基于C++的OpenGL 11 之投光物

1. 引言

本文基于C++语言，描述OpenGL的投光物

前置知识可参考：

• 基于C++的OpenGL 10 之光照贴图 - 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：

• 投光物 - LearnOpenGL CN (learnopengl-cn.github.io)

2. 概述

投光物，即光源，主要有平行光源、点光源和聚光源

平行光源可以使用一个方向向量来模拟

点光源可以使用一个点来模拟，另外，点光源应该有衰减模拟，衰减公式为

\[\begin{equation} F_{att} = \frac{1.0}{K_c + K_l * d + K_q * d^2} \end{equation}

\]

• 常数项通常保持为1.0，它的主要作用是保证分母永远不会比1小，否则的话在某些距离上它反而会增加强度
• 一次项会与距离值相乘，以线性的方式减少强度
• 二次项会与距离的平方相乘，让光源以二次递减的方式减少强度

\(K_l\)与\(K_q\)的取值可以参考实验值：-Point Light Attenuation | Ogre Wiki (ogre3d.org)

聚光源类似于手电筒、聚光灯，只照亮灯光方向的一部分，如下图所示

图中，参数含义如下：

• LightDir：从片段指向光源的向量
• SpotDir：聚光所指向的方向
• Phi\(\phi\)：指定了聚光半径的切光角。落在这个角度之外的物体都不会被这个聚光所照亮
• Theta\(\theta\)：LightDir向量和SpotDir向量之间的夹角。在聚光内部的话θ值应该比ϕ值小

计算LightDir向量和SpotDir向量之间的点积得到两个单位向量夹角的余弦值，并将它与切光角ϕ值对比，即可判断是否被照亮

3. 编码

3.1 平行光

使用一个光线方向向量来模拟平行光

在片段着色器中定义光线的方向向量：

struct Light {
    // vec3 position; // 使用定向光就不再需要了
    vec3 direction;

    vec3 ambient;
    vec3 diffuse;
    vec3 specular;
};
...
void main()
{
  vec3 lightDir = normalize(-light.direction);
  ...
}

输入方向向量：

lightingShader.setVec3("light.direction", -1.0f, 0.0f, 0.0f);

结果如下：

3.2 点光源

给定一个点位置来模拟点光源，并且设置衰减的参数

这里\(K_l\)与\(K_q\)的取值使用的是50米光源的实验值，分别0.09、0.032

在片段着色器中定义参数：

struct Light {
    vec3 position;  

    vec3 ambient;
    vec3 diffuse;
    vec3 specular;

    float constant;
    float linear;
    float quadratic;
};

计算衰减：

float distance    = length(light.position - FragPos);
float attenuation = 1.0 / (light.constant + light.linear * distance +
                light.quadratic * (distance * distance));
ambient  *= attenuation;
diffuse  *= attenuation;
specular *= attenuation;

想GPU输入数据：

lightingShader.setFloat("light.constant",  1.0f);
lightingShader.setFloat("light.linear",    0.09f);
lightingShader.setFloat("light.quadratic", 0.032f);

实现效果如下：

3.3 聚光源

在片段着色器中定义聚光源的参数：

struct Light {
    vec3  position;
    vec3  direction;
    float cutOff;
    ...
};

计算是否照亮：

float theta = dot(lightDir, normalize(-light.direction));

if(theta > light.cutOff)
{
  // 执行光照计算
}
else  // 否则，使用环境光，让场景在聚光之外时不至于完全黑暗
  color = vec4(light.ambient * vec3(texture(material.diffuse, TexCoords)), 1.0);

向GPU传输数据：

lightingShader.setVec3("light.position",  cameraPos);
lightingShader.setVec3("light.direction", cameraFront);
lightingShader.setFloat("light.cutOff",   glm::cos(glm::radians(35.0f)));

结果如下：

目前看起来边缘过渡，使用一个外半径进行边缘过渡是必要的

计算公式为：

\[\begin{equation} I = \frac{\theta - \gamma}{\epsilon} \end{equation}
\]

这里\(\epsilon\)(Epsilon)是内（\(\theta\)）和外圆锥（\(\gamma\)）之间的余弦值差（\(\epsilon = \phi - \gamma\)），最终的\(I\)值就是在当前片段聚光的强度

在片段着色器中定义参数：

struct Light {
    float outerCutOff;
...
};
...
void main()
{
    ....
    float theta     = dot(lightDir, normalize(-light.direction));
    float epsilon   = light.cutOff - light.outerCutOff;
    float intensity = clamp((theta - light.outerCutOff) / epsilon, 0.0, 1.0);
    ...
    // 将不对环境光做出影响，让它总是能有一点光
    diffuse  *= intensity;
    specular *= intensity;
    ...
}

输入数据：

lightingShader.setFloat("light.outerCutOff", glm::cos(glm::radians(40.0f)));

实现效果如下：

4. 完整代码

主要文件caster.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(0.8f, 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, "caster", 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[] = {
    // 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
    };
    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, 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);

    // 纹理
    unsigned int texture;
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    // 为当前绑定的纹理对象设置环绕、过滤方式
    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);
    // 加载并生成纹理
    int width, height, nrChannels;
    unsigned char *data = stbi_load("../container2.png", &width, &height, &nrChannels, 0);
    if (data)
    {
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
        glGenerateMipmap(GL_TEXTURE_2D);
    }
    else
    {
        std::cout << "Failed to load texture" << std::endl;
    }
    stbi_image_free(data);
    lightingShader.setInt("material.diffuse", 0);

