环境

• 系统：ubuntu20.04
• opengl版本：4.6
• glfw版本：3.3
• glad版本：4.6
• cmake版本：3.16.3
• gcc版本：10.3.0

在<opengl学习日记11-opengl的transformtions变换示例>的基础上，进行修改，实现坐标系统变换效果。

代码

CMakeLists.txt

cmake_minimum_required(VERSION 3.5)project(CoordinateSystems )set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)find_package(glfw3 REQUIRED)
message("PROJECT_NAME:${PROJECT_NAME}")
file(GLOB project_file glad.c main.cpp  stb_image.h stbimage.h)add_executable(${PROJECT_NAME} ${project_file})target_link_libraries(${PROJECT_NAME}-lglfw-lGL-lm-lXrandr-lX11-lpthread-ldl)

vertexShaderSource.vs

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec2 aTexCoord;out vec2 TexCoord;uniform mat4 model;
uniform mat4 view;
uniform mat4 projection;void main()
{gl_Position = projection * view * model * vec4(aPos, 1.0f);
//        gl_Position = vec4(aPos, 1.0);TexCoord = vec2(aTexCoord.x,1.0 - aTexCoord.y);
}

main.cpp

#include <glad/glad.h>
#include <GLFW/glfw3.h>#include "stbimage.h"#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <glm/gtc/type_ptr.hpp>#include <fstream>
#include <iosfwd>
#include <iostream>
#include <sstream>
#include <string>//定义窗口大小
int SCR_WIDTH = 800;
int SCR_HEIGHT = 600;std::string readFile(const std::string filename)
{std::string data;data = readFile(filename.c_str());return data;
}std::string readFile(const char* filename)
{std::string data;std::ifstream infile;infile.open(filename);std::stringstream neirong;neirong << infile.rdbuf();infile.close();data = neirong.str();return data;
}int main()
{std::cout << "Hello World!" << std::endl;std::cout << "初始化" << std::endl;glfwInit();glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);GLFWwindow* window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "transformtions", NULL, NULL);if (window == NULL) {std::cout << "创建窗口失败" << std::endl;return -1;}glfwMakeContextCurrent(window);if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress)) {std::cout << "初始化glad失败" << std::endl;return -1;}std::cout << "顶点定义，VAO，VBO，EBO" << std::endl;float vertices[] = {0.5f, 0.5f, 0.0f, 1.0f, 1.0f,0.5f, -0.5f, 0.0f, 1.0f, 0.0f,-0.5f, -0.5f, 0.0f, 0.0f, 0.0f,-0.5f, 0.5f, 0.0f, 0.0f, 1.0f};//索引缓冲区unsigned int indices[] = { 0, 1, 3,1, 2, 3 };unsigned int VAO, VBO, EBO;glGenVertexArrays(1, &VAO);glGenBuffers(1, &VBO);glGenBuffers(1, &EBO);glBindVertexArray(VAO);glBindBuffer(GL_ARRAY_BUFFER, VBO);glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);//定位点关系glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 5 * sizeof(float), (void*)0);glEnableVertexAttribArray(0);glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 5 * sizeof(float), (void*)(3 * sizeof(float)));glEnableVertexAttribArray(1);std::cout << "着色器定义" << std::endl;std::cout << "着色器定义:顶点着色器" << std::endl;std::string vertexShaderSource = readFile("vertexShaderSource.vs");const char* vertexShaderSourceCtr = vertexShaderSource.c_str();unsigned int vertexShader;vertexShader = glCreateShader(GL_VERTEX_SHADER);glShaderSource(vertexShader, 1, &vertexShaderSourceCtr, NULL);glCompileShader(vertexShader);int success;char infolog[512] = { 0 };glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);if (!success) {glGetShaderInfoLog(vertexShader, 512, NULL, infolog);std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n"<< infolog << std::endl;}std::cout << "着色器定义:片段着色器" << std::endl;std::string fragmentShaderSource = readFile("fragmentShaderSource.fs");const char* fragmentShaderSourceCtr = fragmentShaderSource.c_str();unsigned int fragmentShader;fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);glShaderSource(fragmentShader, 1, &fragmentShaderSourceCtr, NULL);glCompileShader(fragmentShader);glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);if (!success) {glGetShaderInfoLog(fragmentShader, 512, NULL, infolog);std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n"<< infolog << std::endl;}std::cout << "着色器定义:启用着色器程序" << std::endl;unsigned int shaderProgram;shaderProgram = glCreateProgram();glAttachShader(shaderProgram, vertexShader);glAttachShader(shaderProgram, fragmentShader);glLinkProgram(shaderProgram);glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);if (!success) {glGetProgramInfoLog(shaderProgram, 512, NULL, infolog);std::cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n"<< infolog << std::endl;}glDeleteShader(vertexShader);glDeleteShader(fragmentShader);std::cout << "纹理定义" << std::endl;unsigned int texture1;glGenTextures(1, &texture1);glBindTexture(GL_TEXTURE_2D, texture1);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);stbi_set_flip_vertically_on_load(true);std::cout << "读取纹理图片" << std::endl;int width, height, nrChannels;unsigned char* texturesData = stbi_load("container.jpg", &width, &height, &nrChannels, 0);if (texturesData) {//绑定和纹理图片数据结合glTexImage2D(GL_TEXTURE_2D,0,GL_RGB,width,height,0,GL_RGB,GL_UNSIGNED_BYTE,texturesData);glGenerateMipmap(GL_TEXTURE_2D);} else {std::cout << "读取图片数据错误" << std::endl;}stbi_image_free(texturesData);std::cout << "纹理定义2" << std::endl;unsigned int texture2;glGenTextures(1, &texture2);glBindTexture(GL_TEXTURE_2D, texture2);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);std::cout << "读取纹理图片" << std::endl;//        int width, height, nrChannels;texturesData = stbi_load("awesomeface.png", &width, &height, &nrChannels, 0);if (texturesData) {//绑定和纹理图片数据结合glTexImage2D(GL_TEXTURE_2D,0,GL_RGB,width,height,0,GL_RGBA,GL_UNSIGNED_BYTE,texturesData);glGenerateMipmap(GL_TEXTURE_2D);} else {std::cout << "读取图片数据错误" << std::endl;}stbi_image_free(texturesData);glUseProgram(shaderProgram);glUniform1i(glGetUniformLocation((GLuint)shaderProgram, "texture1"), 0);glUniform1i(glGetUniformLocation((GLuint)shaderProgram, "texture2"), 1);std::cout << "绘制" << std::endl;while (!glfwWindowShouldClose(window)) {glClearColor(0.2f, 0.3f, 0.3f, 1.0f);glClear(GL_COLOR_BUFFER_BIT);glActiveTexture(GL_TEXTURE0);glBindTexture(GL_TEXTURE_2D, texture1);glActiveTexture(GL_TEXTURE1);glBindTexture(GL_TEXTURE_2D, texture2);glUseProgram(shaderProgram);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.f));glm::mat4 projection = glm::mat4(1.0f);projection = glm::perspective(glm::radians(45.0f), (float)SCR_WIDTH /(float) SCR_HEIGHT, 0.1f, 100.0f);unsigned int transformLoc = glGetUniformLocation(shaderProgram, "model");glUniformMatrix4fv(transformLoc, 1, GL_FALSE, glm::value_ptr(model));unsigned int viewLoc = glGetUniformLocation(shaderProgram, "view");glUniformMatrix4fv(viewLoc, 1, GL_FALSE, glm::value_ptr(view));unsigned int projectionLoc = glGetUniformLocation(shaderProgram, "projection");glUniformMatrix4fv(projectionLoc, 1, GL_FALSE, glm::value_ptr(projection));glBindVertexArray(VAO);glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, (const GLvoid*)0);glfwSwapBuffers(window);glfwPollEvents();}std::cout << "内存释放" << std::endl;glDeleteVertexArrays(1, &VAO);glDeleteBuffers(1, &VBO);glDeleteBuffers(1, &EBO);glDeleteProgram(shaderProgram);glfwTerminate();return 0;
}

