OpenGL基础19：法向量与漫反射

一、法向量

法向量：垂直于当前点所在面的向量

无论是漫反射光照还是镜面光照，我们都需要知道当前反射点的法向量

单纯的对于一个顶点来讲，它肯定是没有法向量的，所以我们需要考虑这个顶点与其周围的顶点（他们可以构成唯一的一个面，即顶点所在的面），并可以通过叉乘来获取法向量，当然对于一个不能再简单的正方体，我们也可以直接将每个顶点的法向量数据加入顶点数据中，也就是类似于离线+存储方法：

GLfloat vertices[] = 
{
    -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
     0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
     0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
     0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
    -0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
    -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,

    -0.5f, -0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
     0.5f, -0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
     0.5f,  0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
     0.5f,  0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
    -0.5f,  0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
    -0.5f, -0.5f,  0.5f,  0.0f,  0.0f,  1.0f,

    -0.5f,  0.5f,  0.5f, -1.0f,  0.0f,  0.0f,
    -0.5f,  0.5f, -0.5f, -1.0f,  0.0f,  0.0f,
    -0.5f, -0.5f, -0.5f, -1.0f,  0.0f,  0.0f,
    -0.5f, -0.5f, -0.5f, -1.0f,  0.0f,  0.0f,
    -0.5f, -0.5f,  0.5f, -1.0f,  0.0f,  0.0f,
    -0.5f,  0.5f,  0.5f, -1.0f,  0.0f,  0.0f,

     0.5f,  0.5f,  0.5f,  1.0f,  0.0f,  0.0f,
     0.5f,  0.5f, -0.5f,  1.0f,  0.0f,  0.0f,
     0.5f, -0.5f, -0.5f,  1.0f,  0.0f,  0.0f,
     0.5f, -0.5f, -0.5f,  1.0f,  0.0f,  0.0f,
     0.5f, -0.5f,  0.5f,  1.0f,  0.0f,  0.0f,
     0.5f,  0.5f,  0.5f,  1.0f,  0.0f,  0.0f,

    -0.5f, -0.5f, -0.5f,  0.0f, -1.0f,  0.0f,
     0.5f, -0.5f, -0.5f,  0.0f, -1.0f,  0.0f,
     0.5f, -0.5f,  0.5f,  0.0f, -1.0f,  0.0f,
     0.5f, -0.5f,  0.5f,  0.0f, -1.0f,  0.0f,
    -0.5f, -0.5f,  0.5f,  0.0f, -1.0f,  0.0f,
    -0.5f, -0.5f, -0.5f,  0.0f, -1.0f,  0.0f,

    -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,
     0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,
     0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,
     0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,
    -0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,
    -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f
};
//……
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)0);
glEnableVertexAttribArray(0);
glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)(3 * sizeof(GLfloat)));
glEnableVertexAttribArray(2);
//……

之后将其传入顶点着色器中，顶点着色器当然要修改

不过。。物体的顶点我们进行了模型变换、视角变换和投影变换，那么对应的法向量需要进行这些变换嘛？

视角变换和投影变换肯定不用，我们唯一需要的就是模型变换，而对于模型变换，又有缩放、位移和旋转

要知道，法向量只表示方向，这样的话位移矩阵对于法向量就没有意义，其次法向量只有3维，而上面的变换矩阵都是齐次矩阵，因此，我们需要给法向量加一维w，并将其设置为0，此时位移转换矩阵对于这个法向量正好也是无效的，并且完全不影响缩放和旋转的计算

我看了很多博客说把模型矩阵左上角的3×3矩阵从模型矩阵中移除也可以，这很明显是不对的！~~但就是不知道怎么那么巧要错一起错还错的完全一样~~，这不重要，我好奇去查了下对应的英文原文，确定是移除第4行第4列的位移部分以只选取左上角的3x3，而并非移除左上角3x3，这显然是两个完全不同的意思

除此之外，如果模型矩阵进行了不等比缩放，那么最后得到的法向量就可能不再垂直于物体表面了，如下：

因此，我们还需要找到另一个矩阵 $G$ ，满足法向量 $N$ 与其相乘后与模型变换后的物体表面仍垂直，其中 $G$ 显然可以由模型矩阵 $M$ 转换而成

对应上图，设 $N$ 为法向量， $T$ 为物体表面向量，它们在对应转换后为 $N^{\prime}$ 和 $T^{\prime}$

我们目的就是找到一个 $G$ 满足 $(G N) \cdot(M T) = N^{\prime} \cdot T^{\prime}=0$

因为①点积可以转换为向量积；②矩阵转置的性质，可以得出： $(G N) \cdot(M T)=(G N)^{T}(M T) =N^{T} G^{T} M T$

又由于 $N^{T} T = 0$ 且 $N^{T} G^{T} M T = 0$ ，那么我们就可以推测出： $G^{T} M = E$

完美：得到 $G=\left(M^{-1}\right)^{T}$

这也是为什么如果模型没有进行不等比缩放，只进行了旋转和等比缩放，那么法向量与矩阵 $M$ 相乘也是OK的，因为此时的 $M$ 必然是正交矩阵，即满足 $M^{-1} = M$

