OpenGL 图形库的使用(十四)—— 光照之光照贴图

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版本记录

版本号 时间
V1.0 2017.12.25

前言

OpenGL ES图形库项目中一直也没用过,最近也想学着使用这个图形库,感觉还是很有意思,也就自然想着好好的总结一下,希望对大家能有所帮助。
1. OpenGL 图形库使用(一) —— 概念基础
2. OpenGL 图形库使用(二) —— 渲染模式、对象、扩展和状态机
3. OpenGL 图形库使用(三) —— 着色器、数据类型与输入输出
4. OpenGL 图形库使用(四) —— Uniform及更多属性
5. OpenGL 图形库使用(五) —— 纹理
6. OpenGL 图形库使用(六) —— 变换
7. OpenGL 图形库的使用(七)—— 坐标系统之五种不同的坐标系统(一)
8. OpenGL 图形库的使用(八)—— 坐标系统之3D效果(二)
9. OpenGL 图形库的使用(九)—— 摄像机(一)
10. OpenGL 图形库的使用(十)—— 摄像机(二)
11. OpenGL 图形库的使用(十一)—— 光照之颜色
12. OpenGL 图形库的使用(十二)—— 光照之基础光照
13. OpenGL 图形库的使用(十三)—— 光照之材质

光照贴图

在上一节中,我们讨论了让每个物体都拥有自己独特的材质从而对光照做出不同的反应的方法。这样子能够很容易在一个光照的场景中给每个物体一个独特的外观,但是这仍不能对一个物体的视觉输出提供足够多的灵活性。

在上一节中,我们将整个物体的材质定义为一个整体,但现实世界中的物体通常并不只包含有一种材质,而是由多种材质所组成。想想一辆汽车:它的外壳非常有光泽,车窗会部分反射周围的环境,轮胎不会那么有光泽,所以它没有镜面高光,轮毂非常闪亮(如果你洗车了的话)。汽车同样会有漫反射和环境光颜色,它们在整个物体上也不会是一样的,汽车有着许多种不同的环境光/漫反射颜色。总之,这样的物体在不同的部件上都有不同的材质属性。

所以,上一节中的那个材质系统是肯定不够的,它只是一个最简单的模型,所以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(以及间接地对环境光分量,它们几乎总是一样的)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。有能够让我们根据片段在物体上的位置来获取颜色值得系统吗?

这可能听起来很熟悉,而且事实上这个系统我们已经使用很长时间了。这听起来很像在之前教程中详细讨论过的纹理,而这基本就是这样:一个纹理。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。

为了演示漫反射贴图,我们将会使用下面的图片,它是一个有钢边框的木箱:

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。

注意sampler2D是所谓的不透明类型(Opaque Type),也就是说我们不能将它实例化,只能通过uniform来定义它。如果我们使用除uniform以外的方法(比如函数的参数)实例化这个结构体,GLSL会抛出一些奇怪的错误。这同样也适用于任何封装了不透明类型的结构体。

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;

如果你非常固执,仍想将环境光颜色设置为一个(漫反射值之外)不同的值,你也可以保留这个环境光的vec3,但整个物体仍只能拥有一个环境光颜色。如果想要对不同片段有不同的环境光值,你需要对环境光值单独使用另外一个纹理。

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));

不要忘记将环境光得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));

这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,也可以直接看下面。

float vertices[] = {
    // positions          // normals           // texture coords
    -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  0.0f, 0.0f,
     0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  1.0f, 0.0f,
     0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  1.0f, 1.0f,
     0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  1.0f, 1.0f,
    -0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  0.0f, 1.0f,
    -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  0.0f, 0.0f,

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

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

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

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

    -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,  0.0f, 1.0f,
     0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,  1.0f, 1.0f,
     0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,  1.0f, 0.0f,
     0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,  1.0f, 0.0f,
    -0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,  0.0f, 0.0f,
    -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,  0.0f, 1.0f
};

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。你的箱子看起来可能像这样:

你可以在这里找到原码,或者直接看下面。

#include <glad/glad.h>
#include <GLFW/glfw3.h>
#include <stb_image.h>

#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <glm/gtc/type_ptr.hpp>

#include <learnopengl/shader_m.h>
#include <learnopengl/camera.h>

#include <iostream>

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); // uncomment this statement to fix compilation on OS X
#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 lampShader("4.1.lamp.vs", "4.1.lamp.fs");

    // set up vertex data (and buffer(s)) and configure vertex attributes
    // ------------------------------------------------------------------
    float vertices[] = {
        // positions          // normals           // texture coords
        -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  0.0f,  0.0f,
         0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  1.0f,  0.0f,
         0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  1.0f,  1.0f,
         0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  1.0f,  1.0f,
        -0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  0.0f,  1.0f,
        -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  0.0f,  0.0f,

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

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

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

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

        -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,  0.0f,  1.0f,
         0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,  1.0f,  1.0f,
         0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,  1.0f,  0.0f,
         0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,  1.0f,  0.0f,
        -0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,  0.0f,  0.0f,
        -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,  0.0f,  1.0f
    };
    // first, configure the cube's VAO (and VBO)
    unsigned int VBO, cubeVAO;
    glGenVertexArrays(1, &cubeVAO);
    glGenBuffers(1, &VBO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glBindVertexArray(cubeVAO);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
    glEnableVertexAttribArray(2);

    // second, configure the light's VAO (VBO stays the same; the vertices are the same for the light object which is also a 3D cube)
    unsigned int lightVAO;
    glGenVertexArrays(1, &lightVAO);
    glBindVertexArray(lightVAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    // note that we update the lamp's position attribute's stride to reflect the updated buffer data
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    // load textures (we now use a utility function to keep the code more organized)
    // -----------------------------------------------------------------------------
    unsigned int diffuseMap = loadTexture(FileSystem::getPath("resources/textures/container2.png").c_str());

    // shader configuration
    // --------------------
    lightingShader.use(); 
    lightingShader.setInt("material.diffuse", 0);


    // render loop
    // -----------
    while (!glfwWindowShouldClose(window))
    {
        // per-frame time logic
        // --------------------
        float currentFrame = glfwGetTime();
        deltaTime = currentFrame - lastFrame;
        lastFrame = currentFrame;

        // input
        // -----
        processInput(window);

        // render
        // ------
        glClearColor(0.1f, 0.1f, 0.1f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        // be sure to activate shader when setting uniforms/drawing objects
        lightingShader.use();
        lightingShader.setVec3("light.position", lightPos);
        lightingShader.setVec3("viewPos", camera.Position);

        // light properties
        lightingShader.setVec3("light.ambient", 0.2f, 0.2f, 0.2f); 
        lightingShader.setVec3("light.diffuse", 0.5f, 0.5f, 0.5f);
        lightingShader.setVec3("light.specular", 1.0f, 1.0f, 1.0f);

        // material properties
        lightingShader.setVec3("material.specular", 0.5f, 0.5f, 0.5f);
        lightingShader.setFloat("material.shininess", 64.0f);

        // view/projection transformations
        glm::mat4 projection = glm::perspective(glm::radians(camera.Zoom), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f);
        glm::mat4 view = camera.GetViewMatrix();
        lightingShader.setMat4("projection", projection);
        lightingShader.setMat4("view", view);

        // world transformation
        glm::mat4 model;
        lightingShader.setMat4("model", model);

        // bind diffuse map
        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, diffuseMap);

        // render the cube
        glBindVertexArray(cubeVAO);
        glDrawArrays(GL_TRIANGLES, 0, 36);


        // also draw the lamp object
        lampShader.use();
        lampShader.setMat4("projection", projection);
        lampShader.setMat4("view", view);
        model = glm::mat4();
        model = glm::translate(model, lightPos);
        model = glm::scale(model, glm::vec3(0.2f)); // a smaller cube
        lampShader.setMat4("model", model);

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


        // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.)
        // -------------------------------------------------------------------------------
        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    // optional: de-allocate all resources once they've outlived their purpose:
    // ------------------------------------------------------------------------
    glDeleteVertexArrays(1, &cubeVAO);
    glDeleteVertexArrays(1, &lightVAO);
    glDeleteBuffers(1, &VBO);

    // glfw: terminate, clearing all previously allocated GLFW resources.
    // ------------------------------------------------------------------
    glfwTerminate();
    return 0;
}

// process all input: query GLFW whether relevant keys are pressed/released this frame and react accordingly
// ---------------------------------------------------------------------------------------------------------
void processInput(GLFWwindow *window)
{
    if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
        glfwSetWindowShouldClose(window, true);

    if (glfwGetKey(window, GLFW_KEY_W) == GLFW_PRESS)
        camera.ProcessKeyboard(FORWARD, deltaTime);
    if (glfwGetKey(window, GLFW_KEY_S) == GLFW_PRESS)
        camera.ProcessKeyboard(BACKWARD, deltaTime);
    if (glfwGetKey(window, GLFW_KEY_A) == GLFW_PRESS)
        camera.ProcessKeyboard(LEFT, deltaTime);
    if (glfwGetKey(window, GLFW_KEY_D) == GLFW_PRESS)
        camera.ProcessKeyboard(RIGHT, deltaTime);
}

// glfw: whenever the window size changed (by OS or user resize) this callback function executes
// ---------------------------------------------------------------------------------------------
void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
    // make sure the viewport matches the new window dimensions; note that width and 
    // height will be significantly larger than specified on retina displays.
    glViewport(0, 0, width, height);
}


// glfw: whenever the mouse moves, this callback is called
// -------------------------------------------------------
void mouse_callback(GLFWwindow* window, double xpos, double ypos)
{
    if (firstMouse)
    {
        lastX = xpos;
        lastY = ypos;
        firstMouse = false;
    }

    float xoffset = xpos - lastX;
    float yoffset = lastY - ypos; // reversed since y-coordinates go from bottom to top

    lastX = xpos;
    lastY = ypos;

    camera.ProcessMouseMovement(xoffset, yoffset);
}

// glfw: whenever the mouse scroll wheel scrolls, this callback is called
// ----------------------------------------------------------------------
void scroll_callback(GLFWwindow* window, double xoffset, double yoffset)
{
    camera.ProcessMouseScroll(yoffset);
}

// utility function for loading a 2D texture from file
// ---------------------------------------------------
unsigned int loadTexture(char const * path)
{
    unsigned int textureID;
    glGenTextures(1, &textureID);
    
    int width, height, nrComponents;
    unsigned char *data = stbi_load(path, &width, &height, &nrComponents, 0);
    if (data)
    {
        GLenum format;
        if (nrComponents == 1)
            format = GL_RED;
        else if (nrComponents == 3)
            format = GL_RGB;
        else if (nrComponents == 4)
            format = GL_RGBA;

        glBindTexture(GL_TEXTURE_2D, textureID);
        glTexImage2D(GL_TEXTURE_2D, 0, format, width, height, 0, format, GL_UNSIGNED_BYTE, data);
        glGenerateMipmap(GL_TEXTURE_2D);

        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_MIPMAP_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

        stbi_image_free(data);
    }
    else
    {
        std::cout << "Texture failed to load at path: " << path << std::endl;
        stbi_image_free(data);
    }

    return textureID;
}

镜面光贴图

你可能会注意到,镜面高光看起来有些奇怪,因为我们的物体大部分都是木头,我们知道木头不应该有这么强的镜面高光的。我们可以将物体的镜面光材质设置为vec3(0.0)来解决这个问题,但这也意味着箱子钢制的边框将不再能够显示镜面高光了,我们知道钢铁应该是有一些镜面高光的。所以,我们想要让物体的某些部分以不同的强度显示镜面高光。这个问题看起来和漫反射贴图非常相似。是巧合吗?我想不是。

我们同样可以使用一个专门用于镜面高光的纹理贴图。这也就意味着我们需要生成一个黑白的(如果你想得话也可以是彩色的)纹理,来定义物体每部分的镜面光强度。下面是一个镜面光贴图(Specular Map)的例子:

镜面高光的强度可以通过图像每个像素的亮度来获取。镜面光贴图上的每个像素都可以由一个颜色向量来表示,比如说黑色代表颜色向量vec3(0.0),灰色代表颜色向量vec3(0.5)。在片段着色器中,我们接下来会取样对应的颜色值并将它乘以光源的镜面强度。一个像素越「白」,乘积就会越大,物体的镜面光分量就会越亮。

由于箱子大部分都由木头所组成,而且木头材质应该没有镜面高光,所以漫反射纹理的整个木头部分全部都转换成了黑色。箱子钢制边框的镜面光强度是有细微变化的,钢铁本身会比较容易受到镜面高光的影响,而裂缝则不会。

从实际角度来说,木头其实也有镜面高光,尽管它的反光度(Shininess)很小(更多的光被散射),影响也比较小,但是为了教学目的,我们可以假设木头不会对镜面光有任何反应。

使用Photoshop或Gimp之类的工具,将漫反射纹理转换为镜面光纹理还是比较容易的,只需要剪切掉一些部分,将图像转换为黑白的,并增加亮度/对比度就好了。


采样镜面光贴图

镜面光贴图和其它的纹理非常类似,所以代码也和漫反射贴图的代码很类似。记得要保证正确地加载图像并生成一个纹理对象。由于我们正在同一个片段着色器中使用另一个纹理采样器,我们必须要对镜面光贴图使用一个不同的纹理单元(见纹理),所以我们在渲染之前先把它绑定到合适的纹理单元上:

lightingShader.setInt("material.specular", 1);
...
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, specularMap);

接下来更新片段着色器的材质属性,让其接受一个sampler2D而不是vec3作为镜面光分量:

struct Material {
    sampler2D diffuse;
    sampler2D specular;
    float     shininess;
};

最后我们希望采样镜面光贴图,来获取片段所对应的镜面光强度:

vec3 ambient  = light.ambient  * vec3(texture(material.diffuse, TexCoords));
vec3 diffuse  = light.diffuse  * diff * vec3(texture(material.diffuse, TexCoords));  
vec3 specular = light.specular * spec * vec3(texture(material.specular, TexCoords));
FragColor = vec4(ambient + diffuse + specular, 1.0);

通过使用镜面光贴图我们可以可以对物体设置大量的细节,比如物体的哪些部分需要有闪闪发光的属性,我们甚至可以设置它们对应的强度。镜面光贴图能够在漫反射贴图之上给予我们更高一层的控制。

如果你想另辟蹊径,你也可以在镜面光贴图中使用真正的颜色,不仅设置每个片段的镜面光强度,还设置了镜面高光的颜色。从现实角度来说,镜面高光的颜色大部分(甚至全部)都是由光源本身所决定的,所以这样并不能生成非常真实的视觉效果(这也是为什么图像通常是黑白的,我们只关心强度)。

如果你现在运行程序的话,你可以清楚地看到箱子的材质现在和真实的钢制边框箱子非常类似了:

你可以在这里找到程序的全部源码。

#include <glad/glad.h>
#include <GLFW/glfw3.h>
#include <stb_image.h>

#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <glm/gtc/type_ptr.hpp>

#include <learnopengl/shader_m.h>
#include <learnopengl/camera.h>

#include <iostream>

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); // uncomment this statement to fix compilation on OS X
#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("4.2.lighting_maps.vs", "4.2.lighting_maps.fs");
    Shader lampShader("4.2.lamp.vs", "4.2.lamp.fs");

