1. 引言

本文基于C++语言，描述OpenGL的基础光照

前置知识可参考：

• 基于C++的OpenGL 07 之颜色 - 当时明月在曾照彩云归 - 博客园 (cnblogs.com)

笔者这里不过多描述每个名词、函数和细节，更详细的文档可以参考：

• 基础光照 - LearnOpenGL CN (learnopengl-cn.github.io)

2. 概述

冯氏光照模型(Phong Lighting Model)：环境(Ambient)、漫反射(Diffuse)和镜面(Specular)光照

漫反射光照的强度由法向量与光的方向向量的余弦值决定，角度越小，光照越强

镜面光照的强度由视线向量与光的反射向量的余弦值决定，角度越小，光照越强

3. 环境光照

使用一个很小的常量因子乘以光的颜色，模拟环境光照

在片段着色器GLSL中：

...

void main()

{

    float ambientStrength = 0.1;

    vec3 ambient = ambientStrength * lightColor;

    vec3 result = ambient * objectColor;

    FragColor = vec4(result, 1.0);

}

结果如下图：

4. 漫反射光照

使用光线与法向量的余弦值作为漫反射因子，再乘以光的颜色来模拟漫反射光照

在这里，法向量由我们手动输入，光的方向由光的位置向量减去平面的位置向量获取

4.1 法向量

设置物体每个平面的法向量：

float 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

};

绑定属性：

//  立方体

glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void *)0);

glEnableVertexAttribArray(0);

glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void *)(3*sizeof(float)));

glEnableVertexAttribArray(1);

// 光源立方体

...

glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void*)0);

glEnableVertexAttribArray(0);

在顶点着色器中将法向量变换为世界坐标并向片段着色器传递数据：

#version 330 core

layout (location = 0) in vec3 aPos;

layout (location = 1) in vec3 aNormal;

...

out vec3 Normal;

void main()

{

    gl_Position = projection * view * model * vec4(aPos, 1.0);

    Normal = mat3(transpose(inverse(model))) * aNormal;

}

在片段着色器接收数据：

in vec3 Normal;

void main()

{

    ...

    vec3 norm = normalize(Normal);

}

4.2 光的方向向量

在片段着色器中设置光源位置向量：

uniform vec3 lightPos;

传输数据：

lightingShader.setVec3("lightPos", lightPos);

将平面位置变换到世界坐标得到位置向量：

out vec3 FragPos;

out vec3 Normal;

void main()

{

    gl_Position = projection * view * model * vec4(aPos, 1.0);

    FragPos = vec3(model * vec4(aPos, 1.0));

    Normal = aNormal;

}

在片段着色器接收数据并计算光的方向向量：

...

in vec3 FragPos;

void main()

{

    ...

    vec3 lightDir = normalize(lightPos - FragPos);

}

4.3 计算漫反射光照

计算方向向量与光的方向向量之间的余弦值作为漫反射因子（小于零的设为零），再乘以光照颜色得到漫反射光照

在片段着色器中：

float diff = max(dot(norm, lightDir), 0.0);

vec3 diffuse = diff * lightColor;

将环境光照与漫反射光照结合：

vec3 result = (ambient + diffuse) * objectColor;

FragColor = vec4(result, 1.0);

结果如下图：

5. 镜面光照

计算视线向量与光的反射向量之间的余弦值作为镜面反射因子（小于零的设为零），再乘以光照颜色得到漫反射光照

在片段着色器中设置观察位置：

uniform vec3 viewPos;

向GPU传输数据：

lightingShader.setVec3("viewPos", cameraPos);

计算视线向量（观察方向）与反射向量：

vec3 viewDir = normalize(viewPos - FragPos);

vec3 reflectDir = reflect(-lightDir, norm);

定义镜面强度，即反射能力：

float specularStrength = 0.5;

计算镜面光照：

float spec = pow(max(dot(viewDir, reflectDir), 0.0), 32);

vec3 specular = specularStrength * spec * lightColor;

将环境光照、漫反射光照、镜面光照结合：

vec3 result = (ambient + diffuse + specular) * objectColor;

