONST, {.i64 = AV_AUDIO_SERVICE_TYPE_MAIN },              INT_MIN, INT_MAX, A|E, "audio_service_type"},
{"ef", "Effects",            0, AV_OPT_TYPE_CONST, {.i64 = AV_AUDIO_SERVICE_TYPE_EFFECTS },           INT_MIN, INT_MAX, A|E, "audio_service_type"},
{"vi", "Visually Impaired",  0, AV_OPT_TYPE_CONST, {.i64 = AV_AUDIO_SERVICE_TYPE_VISUALLY_IMPAIRED }, INT_MIN, INT_MAX, A|E, "audio_service_type"},
{"hi", "Hearing Impaired",   0, AV_OPT_TYPE_CONST, {.i64 = AV_AUDIO_SERVICE_TYPE_HEARING_IMPAIRED },  INT_MIN, INT_MAX, A|E, "audio_service_type"},
{"di", "Dialogue",           0, AV_OPT_TYPE_CONST, {.i64 = AV_AUDIO_SERVICE_TYPE_DIALOGUE },          INT_MIN, INT_MAX, A|E, "audio_service_type"},
{"co", "Commentary",         0, AV_OPT_TYPE_CONST, {.i64 = AV_AUDIO_SERVICE_TYPE_COMMENTARY },        INT_MIN, INT_MAX, A|E, "audio_service_type"},
{"em", "Emergency",          0, AV_OPT_TYPE_CONST, {.i64 = AV_AUDIO_SERVICE_TYPE_EMERGENCY },         INT_MIN, INT_MAX, A|E, "audio_service_type"},
{"vo", "Voice Over",         0, AV_OPT_TYPE_CONST, {.i64 = AV_AUDIO_SERVICE_TYPE_VOICE_OVER },        INT_MIN, INT_MAX, A|E, "audio_service_type"},
{"ka", "Karaoke",            0, AV_OPT_TYPE_CONST, {.i64 = AV_AUDIO_SERVICE_TYPE_KARAOKE },           INT_MIN, INT_MAX, A|E, "audio_service_type"},
{"request_sample_fmt", "sample format audio decoders should prefer", OFFSET(request_sample_fmt), AV_OPT_TYPE_SAMPLE_FMT, {.i64=AV_SAMPLE_FMT_NONE}, -1, INT_MAX, A|D, "request_sample_fmt"},
{"pkt_timebase", NULL, OFFSET(pkt_timebase), AV_OPT_TYPE_RATIONAL, {.dbl = 0 }, 0, INT_MAX, 0},
{"sub_charenc", "set input text subtitles character encoding", OFFSET(sub_charenc), AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, S|D},
{"sub_charenc_mode", "set input text subtitles character encoding mode", OFFSET(sub_charenc_mode), AV_OPT_TYPE_FLAGS, {.i64 = FF_SUB_CHARENC_MODE_AUTOMATIC}, -1, INT_MAX, S|D, "sub_charenc_mode"},
{"do_nothing",  NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_SUB_CHARENC_MODE_DO_NOTHING},  INT_MIN, INT_MAX, S|D, "sub_charenc_mode"},
{"auto",        NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_SUB_CHARENC_MODE_AUTOMATIC},   INT_MIN, INT_MAX, S|D, "sub_charenc_mode"},
{"pre_decoder", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_SUB_CHARENC_MODE_PRE_DECODER}, INT_MIN, INT_MAX, S|D, "sub_charenc_mode"},
{"ignore",      NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_SUB_CHARENC_MODE_IGNORE},      INT_MIN, INT_MAX, S|D, "sub_charenc_mode"},
#if FF_API_SUB_TEXT_FORMAT
{"sub_text_format", "Deprecated, does nothing", OFFSET(sub_text_format), AV_OPT_TYPE_INT, {.i64 = FF_SUB_TEXT_FMT_ASS}, 0, 1, S|D | AV_OPT_FLAG_DEPRECATED, "sub_text_format"},
{"ass",              NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_SUB_TEXT_FMT_ASS},              INT_MIN, INT_MAX, S|D, "sub_text_format"},
#endif