    // set up vertex data (and buffer(s)) and configure vertex attributes
    // ------------------------------------------------------------------
    float vertices[] = {
        // positions          // normals           // texture coords
        -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  0.0f,  0.0f,
         0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  1.0f,  0.0f,
         0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  1.0f,  1.0f,
         0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  1.0f,  1.0f,
        -0.5f,  0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  0.0f,  1.0f,
        -0.5f, -0.5f, -0.5f,  0.0f,  0.0f, -1.0f,  0.0f,  0.0f,

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

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

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

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

        -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,  0.0f,  1.0f,
         0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,  1.0f,  1.0f,
         0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,  1.0f,  0.0f,
         0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,  1.0f,  0.0f,
        -0.5f,  0.5f,  0.5f,  0.0f,  1.0f,  0.0f,  0.0f,  0.0f,
        -0.5f,  0.5f, -0.5f,  0.0f,  1.0f,  0.0f,  0.0f,  1.0f
    };
    // first, configure the cube's VAO (and VBO)
    unsigned int VBO, cubeVAO;
    glGenVertexArrays(1, &cubeVAO);
    glGenBuffers(1, &VBO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glBindVertexArray(cubeVAO);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
    glEnableVertexAttribArray(2);

    // second, configure the light's VAO (VBO stays the same; the vertices are the same for the light object which is also a 3D cube)
    unsigned int lightVAO;
    glGenVertexArrays(1, &lightVAO);
    glBindVertexArray(lightVAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    // note that we update the lamp's position attribute's stride to reflect the updated buffer data
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    // load textures (we now use a utility function to keep the code more organized)
    // -----------------------------------------------------------------------------
    unsigned int diffuseMap = loadTexture(FileSystem::getPath("resources/textures/container2.png").c_str());
    unsigned int specularMap = loadTexture(FileSystem::getPath("resources/textures/container2_specular.png").c_str());

    // shader configuration
    // --------------------
    lightingShader.use();
    lightingShader.setInt("material.diffuse", 0);
    lightingShader.setInt("material.specular", 1);


    // render loop
    // -----------
    while (!glfwWindowShouldClose(window))
    {
        // per-frame time logic
        // --------------------
        float currentFrame = glfwGetTime();
        deltaTime = currentFrame - lastFrame;
        lastFrame = currentFrame;

        // input
        // -----
        processInput(window);

        // render
        // ------
        glClearColor(0.1f, 0.1f, 0.1f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        // be sure to activate shader when setting uniforms/drawing objects
        lightingShader.use();
        lightingShader.setVec3("light.position", lightPos);
        lightingShader.setVec3("viewPos", camera.Position);

        // light properties
        lightingShader.setVec3("light.ambient", 0.2f, 0.2f, 0.2f);
        lightingShader.setVec3("light.diffuse", 0.5f, 0.5f, 0.5f);
        lightingShader.setVec3("light.specular", 1.0f, 1.0f, 1.0f);

        // material properties
        lightingShader.setFloat("material.shininess", 64.0f);

        // view/projection transformations
        glm::mat4 projection = glm::perspective(glm::radians(camera.Zoom), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f);
        glm::mat4 view = camera.GetViewMatrix();
        lightingShader.setMat4("projection", projection);
        lightingShader.setMat4("view", view);

        // world transformation
        glm::mat4 model;
        lightingShader.setMat4("model", model);

        // bind diffuse map
        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, diffuseMap);
        // bind specular map
        glActiveTexture(GL_TEXTURE1);
        glBindTexture(GL_TEXTURE_2D, specularMap);

        // render the cube
        glBindVertexArray(cubeVAO);
        glDrawArrays(GL_TRIANGLES, 0, 36);


        // also draw the lamp object
        lampShader.use();
        lampShader.setMat4("projection", projection);
        lampShader.setMat4("view", view);
        model = glm::mat4();
        model = glm::translate(model, lightPos);
        model = glm::scale(model, glm::vec3(0.2f)); // a smaller cube
        lampShader.setMat4("model", model);

