基于C++的OpenGL 08 之基础光照

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：

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：

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

img

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

img

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

img

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：

image-20220813100722037

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：

image-20220813101924682

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：

image-20220813102500533

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：

7. 参考资料

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

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