{"apply_cropping", NULL, OFFSET(apply_cropping), AV_OPT_TYPE_BOOL, { .i64 = 1 }, 0, 1, V | D },
{"skip_alpha", "Skip processing alpha", OFFSET(skip_alpha), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0, 1, V|D },
{"field_order", "Field order", OFFSET(field_order), AV_OPT_TYPE_INT, {.i64 = AV_FIELD_UNKNOWN }, 0, 5, V|D|E, "field_order" },
{"progressive", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = AV_FIELD_PROGRESSIVE }, 0, 0, V|D|E, "field_order" },
{"tt", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = AV_FIELD_TT }, 0, 0, V|D|E, "field_order" },
{"bb", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = AV_FIELD_BB }, 0, 0, V|D|E, "field_order" },
{"tb", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = AV_FIELD_TB }, 0, 0, V|D|E, "field_order" },
{"bt", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = AV_FIELD_BT }, 0, 0, V|D|E, "field_order" },
{"dump_separator", "set information dump field separator", OFFSET(dump_separator), AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, A|V|S|D|E},
{"codec_whitelist", "List of decoders that are allowed to be used", OFFSET(codec_whitelist), AV_OPT_TYPE_STRING, { .str = NULL },  0, 0, A|V|S|D },
{"pixel_format", "set pixel format", OFFSET(pix_fmt), AV_OPT_TYPE_PIXEL_FMT, {.i64=AV_PIX_FMT_NONE}, -1, INT_MAX, 0 },
{"video_size", "set video size", OFFSET(width), AV_OPT_TYPE_IMAGE_SIZE, {.str=NULL}, 0, INT_MAX, 0 },
{"max_pixels", "Maximum number of pixels", OFFSET(max_pixels), AV_OPT_TYPE_INT64, {.i64 = INT_MAX }, 0, INT_MAX, A|V|S|D|E },
{"max_samples", "Maximum number of samples", OFFSET(max_samples), AV_OPT_TYPE_INT64, {.i64 = INT_MAX }, 0, INT_MAX, A|D|E },
{"hwaccel_flags", NULL, OFFSET(hwaccel_flags), AV_OPT_TYPE_FLAGS, {.i64 = AV_HWACCEL_FLAG_IGNORE_LEVEL }, 0, UINT_MAX, V|D, "hwaccel_flags"},
{"ignore_level", "ignore level even if the codec level used is unknown or higher than the maximum supported level reported by the hardware driver", 0, AV_OPT_TYPE_CONST, { .i64 = AV_HWACCEL_FLAG_IGNORE_LEVEL }, INT_MIN, INT_MAX, V | D, "hwaccel_flags" },
{"allow_high_depth", "allow to output YUV pixel formats with a different chroma sampling than 4:2:0 and/or other than 8 bits per component", 0, AV_OPT_TYPE_CONST, {.i64 = AV_HWACCEL_FLAG_ALLOW_HIGH_DEPTH }, INT_MIN, INT_MAX, V | D, "hwaccel_flags"},
{"allow_profile_mismatch", "attempt to decode anyway if HW accelerated decoder's supported profiles do not exactly match the stream", 0, AV_OPT_TYPE_CONST, {.i64 = AV_HWACCEL_FLAG_ALLOW_PROFILE_MISMATCH }, INT_MIN, INT_MAX, V | D, "hwaccel_flags"},
{"extra_hw_frames", "Number of extra hardware frames to allocate for the user", OFFSET(extra_hw_frames), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, V|D },
{"discard_damaged_percentage", "Percentage of damaged samples to discard a frame", OFFSET(discard_damaged_percentage), AV_OPT_TYPE_INT, {.i64 = 95 }, 0, 100, V|D },
{NULL},
};
 