问题都解决了，顶点着色器修改如下：

inverse(M)：求出对应的逆矩阵
transpose(M)：求出对应的转置矩阵

#version 330 core
layout (location = 0) in vec3 position;
layout (location = 1) in vec2 texture;
layout (location = 2) in vec3 normal;
out vec2 texIn;
out vec3 normalIn;
uniform mat4 model;             //模型矩阵
uniform mat4 view;              //观察矩阵
uniform mat4 projection;        //投影矩阵
void main()
{
    gl_Position = projection * view * model * vec4(position, 1.0);
    //texIn = vec2(texture.x, 1.0f - texture.y);
    normalIn = mat3(transpose(inverse(model))) * normal;
}

二、漫反射光照

了解了法向量后，实现漫反射光照就简单了很多

目的也很简单，求出下图中的 $\Theta$ 角，这个角度越小则亮度越大

为了算出光的方向，我们需要知道光源的位置，并将其传入片段着色器：

GLint lightPosLoc = glGetUniformLocation(shaderObj.Program, "lightPos");
glUniform3f(lightPosLoc, lightPos.x, lightPos.y, lightPos.z);

然后把片段的位置也传入片段着色器（从顶点着色器中传入）：

//……
out vec3 normalIn;
out vec3 FragPosIn;
void main()
{
    gl_Position = projection * view * model * vec4(position, 1.0);
    FragPosIn = vec3(model * vec4(position, 1.0f));
    normalIn = mat3(transpose(inverse(model))) * normal;
}

这样片段着色器中我们想要的变量就都有了，接下来通过光源位置和片段位置得到光照入射向量

注意到 $\mathbf{a} \bullet b=\|a\| b \| \cos \theta$ ，所以我们若要在后面得到 $cos \theta$ ，就必须确保 $a$ 和 $b$ 都是单位长度

vec3 norm = normalize(normalIn);
vec3 lightDir = normalize(lightPos - FragPosIn);

通过对 norm 和 lightDir 向量进行点乘，求出夹角 $cos \theta$ ，它就代表着当前片段的实际的散射影响，得到的结果值再乘以光的颜色，得到散射因子，两个向量之间的角度越大，散射因子就会越小

float diff = max(dot(norm, lightDir), 0.0);
vec3 diffuse = diff * lightColor;

混合并输出片段的颜色，搞定！

vec3 result = (ambient + diffuse) * objectColor;
color = vec4(result, 1.0f);

如果代码完全正确的话，可以得到这样的结果：

最终代码如下：

光源的着色器、摄像机类和着色器类都在之前的文章中能找到

物体着色器部分：

#version 330 core
layout (location = 0) in vec3 position;
layout (location = 1) in vec2 texture;
layout (location = 2) in vec3 normal;
out vec2 texIn;
out vec3 normalIn;
out vec3 FragPosIn;
uniform mat4 model;             //模型矩阵
uniform mat4 view;              //观察矩阵
uniform mat4 projection;        //投影矩阵
void main()
{
    gl_Position = projection * view * model * vec4(position, 1.0);
    //texIn = vec2(texture.x, 1.0f - texture.y);
    FragPosIn = vec3(model * vec4(position, 1.0f));
    normalIn = mat3(transpose(inverse(model))) * normal;
}

////////////////////////////////////////////////////////////////////////

#version 330 core
out vec4 color;
uniform vec3 lightPos;
uniform vec3 objectColor;
uniform vec3 lightColor;
in vec3 FragPosIn;
in vec2 texIn;
in vec3 normalIn;
uniform sampler2D texOutA;
void main()
{
    float ambientStrength = 0.1f;
    vec3 ambient = ambientStrength * lightColor;
    vec3 norm = normalize(normalIn);
    vec3 lightDir = normalize(lightPos - FragPosIn);
    float diff = max(dot(norm, lightDir), 0.0f);
    vec3 diffuse = diff * lightColor;
    vec3 result = (ambient + diffuse) * objectColor;
    color = vec4(result, 1.0f);
}

main.cpp：

#include<iostream>
#include<opengl/glew.h>
#define GLEW_STATIC
#include<GLFW/glfw3.h>
#include"Camera.h"
#include<glm/glm.hpp>
#include<glm/gtc/matrix_transform.hpp>
#include<glm/gtc/type_ptr.hpp>
#include"Shader.h"
#include<opengl/freeglut.h>
#include<SOIL.h>

bool keys[1024];
Camera camera;
GLfloat lastX, lastY;
bool firstMouse = true;
void key_callback(GLFWwindow* window, int key, int scancode, int action, int mode);
void scroll_callback(GLFWwindow* window, double xoffset, double yoffset);
void mouse_callback(GLFWwindow* window, double xpos, double ypos);
void cameraMove();
glm::vec3 lightPos(1.2f, 1.0f, 2.0f);
const GLuint WIDTH = 800, HEIGHT = 600;

int main()
{
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
    glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);

