Java 解析Tiff深入研究

       最近在读取客户发过来的tiff文件是，底层竟然报错了，错误：bandOffsets.length is wrong!   没办法，因为错误消息出现在tiff的read中，因此就对

底层序中tiff读取的代码进行了研究。

       之前有一篇文章，我简单的介绍了Geotools读取Tiff的代码，Java 通过geotools读取tiff，其实通过深入研究发现，原来幕后的大佬竟然是imageio-ext中的TiffImageReader，

imageio做为Java开发的人员肯定都知道，而ImageIO-ext是imageio的扩展类，我们可以到github上看到它的源码，这是一个非常强大的库，对于Java处理各种栅格数据的读写非常有帮助！

      借助这篇文章，我们需要先了解Tiff文件的具体结构，可以参考这篇文章，TIFF文件结构详解 https://blog.csdn.net/oYinHeZhiGuang/article/details/121710467  讲的很好！

      下面我们来看下imageio-ext中的tiff读取代码，主要类TiffImageReader，我们来看下Java程序是如何读取tiff文件的。

      构造方法：

public TIFFImageReader(ImageReaderSpi originatingProvider) {
        super(originatingProvider);
 }

 这个类需要通过一个ImageReaderSpi来实例化，其实这种SPI的设计模式，Java的很多开源项目都在用到，这里我们通过TIFFImageReaderSpi这个类即可。

 其次设置文件的路径，以及其它一些参数，通过该类的如下方法：

public void setInput(Object input,
                         boolean seekForwardOnly,
                         boolean ignoreMetadata)

这个方法，里面有input就是需要读取的文件，seekForwardOnly设置为true表示：只能从这个输入源按升序读取图像和元数据。ignoreMetadata设置为true表示读取忽略元数据

接下来就是对tiff元数据的读取，具体参见getImageMetadata(int imageIndex)这个方法：

public IIOMetadata getImageMetadata(int imageIndex) throws IIOException {
        seekToImage(imageIndex, true);
        TIFFImageMetadata im =
            new TIFFImageMetadata(imageMetadata.getRootIFD().getTagSetList());
        Node root =
            imageMetadata.getAsTree(TIFFImageMetadata.nativeMetadataFormatName);
        im.setFromTree(TIFFImageMetadata.nativeMetadataFormatName, root);
        if (noData != null) {
            im.setNoData(new double[] {noData, noData});
        }
        if (scales != null && offsets != null) {
            im.setScales(scales);
            im.setOffsets(offsets);
        }
        return im;
    }

其中的seekToImage(imageIndex, true)为最主要的逻辑处理，这个方法中，第一个参数，imageIndex为tiff多页中的第几个，第二参数设置标示该tiff页是否已经被解析过

 private void seekToImage(int imageIndex, boolean optimized) throws IIOException {
        checkIndex(imageIndex);

        // TODO we should do this initialization just once!!!
        int index = locateImage(imageIndex);
        if (index != imageIndex) {
            throw new IndexOutOfBoundsException("imageIndex out of bounds!");
        }
        
        final Integer i= Integer.valueOf(index);
        //optimized branch
        if(!optimized){
            
            readMetadata();
            initializeFromMetadata();
            return;
        }
        // in case we have cache the info for this page
        if(pagesInfo.containsKey(i)){
            // initialize from cachedinfo only if needed
            // TODO Improve
            if(imageMetadata == null || !initialized) {// this means the curindex has changed
                final PageInfo info = pagesInfo.get(i);
                final TIFFImageMetadata metadata = info.imageMetadata.get();
                if (metadata != null) {
                    initializeFromCachedInfo(info, metadata);
                    return;
                }
                pagesInfo.put(i,null);
                    
            }
        }
        
        readMetadata();
        initializeFromMetadata();
    }

这个方法当中，第一次加载tiff，通过readMetadata()和initializeFromMetadata()将tiff的元信息缓存起来，方便后面再次读取。

读取过程

主要是要结合Tiff的格式进行理解，大体主要是解析tiff头，然后获取到IFD(tiff的图像目录信息)，然后再依次去解析每个目录的具体内容，代码就不再这里罗列了。

这里主要说下，解析目录信息是获取tiff的元信息的过程，通常是解析每个tag的信息，解析代码TIFFIFD类的initialize(ImageInputStream stream,  boolean ignoreUnknownFields, final boolean isBTIFF)方法中

public void initialize(ImageInputStream stream,
            boolean ignoreUnknownFields, final boolean isBTIFF) throws IOException {
        removeTIFFFields();

        List tagSetList = getTagSetList();

       
        final long numEntries;
        if(isBTIFF)
            numEntries= stream.readLong();
        else
            numEntries= stream.readUnsignedShort();
        
        for (int i = 0; i < numEntries; i++) {
            // Read tag number, value type, and value count.
            int tag = stream.readUnsignedShort();
            int type = stream.readUnsignedShort();
            int count;
            if(isBTIFF)
            {
                long count_=stream.readLong();
                count = (int)count_;
                if(count!=count_)
                    throw new IllegalArgumentException("unable to use long number of values");
            }
            else            
                count = (int)stream.readUnsignedInt();

            // Get the associated TIFFTag.
            TIFFTag tiffTag = getTag(tag, tagSetList);

            // Ignore unknown fields.
            if(ignoreUnknownFields && tiffTag == null) {
                // Skip the value/offset so as to leave the stream
                // position at the start of the next IFD entry.

                if(isBTIFF)
                    stream.skipBytes(8);
                else
                    stream.skipBytes(4);

                // XXX Warning message ...