#undef A
#undef V
#undef S
#undef E
#undef D
#undef CC
#undef DEFAULT
#undef OFFSET
 
#endif /* AVCODEC_OPTIONS_TABLE_H */

AVFrame中的AVClass

• AVFrame 中的AVClass定义位于libavcodec\options.c中，是一个名称为av_frame_class的静态结构体。如下所示。

static const AVClass av_frame_class = {
    .class_name              = "AVFrame",
    .item_name               = NULL,
    .option                  = frame_options,
    .version                 = LIBAVUTIL_VERSION_INT,
};

• option字段则指向一个元素个数极多的静态数组frame_options。frame_options定义如下所示。

static const AVOption frame_options[]={
{"best_effort_timestamp", "", FOFFSET(best_effort_timestamp), AV_OPT_TYPE_INT64, {.i64 = AV_NOPTS_VALUE }, INT64_MIN, INT64_MAX, 0},
{"pkt_pos", "", FOFFSET(pkt_pos), AV_OPT_TYPE_INT64, {.i64 = -1 }, INT64_MIN, INT64_MAX, 0},
{"pkt_size", "", FOFFSET(pkt_size), AV_OPT_TYPE_INT64, {.i64 = -1 }, INT64_MIN, INT64_MAX, 0},
{"sample_aspect_ratio", "", FOFFSET(sample_aspect_ratio), AV_OPT_TYPE_RATIONAL, {.dbl = 0 }, 0, INT_MAX, 0},
{"width", "", FOFFSET(width), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, 0},
{"height", "", FOFFSET(height), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, 0},
{"format", "", FOFFSET(format), AV_OPT_TYPE_INT, {.i64 = -1 }, 0, INT_MAX, 0},
#if FF_API_OLD_CHANNEL_LAYOUT
{"channel_layout", "", FOFFSET(channel_layout), AV_OPT_TYPE_INT64, {.i64 = 0 }, 0, INT64_MAX, 0},
#endif
{"sample_rate", "", FOFFSET(sample_rate), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, 0},
{NULL},
};

• 可以看出AVFrame的选项数组中包含了“width”，“height”这类用于视频帧的选项，以及“channel_layout”，“sample_rate”这类用于音频帧的选项。

各种组件特有的AVClass

• 除了FFmpeg中通用的AVFormatContext，AVCodecContext，AVFrame这类的结构体之外，每种特定的组件也包含自己的AVClass。
• 各种组件（libRTMP，libx264，libx265）里面特有的AVClass。
• 下面举例几个。

LibRTMP

• libRTMP中根据协议类型的不同定义了多种的AVClass。由于这些AVClass除了名字不一样之外，其他的字段一模一样，所以AVClass的声明写成了一个名称为RTMP_CLASS的宏。

#define RTMP_CLASS(flavor)\
static const AVClass lib ## flavor ## _class = {\
    .class_name = "lib" #flavor " protocol",\
    .item_name  = av_default_item_name,\
    .option     = options,\
    .version    = LIBAVUTIL_VERSION_INT,\
};

• 而后定义了多种AVCLass：
  • RTMP_CLASS(rtmp)
  • RTMP_CLASS(rtmpt)
  • RTMP_CLASS(rtmpe)
  • RTMP_CLASS(rtmpte)
  • RTMP_CLASS(rtmps)

RTMP_CLASS(rtmp)
const URLProtocol ff_librtmp_protocol = {
    .name                = "rtmp",
    .url_open            = rtmp_open,
    .url_read            = rtmp_read,
    .url_write           = rtmp_write,
    .url_close           = rtmp_close,
    .url_read_pause      = rtmp_read_pause,
    .url_read_seek       = rtmp_read_seek,
    .url_get_file_handle = rtmp_get_file_handle,
    .priv_data_size      = sizeof(LibRTMPContext),
    .priv_data_class     = &librtmp_class,
    .flags               = URL_PROTOCOL_FLAG_NETWORK,
};
 