    GLFWwindow* window = glfwCreateWindow(WIDTH, HEIGHT, "LearnOpenGL", nullptr, nullptr);
    glfwMakeContextCurrent(window);
    glfwSetKeyCallback(window, key_callback);
    glfwSetCursorPosCallback(window, mouse_callback);
    glfwSetScrollCallback(window, scroll_callback);

    glfwSetInputMode(window, GLFW_CURSOR, GLFW_CURSOR_DISABLED);
    glewExperimental = GL_TRUE;
    glewInit();

    int width, height;
    glfwGetFramebufferSize(window, &width, &height);
    glViewport(0, 0, width, height);

    Shader shaderObj("ObjVShader.txt", "ObjFShader.txt");
    Shader shaderLight("LightVShader.txt", "LightFShader.txt");

    GLfloat vertices[] = 
    {
        -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
         0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
         0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
         0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
        -0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,
        -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,

        -0.5f, -0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
         0.5f, -0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
         0.5f,  0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
         0.5f,  0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
        -0.5f,  0.5f,  0.5f,  0.0f,  0.0f,  1.0f,
        -0.5f, -0.5f,  0.5f,  0.0f,  0.0f,  1.0f,

        -0.5f,  0.5f,  0.5f, -1.0f,  0.0f,  0.0f,
        -0.5f,  0.5f, -0.5f, -1.0f,  0.0f,  0.0f,
        -0.5f, -0.5f, -0.5f, -1.0f,  0.0f,  0.0f,
        -0.5f, -0.5f, -0.5f, -1.0f,  0.0f,  0.0f,
        -0.5f, -0.5f,  0.5f, -1.0f,  0.0f,  0.0f,
        -0.5f,  0.5f,  0.5f, -1.0f,  0.0f,  0.0f,

         0.5f,  0.5f,  0.5f,  1.0f,  0.0f,  0.0f,
         0.5f,  0.5f, -0.5f,  1.0f,  0.0f,  0.0f,
         0.5f, -0.5f, -0.5f,  1.0f,  0.0f,  0.0f,
         0.5f, -0.5f, -0.5f,  1.0f,  0.0f,  0.0f,
         0.5f, -0.5f,  0.5f,  1.0f,  0.0f,  0.0f,
         0.5f,  0.5f,  0.5f,  1.0f,  0.0f,  0.0f,

        -0.5f, -0.5f, -0.5f,  0.0f, -1.0f,  0.0f,
         0.5f, -0.5f, -0.5f,  0.0f, -1.0f,  0.0f,
         0.5f, -0.5f,  0.5f,  0.0f, -1.0f,  0.0f,
         0.5f, -0.5f,  0.5f,  0.0f, -1.0f,  0.0f,
        -0.5f, -0.5f,  0.5f,  0.0f, -1.0f,  0.0f,
        -0.5f, -0.5f, -0.5f,  0.0f, -1.0f,  0.0f,

        -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,
         0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,
         0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,
         0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,
        -0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,
        -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f
    };
    GLuint VBO, VAO, texture;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenTextures(1, &texture);

    glBindVertexArray(VAO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBindTexture(GL_TEXTURE_2D, texture);

    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)0);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)(3 * sizeof(GLfloat)));
    glEnableVertexAttribArray(2);
    /*glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 5 * sizeof(GLfloat), (GLvoid*)(3 * sizeof(GLfloat)));
    glEnableVertexAttribArray(1);

    int picWidth, picHeight;
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    unsigned char* image = SOIL_load_image("Texture/wood.jpg", &picWidth, &picHeight, 0, SOIL_LOAD_RGB);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, picWidth, picHeight, 0, GL_RGB, GL_UNSIGNED_BYTE, image);
    glGenerateMipmap(GL_TEXTURE_2D);
    SOIL_free_image_data(image);
    glBindTexture(GL_TEXTURE_2D, 0);*/

    GLuint lightVAO;
    glGenVertexArrays(1, &lightVAO);
    glBindVertexArray(lightVAO);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)0);
    //VBO数据已经绑定且我们就用之前的顶点数据，所以无需再管理VBO
    glEnableVertexAttribArray(0);

    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);

    glEnable(GL_DEPTH_TEST);
    while (!glfwWindowShouldClose(window))
    {
        glfwPollEvents();
        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);
        glClear(GL_DEPTH_BUFFER_BIT);
        cameraMove();