                // Continue with the next IFD entry.
                continue;
            }
       
            long nextTagOffset;
            
            if(isBTIFF){
                nextTagOffset = stream.getStreamPosition() + 8;
                int sizeOfType = TIFFTag.getSizeOfType(type);
                if (count*sizeOfType > 8) {
                    long value = stream.readLong();
                    stream.seek(value);
                 }
            }
            else{                
                nextTagOffset = stream.getStreamPosition() + 4;
                int sizeOfType = TIFFTag.getSizeOfType(type);
                 if (count*sizeOfType > 4) {
                    long value = stream.readUnsignedInt();
                    stream.seek(value);
                 }
            }
            
            if (tag == BaselineTIFFTagSet.TAG_STRIP_BYTE_COUNTS ||
                tag == BaselineTIFFTagSet.TAG_TILE_BYTE_COUNTS ||
                tag == BaselineTIFFTagSet.TAG_JPEG_INTERCHANGE_FORMAT_LENGTH) {
                this.stripOrTileByteCountsPosition =
                    stream.getStreamPosition();
                if (LAZY_LOADING) {
                    type = type == TIFFTag.TIFF_LONG ? TIFFTag.TIFF_LAZY_LONG : TIFFTag.TIFF_LAZY_LONG8;
                }
            } else if (tag == BaselineTIFFTagSet.TAG_STRIP_OFFSETS ||
                       tag == BaselineTIFFTagSet.TAG_TILE_OFFSETS ||
                       tag == BaselineTIFFTagSet.TAG_JPEG_INTERCHANGE_FORMAT) {
                this.stripOrTileOffsetsPosition =
                    stream.getStreamPosition();
                if (LAZY_LOADING) {
                    type = type == TIFFTag.TIFF_LONG ? TIFFTag.TIFF_LAZY_LONG : TIFFTag.TIFF_LAZY_LONG8;
                }
            }

            Object obj = null;

            try {
                switch (type) {
                case TIFFTag.TIFF_BYTE:
                case TIFFTag.TIFF_SBYTE:
                case TIFFTag.TIFF_UNDEFINED:
                case TIFFTag.TIFF_ASCII:
                    byte[] bvalues = new byte[count];
                    stream.readFully(bvalues, 0, count);
                
                    if (type == TIFFTag.TIFF_ASCII) {
                        // Can be multiple strings
                        final List v = new ArrayList();
                        boolean inString = false;
                        int prevIndex = 0;
                        for (int index = 0; index <= count; index++) {
                            if (index < count && bvalues[index] != 0) {
                                if (!inString) {
                                // start of string
                                    prevIndex = index;
                                    inString = true;
                                }
                            } else { // null or special case at end of string
                                if (inString) {
                                // end of string
                                    final String s = new String(bvalues, prevIndex,index - prevIndex);
                                    v.add(s);
                                    inString = false;
                                }
                            }
                        }

                        count = v.size();
                        String[] strings;
                        if(count != 0) {
                            strings = new String[count];
                            for (int c = 0 ; c < count; c++) {
                                strings[c] = v.get(c);
                            }
                        } else {
                            // This case has been observed when the value of
                            // 'count' recorded in the field is non-zero but
                            // the value portion contains all nulls.
                            count = 1;
                            strings = new String[] {""};
                        }
                    
                        obj = strings;
                    } else {
                        obj = bvalues;
                    }
                    break;
                
                case TIFFTag.TIFF_SHORT:
                    char[] cvalues = new char[count];
                    for (int j = 0; j < count; j++) {
                        cvalues[j] = (char)(stream.readUnsignedShort());
                    }
                    obj = cvalues;
                    break;
                
                case TIFFTag.TIFF_LONG:
                case TIFFTag.TIFF_IFD_POINTER:
                    long[] lvalues = new long[count];
                    for (int j = 0; j < count; j++) {
                        lvalues[j] = stream.readUnsignedInt();
                    }
                    obj = lvalues;
                    break;
                
                case TIFFTag.TIFF_RATIONAL:
                    long[][] llvalues = new long[count][2];
                    for (int j = 0; j < count; j++) {
                        llvalues[j][0] = stream.readUnsignedInt();
                        llvalues[j][1] = stream.readUnsignedInt();
                    }
                    obj = llvalues;
                    break;
                
                case TIFFTag.TIFF_SSHORT:
                    short[] svalues = new short[count];
                    for (int j = 0; j < count; j++) {
                        svalues[j] = stream.readShort();
                    }
                    obj = svalues;
                    break;
                
                case TIFFTag.TIFF_SLONG:
                    int[] ivalues = new int[count];
                    for (int j = 0; j < count; j++) {
                        ivalues[j] = stream.readInt();
                    }
                    obj = ivalues;
                    break;
                
                case TIFFTag.TIFF_SRATIONAL:
                    int[][] iivalues = new int[count][2];
                    for (int j = 0; j < count; j++) {
                        iivalues[j][0] = stream.readInt();
                        iivalues[j][1] = stream.readInt();
                    }
                    obj = iivalues;
                    break;
                
                case TIFFTag.TIFF_FLOAT:
                    float[] fvalues = new float[count];
                    for (int j = 0; j < count; j++) {
                        fvalues[j] = stream.readFloat();
                    }
                    obj = fvalues;
                    break;
                
                case TIFFTag.TIFF_DOUBLE:
                    double[] dvalues = new double[count];
                    for (int j = 0; j < count; j++) {
                        dvalues[j] = stream.readDouble();
                    }
                    obj = dvalues;
                    break;
                           
                case TIFFTag.TIFF_LONG8:
                case TIFFTag.TIFF_SLONG8:    
                case TIFFTag.TIFF_IFD8:
                    long[] lBvalues = new long[count];
                    for (int j = 0; j < count; j++) {
                        lBvalues[j] = stream.readLong();
                    }
                    obj = lBvalues;
                    break;
                
                case TIFFTag.TIFF_LAZY_LONG8:   
                case TIFFTag.TIFF_LAZY_LONG:   
                    obj = new TIFFLazyData(stream, type, count);
                    break;
                default:
                    // XXX Warning
                    break;
                }
            } catch(EOFException eofe) {
                // The TIFF 6.0 fields have tag numbers less than or equal
                // to 532 (ReferenceBlackWhite) or equal to 33432 (Copyright).
                // If there is an error reading a baseline tag, then re-throw
                // the exception and fail; otherwise continue with the next
                // field.
                if(BaselineTIFFTagSet.getInstance().getTag(tag) == null) {
                    throw eofe;
                }
            }
            
            if (tiffTag == null) {
                // XXX Warning: unknown tag
            } else if (!tiffTag.isDataTypeOK(type)) {
                // XXX Warning: bad data type
            } else if (tiffTag.isIFDPointer() && obj != null) {
                stream.mark();
                stream.seek(((long[])obj)[0]);

                List tagSets = new ArrayList(1);
                tagSets.add(tiffTag.getTagSet());
                TIFFIFD subIFD = new TIFFIFD(tagSets);

                // XXX Use same ignore policy for sub-IFD fields?
                subIFD.initialize(stream, ignoreUnknownFields);
                obj = subIFD;
                stream.reset();
            }

            if (tiffTag == null) {
                tiffTag = new TIFFTag(null, tag, 1 << type, null);
            }

            // Add the field if its contents have been initialized which
            // will not be the case if an EOF was ignored above.
            if(obj != null) {
                TIFFField f = new TIFFField(tiffTag, type, count, obj);
                addTIFFField(f);
            }

            stream.seek(nextTagOffset);
        }

        this.lastPosition = stream.getStreamPosition();
    }

Tiff常用的Tag标签类有BaseLineTiffTagSet、FaxTiffTagSet、GeoTiffTagSet、EXIFPTiffTagSet、PrivateTIFFTagSet等。

其中的GeoTiffTagSet用于geotiff的额外存储信息，在这里说明下，Geotiff是Tiff格式对Gis数据的一种存储支持，而PrivateTIFFTagSet是对gdal的支持，增加了NODATA、MEATADATA的信息。

 对于文章开头提的关于bandOffsets.length is wrong!，主要原因出现在getImageTypes(int imageIndex)这个方法的下面这个实现中。

ImageTypeSpecifier itsRaw = 
            TIFFDecompressor.getRawImageTypeSpecifier
                (photometricInterpretation,
                 compression,
                 samplesPerPixel,
                 bitsPerSample,
                 sampleFormat,
                 extraSamples,
                 colorMap);

最终我们在ImageTypeSpecifier这个类的Interleaved(ColorSpace colorSpace,int[] bandOffsets,int dataType,boolean hasAlpha,boolean isAlphaPremultiplied) 方法中发现问题。

public Interleaved(ColorSpace colorSpace,
                           int[] bandOffsets,
                           int dataType,
                           boolean hasAlpha,
                           boolean isAlphaPremultiplied) {
            if (colorSpace == null) {
                throw new IllegalArgumentException("colorSpace == null!");
            }
            if (bandOffsets == null) {
                throw new IllegalArgumentException("bandOffsets == null!");
            }
            int numBands = colorSpace.getNumComponents() +
                (hasAlpha ? 1 : 0);
            if (bandOffsets.length != numBands) {
                throw new IllegalArgumentException
                    ("bandOffsets.length is wrong!");
            }

我们发现只有当我们的图像偏移数量和我们的通道数不一致的时候，就会报这个错误！

总结

通过研究这个问题，基本上梳理了Java基于ImageIO-ext读取tiff的过程，基本跟tiff的数据结构对应起来。