RTMP_CLASS(rtmpt)
const URLProtocol ff_librtmpt_protocol = {
    .name                = "rtmpt",
    .url_open            = rtmp_open,
    .url_read            = rtmp_read,
    .url_write           = rtmp_write,
    .url_close           = rtmp_close,
    .url_read_pause      = rtmp_read_pause,
    .url_read_seek       = rtmp_read_seek,
    .url_get_file_handle = rtmp_get_file_handle,
    .priv_data_size      = sizeof(LibRTMPContext),
    .priv_data_class     = &librtmpt_class,
    .flags               = URL_PROTOCOL_FLAG_NETWORK,
};
 
RTMP_CLASS(rtmpe)
const URLProtocol ff_librtmpe_protocol = {
    .name                = "rtmpe",
    .url_open            = rtmp_open,
    .url_read            = rtmp_read,
    .url_write           = rtmp_write,
    .url_close           = rtmp_close,
    .url_read_pause      = rtmp_read_pause,
    .url_read_seek       = rtmp_read_seek,
    .url_get_file_handle = rtmp_get_file_handle,
    .priv_data_size      = sizeof(LibRTMPContext),
    .priv_data_class     = &librtmpe_class,
    .flags               = URL_PROTOCOL_FLAG_NETWORK,
};
 
RTMP_CLASS(rtmpte)
const URLProtocol ff_librtmpte_protocol = {
    .name                = "rtmpte",
    .url_open            = rtmp_open,
    .url_read            = rtmp_read,
    .url_write           = rtmp_write,
    .url_close           = rtmp_close,
    .url_read_pause      = rtmp_read_pause,
    .url_read_seek       = rtmp_read_seek,
    .url_get_file_handle = rtmp_get_file_handle,
    .priv_data_size      = sizeof(LibRTMPContext),
    .priv_data_class     = &librtmpte_class,
    .flags               = URL_PROTOCOL_FLAG_NETWORK,
};
 
RTMP_CLASS(rtmps)
const URLProtocol ff_librtmps_protocol = {
    .name                = "rtmps",
    .url_open            = rtmp_open,
    .url_read            = rtmp_read,
    .url_write           = rtmp_write,
    .url_close           = rtmp_close,
    .url_read_pause      = rtmp_read_pause,
    .url_read_seek       = rtmp_read_seek,
    .url_get_file_handle = rtmp_get_file_handle,
    .priv_data_size      = sizeof(LibRTMPContext),
    .priv_data_class     = &librtmps_class,
    .flags               = URL_PROTOCOL_FLAG_NETWORK,
};

• 这些AVClass的option字段指向的数组是一样的，如下所示。

#define OFFSET(x) offsetof(LibRTMPContext, x)
#define DEC AV_OPT_FLAG_DECODING_PARAM
#define ENC AV_OPT_FLAG_ENCODING_PARAM
static const AVOption options[] = {
    {"rtmp_app", "Name of application to connect to on the RTMP server", OFFSET(app), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_buffer", "Set buffer time in milliseconds. The default is 3000.", OFFSET(client_buffer_time), AV_OPT_TYPE_STRING, {.str = "3000"}, 0, 0, DEC|ENC},
    {"rtmp_conn", "Append arbitrary AMF data to the Connect message", OFFSET(conn), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_flashver", "Version of the Flash plugin used to run the SWF player.", OFFSET(flashver), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_live", "Specify that the media is a live stream.", OFFSET(live), AV_OPT_TYPE_INT, {.i64 = 0}, INT_MIN, INT_MAX, DEC, "rtmp_live"},
    {"any", "both", 0, AV_OPT_TYPE_CONST, {.i64 = -2}, 0, 0, DEC, "rtmp_live"},
    {"live", "live stream", 0, AV_OPT_TYPE_CONST, {.i64 = -1}, 0, 0, DEC, "rtmp_live"},
    {"recorded", "recorded stream", 0, AV_OPT_TYPE_CONST, {.i64 = 0}, 0, 0, DEC, "rtmp_live"},
    {"rtmp_pageurl", "URL of the web page in which the media was embedded. By default no value will be sent.", OFFSET(pageurl), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC},
    {"rtmp_playpath", "Stream identifier to play or to publish", OFFSET(playpath), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_subscribe", "Name of live stream to subscribe to. Defaults to rtmp_playpath.", OFFSET(subscribe), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC},
    {"rtmp_swfurl", "URL of the SWF player. By default no value will be sent", OFFSET(swfurl), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
    {"rtmp_swfverify", "URL to player swf file, compute hash/size automatically. (unimplemented)", OFFSET(swfverify), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC},
    {"rtmp_tcurl", "URL of the target stream. Defaults to proto://host[:port]/app.", OFFSET(tcurl), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
#if CONFIG_NETWORK
    {"rtmp_buffer_size", "set buffer size in bytes", OFFSET(buffer_size), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX, DEC|ENC },
#endif
    { NULL },
};