总结

1. 之前注意到需要把下面两个头文件引入放在头文件的最前端，否则报错，今天发现，不仅这两个头文件需要放在最前端，而且引入顺序也不能更改，否则同样会报错，有函数找不到

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

2. 世界空间的变换(模型矩阵)

物体的坐标将会从局部变换到世界空间；该变换是由模型矩阵(Model Matrix)实现的。模型矩阵是一种变换矩阵，它能通过对物体进行位移、缩放、旋转来将它置于它本应该在的位置或朝向。

所以，局部空间到世界空间的变换可以用以下glm::rotate函数实现,只要最后得到一个mat4类型的结构就可以

glm::mat4 model;
model = glm::rotate(model, glm::radians(-55.0f), glm::vec3(1.0f, 0.0f, 0.0f));

所以，这里我们可以引申一下，就是位移、缩放、旋转这三个功能函数都可以得到mat4结构，即将局部空间转换为世界空间

3. 观察空间变换(观察矩阵)

观察空间经常被人们称之OpenGL的摄像机(Camera)（所以有时也称为摄像机空间(Camera Space)或视觉空间(Eye Space)）。观察空间是将世界空间坐标转化为用户视野前方的坐标而产生的结果。这通常是由一系列的位移和旋转的组合来完成，平移/旋转场景从而使得特定的对象被变换到摄像机的前方。这些组合在一起的变换通常存储在一个观察矩阵(View Matrix)里，它被用来将世界坐标变换到观察空间。

通过以上解释，我理解的是观察空间的变换也是由位移和旋转,平移/旋转这些变换来的，所以这些个函数也可以用作观察空间的变换

glm::mat4 view;
// 注意，我们将矩阵向我们要进行移动场景的反方向移动。
view = glm::translate(view, glm::vec3(0.0f, 0.0f, -3.0f));

4. 裁剪空间(投影矩阵)

在一个顶点着色器运行的最后，OpenGL期望所有的坐标都能落在一个特定的范围内，且任何在这个范围之外的点都应该被裁剪掉(Clipped)。被裁剪掉的坐标就会被忽略，所以剩下的坐标就将变为屏幕上可见的片段。这也就是裁剪空间(Clip Space)名字的由来。投影矩阵(Projection Matrix)，它指定了一个范围的坐标

透视投影glm创建函数为

glm::mat4 proj = glm::perspective(glm::radians(45.0f), (float)width/(float)height, 0.1f, 100.0f);

5. 顶点坐标变换为裁剪坐标的公式

{V}_{clip} = M_{projection} ⋅ M_{view} ⋅ M_{model} ