OpenGL之光照贴图

 我们需要拓展之前的系统，引入漫反射和镜面光贴图(Map)。这允许我们对物体的漫反射分量和镜面光分量有着更精确的控制。

漫反射贴图

我们希望通过某种方式对物体的每个片段单独设置漫反射颜色。我们仅仅是对同样的原理使用了不同的名字：其实都是使用一张覆盖物体的图像，让我们能够逐片段索引其独立的颜色值。在光照场景中，它通常叫做一个漫反射贴图(Diffuse Map)（3D艺术家通常都这么叫它），它是一个表现了物体所有的漫反射颜色的纹理图像。

在着色器中使用漫反射贴图的方法和纹理教程中是完全一样的。但这次我们会将纹理储存为Material结构体中的一个sampler2D。我们将之前定义的vec3漫反射颜色向量替换为漫反射贴图。 

我们也移除了环境光材质颜色向量，因为环境光颜色在几乎所有情况下都等于漫反射颜色，所以我们不需要将它们分开储存：

struct Material {
    sampler2D diffuse;
    vec3      specular;
    float     shininess;
}; 
...
in vec2 TexCoords;

 注意我们将在片段着色器中再次需要纹理坐标，所以我们声明一个额外的输入变量。接下来我们只需要从纹理中采样片段的漫反射颜色值即可：

vec3 diffuse = light.diffuse * diff * vec3(texture(material.diffuse, TexCoords));

不要忘记将环境光的材质颜色设置为漫反射材质颜色同样的值。

vec3 ambient = light.ambient * vec3(texture(material.diffuse, TexCoords));

就是使用漫反射贴图的全部步骤了。你可以看到，这并不是什么新的东西，但这能够极大地提高视觉品质。为了让它正常工作，我们还需要使用纹理坐标更新顶点数据，将它们作为顶点属性传递到片段着色器，加载材质并绑定材质到合适的纹理单元。

更新后的顶点数据可以在这里找到。顶点数据现在包含了顶点位置、法向量和立方体顶点处的纹理坐标。让我们更新顶点着色器来以顶点属性的形式接受纹理坐标，并将它们传递到片段着色器中：

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aNormal;
layout (location = 2) in vec2 aTexCoords;
...
out vec2 TexCoords;
 
void main()
{
    ...
    TexCoords = aTexCoords;
}

记得去更新两个VAO的顶点属性指针来匹配新的顶点数据，并加载箱子图像为一个纹理。在绘制箱子之前，我们希望将要用的纹理单元赋值到material.diffuse这个uniform采样器，并绑定箱子的纹理到这个纹理单元：

lightingShader.setInt("material.diffuse", 0);
...
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, diffuseMap);

使用了漫反射贴图之后，细节再一次得到惊人的提升，这次箱子有了光照开始闪闪发光（字面意思也是）了。箱子看起来可能像这样：

4.1.lighting_maps.vs

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aNormal;
layout (location = 2) in vec2 aTexCoords;
 
out vec3 FragPos;
out vec3 Normal;
out vec2 TexCoords;
 
uniform mat4 model;
uniform mat4 view;
uniform mat4 projection;
 
void main()
{
    FragPos = vec3(model * vec4(aPos, 1.0));
    Normal = mat3(transpose(inverse(model))) * aNormal;  
    TexCoords = aTexCoords;
    
    gl_Position = projection * view * vec4(FragPos, 1.0);
}

4.1.lighting_maps.fs

#version 330 core
out vec4 FragColor;
 
struct Material {
    sampler2D diffuse;
    vec3 specular;    
    float shininess;
}; 
 
struct Light {
    vec3 position;
 
    vec3 ambient;
    vec3 diffuse;
    vec3 specular;
};
 
in vec3 FragPos;  
in vec3 Normal;  
in vec2 TexCoords;
  
uniform vec3 viewPos;
uniform Material material;
uniform Light light;
 
void main()
{
    // ambient
    vec3 ambient = light.ambient * texture(material.diffuse, TexCoords).rgb;
  	
    // diffuse 
    vec3 norm = normalize(Normal);
    vec3 lightDir = normalize(light.position - FragPos);
    float diff = max(dot(norm, lightDir), 0.0);
    vec3 diffuse = light.diffuse * diff * texture(material.diffuse, TexCoords).rgb;  
    
    // specular
    vec3 viewDir = normalize(viewPos - FragPos);
    vec3 reflectDir = reflect(-lightDir, norm);  
    float spec = pow(max(dot(viewDir, reflectDir), 0.0), material.shininess);
    vec3 specular = light.specular * (spec * material.specular);  
        
    vec3 result = ambient + diffuse + specular;
    FragColor = vec4(result, 1.0);
} 

4.1.light_cube.vs

#version 330 core
layout (location = 0) in vec3 aPos;
 
uniform mat4 model;
uniform mat4 view;
uniform mat4 projection;
 
void main()
{
    gl_Position = projection * view * model * vec4(aPos, 1.0);
}

4.1.light_cube.fs 

#version 330 core
out vec4 FragColor;
 
void main()
{
    FragColor = vec4(1.0); // set all 4 vector values to 1.0
}

#include 
#include 
#include 
 
#include 
#include 
#include 
 
#include 
#include 
 
#include 
 
void framebuffer_size_callback(GLFWwindow* window, int width, int height);
void mouse_callback(GLFWwindow* window, double xpos, double ypos);
void scroll_callback(GLFWwindow* window, double xoffset, double yoffset);
void processInput(GLFWwindow *window);
unsigned int loadTexture(const char *path);
 
// settings
const unsigned int SCR_WIDTH = 800;
const unsigned int SCR_HEIGHT = 600;
 
// camera
Camera camera(glm::vec3(0.0f, 0.0f, 3.0f));
float lastX = SCR_WIDTH / 2.0f;
float lastY = SCR_HEIGHT / 2.0f;
bool firstMouse = true;
 
// timing
float deltaTime = 0.0f;
float lastFrame = 0.0f;
 
// lighting
glm::vec3 lightPos(1.2f, 1.0f, 2.0f);
 
int main()
{
    // glfw: initialize and configure
    // ------------------------------
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
 
#ifdef __APPLE__
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
#endif
 
    // glfw window creation
    // --------------------
    GLFWwindow* window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "LearnOpenGL", NULL, NULL);
    if (window == NULL)
    {
        std::cout << "Failed to create GLFW window" << std::endl;
        glfwTerminate();
        return -1;
    }
    glfwMakeContextCurrent(window);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);
    glfwSetCursorPosCallback(window, mouse_callback);
    glfwSetScrollCallback(window, scroll_callback);
 
    // tell GLFW to capture our mouse
    glfwSetInputMode(window, GLFW_CURSOR, GLFW_CURSOR_DISABLED);
 
    // glad: load all OpenGL function pointers
    // ---------------------------------------
    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        std::cout << "Failed to initialize GLAD" << std::endl;
        return -1;
    }
 
    // configure global opengl state
    // -----------------------------
    glEnable(GL_DEPTH_TEST);
 
    // build and compile our shader zprogram
    // ------------------------------------
    Shader lightingShader("4.1.lighting_maps.vs", "4.1.lighting_maps.fs");
    Shader lightCubeShader("4.1.light_cube.vs", "4.1.light_cube.fs");
 
    // set up vertex data (and buffer(s)) and configure vertex attributes
    // --------------------------------------------------