Libx264

• Libx264的AVClass定义如下所示。

#if CONFIG_LIBX264_ENCODER
static const AVClass x264_class = {
    .class_name = "libx264",
    .item_name  = av_default_item_name,
    .option     = options,
    .version    = LIBAVUTIL_VERSION_INT,
};

• 其中option字段指向的数组定义如下所示。这些option的使用频率还是比较高的。

static const AVOption options[] = {
    { "preset",        "Set the encoding preset (cf. x264 --fullhelp)",   OFFSET(preset),        AV_OPT_TYPE_STRING, { .str = "medium" }, 0, 0, VE},
    { "tune",          "Tune the encoding params (cf. x264 --fullhelp)",  OFFSET(tune),          AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
    { "profile",       "Set profile restrictions (cf. x264 --fullhelp) ", OFFSET(profile),       AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
    { "fastfirstpass", "Use fast settings when encoding first pass",      OFFSET(fastfirstpass), AV_OPT_TYPE_BOOL, { .i64 = 1 }, 0, 1, VE},
    {"level", "Specify level (as defined by Annex A)", OFFSET(level), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
    {"passlogfile", "Filename for 2 pass stats", OFFSET(stats), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
    {"wpredp", "Weighted prediction for P-frames", OFFSET(wpredp), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
    {"a53cc",          "Use A53 Closed Captions (if available)",          OFFSET(a53_cc),        AV_OPT_TYPE_BOOL,   {.i64 = 1}, 0, 1, VE},
    {"x264opts", "x264 options", OFFSET(x264opts), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
    { "crf",           "Select the quality for constant quality mode",    OFFSET(crf),           AV_OPT_TYPE_FLOAT,  {.dbl = -1 }, -1, FLT_MAX, VE },
    { "crf_max",       "In CRF mode, prevents VBV from lowering quality beyond this point.",OFFSET(crf_max), AV_OPT_TYPE_FLOAT, {.dbl = -1 }, -1, FLT_MAX, VE },
    { "qp",            "Constant quantization parameter rate control method",OFFSET(cqp),        AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE },
    { "aq-mode",       "AQ method",                                       OFFSET(aq_mode),       AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "aq_mode"},
    { "none",          NULL,                              0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_NONE},         INT_MIN, INT_MAX, VE, "aq_mode" },
    { "variance",      "Variance AQ (complexity mask)",   0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_VARIANCE},     INT_MIN, INT_MAX, VE, "aq_mode" },
    { "autovariance",  "Auto-variance AQ",                0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_AUTOVARIANCE}, INT_MIN, INT_MAX, VE, "aq_mode" },
#if X264_BUILD >= 144
    { "autovariance-biased", "Auto-variance AQ with bias to dark scenes", 0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_AUTOVARIANCE_BIASED}, INT_MIN, INT_MAX, VE, "aq_mode" },
#endif
    { "aq-strength",   "AQ strength. Reduces blocking and blurring in flat and textured areas.", OFFSET(aq_strength), AV_OPT_TYPE_FLOAT, {.dbl = -1}, -1, FLT_MAX, VE},
    { "psy",           "Use psychovisual optimizations.",                 OFFSET(psy),           AV_OPT_TYPE_BOOL,   { .i64 = -1 }, -1, 1, VE },
    { "psy-rd",        "Strength of psychovisual optimization, in <psy-rd>:<psy-trellis> format.", OFFSET(psy_rd), AV_OPT_TYPE_STRING,  {0 }, 0, 0, VE},
    { "rc-lookahead",  "Number of frames to look ahead for frametype and ratecontrol", OFFSET(rc_lookahead), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE },
    { "weightb",       "Weighted prediction for B-frames.",               OFFSET(weightb),       AV_OPT_TYPE_BOOL,   { .i64 = -1 }, -1, 1, VE },
    { "weightp",       "Weighted prediction analysis method.",            OFFSET(weightp),       AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "weightp" },