    // 镜面反射纹理
    unsigned int texture1;
    glGenTextures(1, &texture1);
    glBindTexture(GL_TEXTURE_2D, texture1);
    // 为当前绑定的纹理对象设置环绕、过滤方式
    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);
    // 加载并生成纹理
    data = stbi_load("../container2_specular.png", &width, &height, &nrChannels, 0);
    if (data)
    {
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
        glGenerateMipmap(GL_TEXTURE_2D);
    }
    else
    {
        std::cout << "Failed to load texture" << std::endl;
    }
    stbi_image_free(data);
    lightingShader.setInt("material.diffuse", 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, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    // positions all containers
    glm::vec3 cubePositions[] = {
        glm::vec3( 0.0f,  0.0f,  0.0f),
        glm::vec3( 2.0f,  5.0f, -15.0f),
        glm::vec3(-1.5f, -2.2f, -2.5f),
        glm::vec3(-3.8f, -2.0f, -12.3f),
        glm::vec3( 2.4f, -0.4f, -3.5f),
        glm::vec3(-1.7f,  3.0f, -7.5f),
        glm::vec3( 1.3f, -2.0f, -2.5f),
        glm::vec3( 1.5f,  2.0f, -2.5f),
        glm::vec3( 1.5f,  0.2f, -1.5f),
        glm::vec3(-1.3f,  1.0f, -1.5f)
    };

    while (!glfwWindowShouldClose(window))
    {
        process_input(window);

        glClearColor(0.0, 0.0, 0.0, 1.0);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, texture);
        glActiveTexture(GL_TEXTURE1);
        glBindTexture(GL_TEXTURE_2D, texture1);

        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);
        lightingShader.setFloat("material.shininess", 32.0f);
        lightingShader.setVec3("light.ambient",  0.2f, 0.2f, 0.2f);
        lightingShader.setVec3("light.diffuse",  0.5f, 0.5f, 0.5f); // 将光照调暗了一些以搭配场景
        lightingShader.setVec3("light.specular", 1.0f, 1.0f, 1.0f);
        // lightingShader.setVec3("light.direction", -1.0f, 0.0f, 0.0f);
        // lightingShader.setFloat("light.constant",  1.0f);
        // lightingShader.setFloat("light.linear",    0.09f);
        // lightingShader.setFloat("light.quadratic", 0.032f);
        lightingShader.setVec3("light.position",  cameraPos);
        lightingShader.setVec3("light.direction", cameraFront);
        lightingShader.setFloat("light.cutOff", glm::cos(glm::radians(35.0f)));
        lightingShader.setFloat("light.outerCutOff", glm::cos(glm::radians(40.0f)));

        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);
        for (unsigned int i = 0; i < 10; i++)
        {
            // calculate the model matrix for each object and pass it to shader before drawing
            glm::mat4 model = glm::mat4(1.0f);
            model = glm::translate(model, cubePositions[i]);
            float angle = 20.0f * i;
            model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f));
            lightingShader.setMat4("model", model);

            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GLSLcube.vs.glsl：

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aNormal;
layout (location = 2) in vec2 aTexCoords;

out vec3 Normal;
out vec3 FragPos;
out vec2 TexCoords;

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;
    TexCoords = aTexCoords;
}

立方体片段着色器GLSLcube.fs.glsl：

#version 330 core
struct Material {
    sampler2D diffuse;
    sampler2D specular;
    float     shininess;
};
struct Light {
    vec3  position;
    vec3  direction;
    float cutOff;
    float outerCutOff;

    vec3 ambient;
    vec3 diffuse;
    vec3 specular;

    // float constant;
    // float linear;
    // float quadratic;
};
in vec3 Normal;
in vec3 FragPos;
in vec2 TexCoords;

out vec4 FragColor;

uniform vec3 objectColor;
uniform vec3 lightColor;
uniform vec3 lightPos;
uniform vec3 viewPos;
uniform Material material;
uniform Light light;

void main()
{
    // 环境光
    // 将环境光下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));

    // 漫反射
    vec3 norm = normalize(Normal);
    vec3 lightDir = normalize(lightPos - FragPos);
    // vec3 lightDir = normalize(-light.direction);
    float diff = max(dot(norm, lightDir), 0.0);
    vec3 diffuse = light.diffuse * diff * vec3(texture(material.diffuse, TexCoords));

    // 镜面光
    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 * vec3(texture(material.specular, TexCoords));  

    // 计算衰减
    // float distance    = length(light.position - FragPos);
    // float attenuation = 1.0 / (light.constant + light.linear * distance +
    //                 light.quadratic * (distance * distance));
    // ambient  *= attenuation;
    // diffuse  *= attenuation;
    // specular *= attenuation;

    float theta = dot(lightDir, normalize(-light.direction));

    if(theta > light.cutOff)
    {
        // 执行光照计算
        vec3 result = ambient + diffuse + specular;
        FragColor = vec4(result, 1.0);
    }
    else{  // 否则，使用环境光，让场景在聚光之外时不至于完全黑暗
        float epsilon   = light.cutOff - light.outerCutOff;
        float intensity = clamp((theta - light.outerCutOff) / epsilon, 0.0, 1.0);
        // 将不对环境光做出影响，让它总是能有一点光
        diffuse  *= intensity;
        specular *= intensity;
        vec3 result = ambient + diffuse + specular;
        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)

5. 参考资料

[1]投光物 - LearnOpenGL CN (learnopengl-cn.github.io)