FragColor = vec4(result, 1.0);

结果如下图：

6. 完整代码

主要文件Lighting.cpp：

#include <glad/glad.h>

#include <GLFW/glfw3.h>

#include <iostream>

#include <math.h>

#include "Shader.hpp"

#define STB_IMAGE_IMPLEMENTATION

#include "stb_image.h"

#include <glm/glm.hpp>

#include <glm/ext/matrix_transform.hpp>  // glm::translate, glm::rotate, glm::scale

#include <glm/ext/matrix_clip_space.hpp> // glm::perspective

#include <glm/gtc/type_ptr.hpp>

//全局变量

glm::vec3 cameraPos = glm::vec3(0.0f, 0.0f, 10.0f);

glm::vec3 cameraFront = glm::vec3(0.0f, 0.0f, -1.0f);

glm::vec3 cameraUp = glm::vec3(0.0f, 1.0f, 0.0f);

glm::vec3 lightPos(1.2f, 1.0f, 2.0f);

// 函数声明

void framebuffer_size_callback(GLFWwindow *window, int width, int height);

void process_input(GLFWwindow *window);

int main()

{

    glfwInit();

    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);

    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);

    GLFWwindow *window = glfwCreateWindow(800, 600, "lighting", nullptr, nullptr);

    if (window == nullptr)

    {

        std::cout << "Faild to create window" << std::endl;

        glfwTerminate();

    }

    glfwMakeContextCurrent(window);

    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))

    {

        std::cout << "Faild to initialize glad" << std::endl;

        return -1;

    }

    glad_glViewport(0, 0, 800, 600);

    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    //配置项

    glEnable(GL_DEPTH_TEST);

    Shader lightCubeShader("../light_cube.vs.glsl", "../light_cube.fs.glsl");

    Shader lightingShader("../cube.vs.glsl", "../cube.fs.glsl");

    unsigned int cubeVAO;

    glGenVertexArrays(1, &cubeVAO);

    glBindVertexArray(cubeVAO);

    float 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

    };

    unsigned int VBO;

    glGenBuffers(1, &VBO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);

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

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void *)0);

    glEnableVertexAttribArray(0);

    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void *)(3*sizeof(float)));

    glEnableVertexAttribArray(1);

    unsigned int lightCubeVAO;

    glGenVertexArrays(1, &lightCubeVAO);

    glBindVertexArray(lightCubeVAO);

    // 只需要绑定VBO不用再次设置VBO的数据，因为箱子的VBO数据中已经包含了正确的立方体顶点数据

    glBindBuffer(GL_ARRAY_BUFFER, VBO);

    // 设置灯立方体的顶点属性（对我们的灯来说仅仅只有位置数据）

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void*)0);

    glEnableVertexAttribArray(0);

    while (!glfwWindowShouldClose(window))

    {

        process_input(window);

        glClearColor(0.0, 0.0, 0.0, 1.0);

        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        lightingShader.use();

        lightingShader.setVec3("objectColor", 1.0f, 0.5f, 0.31f);

        lightingShader.setVec3("lightColor", 1.0f, 1.0f, 1.0f);

        lightingShader.setVec3("lightPos", lightPos);

        lightingShader.setVec3("viewPos", cameraPos);

        glm::mat4 model = glm::mat4(1.0f);

        model = glm::rotate(model, glm::radians(-55.0f), glm::vec3(1.0f, 0.0f, 0.0f));

        glm::mat4 view = glm::mat4(1.0f);

        // view = glm::translate(view, glm::vec3(0.0f, 0.0f, -3.0f));

        view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp);

        glm::mat4 projection = glm::mat4(1.0f);

        projection = glm::perspective(glm::radians(45.0f), 800.0f / 600.0f, 0.1f, 100.0f);

        // 模型矩阵

        int modelLoc = glGetUniformLocation(lightingShader.ID, "model");

        glUniformMatrix4fv(modelLoc, 1, GL_FALSE, glm::value_ptr(model));

        // 观察矩阵和投影矩阵与之类似

        int viewLoc = glGetUniformLocation(lightingShader.ID, "view");

        glUniformMatrix4fv(viewLoc, 1, GL_FALSE, glm::value_ptr(view));

        int projectionLoc = glGetUniformLocation(lightingShader.ID, "projection");

        glUniformMatrix4fv(projectionLoc, 1, GL_FALSE, glm::value_ptr(projection));

        // render the cube

        glBindVertexArray(cubeVAO);

        glDrawArrays(GL_TRIANGLES, 0, 36);

        // also draw the lamp object

        lightCubeShader.use();

        lightCubeShader.setMat4("projection", projection);

        lightCubeShader.setMat4("view", view);

        model = glm::mat4(1.0f);

        model = glm::translate(model, lightPos);

        model = glm::scale(model, glm::vec3(0.2f)); // a smaller cube

        lightCubeShader.setMat4("model", model);

        glBindVertexArray(lightCubeVAO);

        glDrawArrays(GL_TRIANGLES, 0, 36);

        glfwSwapBuffers(window);

        glfwPollEvents();

    }

    glfwTerminate();

    return 0;

}

void framebuffer_size_callback(GLFWwindow *window, int width, int height)

{

    glViewport(0, 0, width, height);

}

void process_input(GLFWwindow *window)

{

    if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)

    {

        glfwSetWindowShouldClose(window, true);

    }

    float cameraSpeed = 0.05f; // adjust accordingly

    if (glfwGetKey(window, GLFW_KEY_W) == GLFW_PRESS)

        cameraPos += cameraSpeed * cameraFront;

    if (glfwGetKey(window, GLFW_KEY_S) == GLFW_PRESS)

        cameraPos -= cameraSpeed * cameraFront;

    if (glfwGetKey(window, GLFW_KEY_A) == GLFW_PRESS)

        cameraPos -= glm::normalize(glm::cross(cameraFront, cameraUp)) * cameraSpeed;

    if (glfwGetKey(window, GLFW_KEY_D) == GLFW_PRESS)

        cameraPos += glm::normalize(glm::cross(cameraFront, cameraUp)) * cameraSpeed;

}

立方体顶点着色器GLSLcue.vs.glsl：

#version 330 core

layout (location = 0) in vec3 aPos;

layout (location = 1) in vec3 aNormal;

out vec3 Normal;

out vec3 FragPos;  

uniform mat4 model;

uniform mat4 view;

uniform mat4 projection;

void main()

{

    gl_Position = projection * view * model * vec4(aPos, 1.0);

    FragPos = vec3(model * vec4(aPos, 1.0));

    Normal = aNormal;

}

立方体片段着色器GLSLcube.fs.glsl：

#version 330 core

in vec3 Normal;

in vec3 FragPos;

out vec4 FragColor;

uniform vec3 objectColor;

uniform vec3 lightColor;

uniform vec3 lightPos;

uniform vec3 viewPos;

void main()

{

    float ambientStrength = 0.1;

    vec3 ambient = ambientStrength * lightColor;

    vec3 norm = normalize(Normal);

    vec3 lightDir = normalize(lightPos - FragPos);

    float diff = max(dot(norm, lightDir), 0.0);

    vec3 diffuse = diff * lightColor;

    vec3 viewDir = normalize(viewPos - FragPos);

    vec3 reflectDir = reflect(-lightDir, norm);

    float specularStrength = 0.8;

    float spec = pow(max(dot(viewDir, reflectDir), 0.0), 32);

    vec3 specular = specularStrength * spec * lightColor;

    vec3 result = (ambient + diffuse + specular) * objectColor;

    FragColor = vec4(result, 1.0);

}

着色器Shader.hpp、光源顶点着色器GLSLlight_cube.vs.glsl、光源片段着色器GLSLlight_cube.fs.glsl见：

• 基于C++的OpenGL 07 之颜色 - 当时明月在曾照彩云归 - 博客园 (cnblogs.com)

7. 参考资料

[1]基础光照 - LearnOpenGL CN (learnopengl-cn.github.io)

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