    { "none",          NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_WEIGHTP_NONE},   INT_MIN, INT_MAX, VE, "weightp" },
    { "simple",        NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_WEIGHTP_SIMPLE}, INT_MIN, INT_MAX, VE, "weightp" },
    { "smart",         NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_WEIGHTP_SMART},  INT_MIN, INT_MAX, VE, "weightp" },
    { "ssim",          "Calculate and print SSIM stats.",                 OFFSET(ssim),          AV_OPT_TYPE_BOOL,   { .i64 = -1 }, -1, 1, VE },
    { "intra-refresh", "Use Periodic Intra Refresh instead of IDR frames.",OFFSET(intra_refresh),AV_OPT_TYPE_BOOL,   { .i64 = -1 }, -1, 1, VE },
    { "bluray-compat", "Bluray compatibility workarounds.",               OFFSET(bluray_compat) ,AV_OPT_TYPE_BOOL,   { .i64 = -1 }, -1, 1, VE },
    { "b-bias",        "Influences how often B-frames are used",          OFFSET(b_bias),        AV_OPT_TYPE_INT,    { .i64 = INT_MIN}, INT_MIN, INT_MAX, VE },
    { "b-pyramid",     "Keep some B-frames as references.",               OFFSET(b_pyramid),     AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "b_pyramid" },
    { "none",          NULL,                                  0, AV_OPT_TYPE_CONST, {.i64 = X264_B_PYRAMID_NONE},   INT_MIN, INT_MAX, VE, "b_pyramid" },
    { "strict",        "Strictly hierarchical pyramid",       0, AV_OPT_TYPE_CONST, {.i64 = X264_B_PYRAMID_STRICT}, INT_MIN, INT_MAX, VE, "b_pyramid" },
    { "normal",        "Non-strict (not Blu-ray compatible)", 0, AV_OPT_TYPE_CONST, {.i64 = X264_B_PYRAMID_NORMAL}, INT_MIN, INT_MAX, VE, "b_pyramid" },
    { "mixed-refs",    "One reference per partition, as opposed to one reference per macroblock", OFFSET(mixed_refs), AV_OPT_TYPE_BOOL, { .i64 = -1}, -1, 1, VE },
    { "8x8dct",        "High profile 8x8 transform.",                     OFFSET(dct8x8),        AV_OPT_TYPE_BOOL,   { .i64 = -1 }, -1, 1, VE},
    { "fast-pskip",    NULL,                                              OFFSET(fast_pskip),    AV_OPT_TYPE_BOOL,   { .i64 = -1 }, -1, 1, VE},
    { "aud",           "Use access unit delimiters.",                     OFFSET(aud),           AV_OPT_TYPE_BOOL,   { .i64 = -1 }, -1, 1, VE},
    { "mbtree",        "Use macroblock tree ratecontrol.",                OFFSET(mbtree),        AV_OPT_TYPE_BOOL,   { .i64 = -1 }, -1, 1, VE},
    { "deblock",       "Loop filter parameters, in <alpha:beta> form.",   OFFSET(deblock),       AV_OPT_TYPE_STRING, { 0 },  0, 0, VE},
    { "cplxblur",      "Reduce fluctuations in QP (before curve compression)", OFFSET(cplxblur), AV_OPT_TYPE_FLOAT,  {.dbl = -1 }, -1, FLT_MAX, VE},
    { "partitions",    "A comma-separated list of partitions to consider. "
                       "Possible values: p8x8, p4x4, b8x8, i8x8, i4x4, none, all", OFFSET(partitions), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
    { "direct-pred",   "Direct MV prediction mode",                       OFFSET(direct_pred),   AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "direct-pred" },
    { "none",          NULL,      0,    AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_NONE },     0, 0, VE, "direct-pred" },
    { "spatial",       NULL,      0,    AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_SPATIAL },  0, 0, VE, "direct-pred" },
    { "temporal",      NULL,      0,    AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_TEMPORAL }, 0, 0, VE, "direct-pred" },
    { "auto",          NULL,      0,    AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_AUTO },     0, 0, VE, "direct-pred" },
    { "slice-max-size","Limit the size of each slice in bytes",           OFFSET(slice_max_size),AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE },
    { "stats",         "Filename for 2 pass stats",                       OFFSET(stats),         AV_OPT_TYPE_STRING, { 0 },  0,       0, VE },
    { "nal-hrd",       "Signal HRD information (requires vbv-bufsize; "
                       "cbr not allowed in .mp4)",                        OFFSET(nal_hrd),       AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE, "nal-hrd" },
    { "none",          NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_NAL_HRD_NONE}, INT_MIN, INT_MAX, VE, "nal-hrd" },
    { "vbr",           NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_NAL_HRD_VBR},  INT_MIN, INT_MAX, VE, "nal-hrd" },
    { "cbr",           NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_NAL_HRD_CBR},  INT_MIN, INT_MAX, VE, "nal-hrd" },
    { "avcintra-class","AVC-Intra class 50/100/200/300/480",              OFFSET(avcintra_class),AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, 480   , VE},
    { "me_method",    "Set motion estimation method",                     OFFSET(motion_est),    AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, X264_ME_TESA, VE, "motion-est"},
    { "motion-est",   "Set motion estimation method",                     OFFSET(motion_est),    AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, X264_ME_TESA, VE, "motion-est"},
    { "dia",           NULL, 0, AV_OPT_TYPE_CONST, { .i64 = X264_ME_DIA },  INT_MIN, INT_MAX, VE, "motion-est" },
    { "hex",           NULL, 0, AV_OPT_TYPE_CONST, { .i64 = X264_ME_HEX },  INT_MIN, INT_MAX, VE, "motion-est" },
    { "umh",           NULL, 0, AV_OPT_TYPE_CONST, { .i64 = X264_ME_UMH },  INT_MIN, INT_MAX, VE, "motion-est" },
    { "esa",           NULL, 0, AV_OPT_TYPE_CONST, { .i64 = X264_ME_ESA },  INT_MIN, INT_MAX, VE, "motion-est" },
    { "tesa",          NULL, 0, AV_OPT_TYPE_CONST, { .i64 = X264_ME_TESA }, INT_MIN, INT_MAX, VE, "motion-est" },
    { "forced-idr",   "If forcing keyframes, force them as IDR frames.",                                  OFFSET(forced_idr),  AV_OPT_TYPE_BOOL,   { .i64 = 0 }, -1, 1, VE },
    { "coder",    "Coder type",                                           OFFSET(coder), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE, "coder" },
    { "default",          NULL, 0, AV_OPT_TYPE_CONST, { .i64 = -1 }, INT_MIN, INT_MAX, VE, "coder" },
    { "cavlc",            NULL, 0, AV_OPT_TYPE_CONST, { .i64 = 0 },  INT_MIN, INT_MAX, VE, "coder" },
    { "cabac",            NULL, 0, AV_OPT_TYPE_CONST, { .i64 = 1 },  INT_MIN, INT_MAX, VE, "coder" },
    { "vlc",              NULL, 0, AV_OPT_TYPE_CONST, { .i64 = 0 },  INT_MIN, INT_MAX, VE, "coder" },
    { "ac",               NULL, 0, AV_OPT_TYPE_CONST, { .i64 = 1 },  INT_MIN, INT_MAX, VE, "coder" },
    { "b_strategy",   "Strategy to choose between I/P/B-frames",          OFFSET(b_frame_strategy), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 2, VE },
    { "chromaoffset", "QP difference between chroma and luma",            OFFSET(chroma_offset), AV_OPT_TYPE_INT, { .i64 = 0 }, INT_MIN, INT_MAX, VE },
    { "sc_threshold", "Scene change threshold",                           OFFSET(scenechange_threshold), AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX, VE },
    { "noise_reduction", "Noise reduction",                               OFFSET(noise_reduction), AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX, VE },
    { "udu_sei",      "Use user data unregistered SEI if available",      OFFSET(udu_sei),  AV_OPT_TYPE_BOOL,   { .i64 = 0 }, 0, 1, VE },
    { "x264-params",  "Override the x264 configuration using a :-separated list of key=value parameters", OFFSET(x264_params), AV_OPT_TYPE_DICT, { 0 }, 0, 0, VE },
    { NULL },
};