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


        // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.)
        // -------------------------------------------------------------------------------
        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    // optional: de-allocate all resources once they've outlived their purpose:
    // ------------------------------------------------------------------------
    glDeleteVertexArrays(1, &cubeVAO);
    glDeleteVertexArrays(1, &lightVAO);
    glDeleteBuffers(1, &VBO);

    // glfw: terminate, clearing all previously allocated GLFW resources.
    // ------------------------------------------------------------------
    glfwTerminate();
    return 0;
}

// process all input: query GLFW whether relevant keys are pressed/released this frame and react accordingly
// ---------------------------------------------------------------------------------------------------------
void processInput(GLFWwindow *window)
{
    if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
        glfwSetWindowShouldClose(window, true);

    if (glfwGetKey(window, GLFW_KEY_W) == GLFW_PRESS)
        camera.ProcessKeyboard(FORWARD, deltaTime);
    if (glfwGetKey(window, GLFW_KEY_S) == GLFW_PRESS)
        camera.ProcessKeyboard(BACKWARD, deltaTime);
    if (glfwGetKey(window, GLFW_KEY_A) == GLFW_PRESS)
        camera.ProcessKeyboard(LEFT, deltaTime);
    if (glfwGetKey(window, GLFW_KEY_D) == GLFW_PRESS)
        camera.ProcessKeyboard(RIGHT, deltaTime);
}

// glfw: whenever the window size changed (by OS or user resize) this callback function executes
// ---------------------------------------------------------------------------------------------
void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
    // make sure the viewport matches the new window dimensions; note that width and 
    // height will be significantly larger than specified on retina displays.
    glViewport(0, 0, width, height);
}

// glfw: whenever the mouse moves, this callback is called
// -------------------------------------------------------
void mouse_callback(GLFWwindow* window, double xpos, double ypos)
{
    if (firstMouse)
    {
        lastX = xpos;
        lastY = ypos;
        firstMouse = false;
    }

    float xoffset = xpos - lastX;
    float yoffset = lastY - ypos; // reversed since y-coordinates go from bottom to top

    lastX = xpos;
    lastY = ypos;

    camera.ProcessMouseMovement(xoffset, yoffset);
}

// glfw: whenever the mouse scroll wheel scrolls, this callback is called
// ----------------------------------------------------------------------
void scroll_callback(GLFWwindow* window, double xoffset, double yoffset)
{
    camera.ProcessMouseScroll(yoffset);
}

// utility function for loading a 2D texture from file
// ---------------------------------------------------
unsigned int loadTexture(char const * path)
{
    unsigned int textureID;
    glGenTextures(1, &textureID);

    int width, height, nrComponents;
    unsigned char *data = stbi_load(path, &width, &height, &nrComponents, 0);
    if (data)
    {
        GLenum format;
        if (nrComponents == 1)
            format = GL_RED;
        else if (nrComponents == 3)
            format = GL_RGB;
        else if (nrComponents == 4)
            format = GL_RGBA;

        glBindTexture(GL_TEXTURE_2D, textureID);
        glTexImage2D(GL_TEXTURE_2D, 0, format, width, height, 0, format, GL_UNSIGNED_BYTE, data);
        glGenerateMipmap(GL_TEXTURE_2D);

        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_MIPMAP_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

        stbi_image_free(data);
    }
    else
    {
        std::cout << "Texture failed to load at path: " << path << std::endl;
        stbi_image_free(data);
    }

    return textureID;
}

通过使用漫反射和镜面光贴图,我们可以给相对简单的物体添加大量的细节。我们甚至可以使用法线/凹凸贴图(Normal/Bump Map)或者反射贴图(Reflection Map)给物体添加更多的细节,但这些将会留到之后的教程中。把你的箱子给你的朋友或者家人看看,并且坚信我们的箱子有一天会比现在更加漂亮!

后记

圣诞节快乐,未完,待续~~~

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