        //glBindTexture(GL_TEXTURE_2D, texture);
        shaderObj.Use();
        GLint objectColorLoc = glGetUniformLocation(shaderObj.Program, "objectColor");
        GLint lightColorLoc = glGetUniformLocation(shaderObj.Program, "lightColor");
        GLint lightPosLoc = glGetUniformLocation(shaderObj.Program, "lightPos");
        glUniform3f(lightPosLoc, lightPos.x, lightPos.y, lightPos.z);
        glUniform3f(objectColorLoc, 1.0f, 0.5f, 0.31f);     //物体反射颜色
        glUniform3f(lightColorLoc, 1.0f, 1.0f, 1.0f);       //光源：默认为白色
        glm::mat4 model = glm::mat4(1.0f);
        glm::mat4 view = glm::mat4(1.0f);
        glm::mat4 projection = glm::mat4(1.0f);
        model = glm::rotate(model, glm::radians(57.0f), glm::vec3(-0.5f, 1.0f, 0.0f));
        view = camera.GetViewMatrix();
        projection = glm::perspective(glm::radians(camera.Zoom), (GLfloat)WIDTH / (GLfloat)HEIGHT, 0.1f, 100.0f);
        GLint modelLoc = glGetUniformLocation(shaderObj.Program, "model");
        GLint viewLoc = glGetUniformLocation(shaderObj.Program, "view");
        GLint projLoc = glGetUniformLocation(shaderObj.Program, "projection");
        glUniformMatrix4fv(modelLoc, 1, GL_FALSE, glm::value_ptr(model));
        glUniformMatrix4fv(viewLoc, 1, GL_FALSE, glm::value_ptr(view));
        glUniformMatrix4fv(projLoc, 1, GL_FALSE, glm::value_ptr(projection));

        glBindVertexArray(VAO);
        glDrawArrays(GL_TRIANGLES, 0, 36);

        //glBindTexture(GL_TEXTURE_2D, 0);
        shaderLight.Use();
        model = glm::translate(glm::mat4(1.0f), lightPos);
        model = glm::scale(model, glm::vec3(0.2f));
        modelLoc = glGetUniformLocation(shaderLight.Program, "model");
        viewLoc = glGetUniformLocation(shaderLight.Program, "view");
        projLoc = glGetUniformLocation(shaderLight.Program, "projection");
        glUniformMatrix4fv(modelLoc, 1, GL_FALSE, glm::value_ptr(model));
        glUniformMatrix4fv(viewLoc, 1, GL_FALSE, glm::value_ptr(view));
        glUniformMatrix4fv(projLoc, 1, GL_FALSE, glm::value_ptr(projection));

        glBindVertexArray(lightVAO);
        glDrawArrays(GL_TRIANGLES, 0, 36);

        glBindVertexArray(0);
        glfwSwapBuffers(window);
    }
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    glfwTerminate();
    return 0;
}


GLfloat deltaTime = 0.0f;
GLfloat lastFrame = 0.0f;
void cameraMove()
{
    GLfloat currentFrame = glfwGetTime();
    deltaTime = currentFrame - lastFrame;
    lastFrame = currentFrame;

    GLfloat cameraSpeed = 1.0f * deltaTime;
    if (keys[GLFW_KEY_W])
        camera.ProcessKeyboard(Camera_Movement(FORWARD), deltaTime);
    if (keys[GLFW_KEY_S])
        camera.ProcessKeyboard(Camera_Movement(BACKWARD), deltaTime);
    if (keys[GLFW_KEY_A])
        camera.ProcessKeyboard(Camera_Movement(LEFT), deltaTime);
    if (keys[GLFW_KEY_D])
        camera.ProcessKeyboard(Camera_Movement(RIGHT), deltaTime);
}

void key_callback(GLFWwindow* window, int key, int scancode, int action, int mode)
{
    if (key == GLFW_KEY_ESCAPE && action == GLFW_PRESS)
        glfwSetWindowShouldClose(window, GL_TRUE);
    if (action == GLFW_PRESS)           //如果当前是按下操作
        keys[key] = true;
    else if (action == GLFW_RELEASE)            //松开键盘
        keys[key] = false;
}

void scroll_callback(GLFWwindow* window, double xoffset, double yoffset)
{
    camera.ProcessMouseScroll(yoffset);
}

void mouse_callback(GLFWwindow* window, double xpos, double ypos)
{
    if (firstMouse)
    {
        lastX = xpos;
        lastY = ypos;
        firstMouse = false;
    }
    GLfloat xoffset = xpos - lastX;
    GLfloat yoffset = lastY - ypos;
    lastX = xpos;
    lastY = ypos;
    
    GLfloat sensitivity = 0.05;
    xoffset *= sensitivity;
    yoffset *= sensitivity;
    
    camera.ProcessMouseMovement(xoffset, yoffset);
}