Libx265

• Libx265的AVClass定义如下所示。

static const AVClass class = {
    .class_name = "libx265",
    .item_name  = av_default_item_name,
    .option     = options,
    .version    = LIBAVUTIL_VERSION_INT,
};

• 其中option字段指向的数组定义如下所示。

static const AVOption options[] = {
    { "crf",         "set the x265 crf",                                                            OFFSET(crf),       AV_OPT_TYPE_FLOAT,  { .dbl = -1 }, -1, FLT_MAX, VE },
    { "qp",          "set the x265 qp",                                                             OFFSET(cqp),       AV_OPT_TYPE_INT,    { .i64 = -1 }, -1, INT_MAX, VE },
    { "forced-idr",  "if forcing keyframes, force them as IDR frames",                              OFFSET(forced_idr),AV_OPT_TYPE_BOOL,   { .i64 =  0 },  0,       1, VE },
    { "preset",      "set the x265 preset",                                                         OFFSET(preset),    AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
    { "tune",        "set the x265 tune parameter",                                                 OFFSET(tune),      AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
    { "profile",     "set the x265 profile",                                                        OFFSET(profile),   AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
    { "udu_sei",      "Use user data unregistered SEI if available",                                OFFSET(udu_sei),   AV_OPT_TYPE_BOOL,   { .i64 = 0 }, 0, 1, VE },
    { "x265-params", "set the x265 configuration using a :-separated list of key=value parameters", OFFSET(x265_opts), AV_OPT_TYPE_DICT,   { 0 }, 0, 0, VE },
    { NULL }
};

官方代码中有关AVClass和AVOption的示例

• 官方代码中给出了一小段示例代码，演示了如何给一个普通的结构体添加AVOption的支持。如下所示。

typedef struct test_struct {
    AVClass  *class;
    int      int_opt;
    char    str_opt;
    uint8_t  bin_opt;
    int      bin_len;
} test_struct;
 
static const AVOption test_options[] = {
  { "test_int", "This is a test option of int type.", offsetof(test_struct, int_opt),
    AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX },
  { "test_str", "This is a test option of string type.", offsetof(test_struct, str_opt),
    AV_OPT_TYPE_STRING },
  { "test_bin", "This is a test option of binary type.", offsetof(test_struct, bin_opt),
    AV_OPT_TYPE_BINARY },
  { NULL },
};
 
static const AVClass test_class = {
    .class_name = "test class",
    .item_name  = av_default_item_name,
    .option     = test_options,
    .version    = LIBAVUTIL_VERSION_INT,
};

AVClass有关的API

• 与AVClass相关的API很少。AVFormatContext提供了一个获取当前AVClass的函数avformat_get_class()。
• 它的代码很简单，直接返回全局静态变量av_format_context_class。
• 定义如下所示。

const AVClass *avformat_get_class(void)
{
    return &av_format_context_class;
}

• 同样，AVCodecContext也提供了一个获取当前AVClass的函数avcodec_get_class()。
• 它直接返回静态变量av_codec_context_class。
• 定义如下所示。

const AVClass *avcodec_get_class(void)
{
    return &av_codec_context_class;
}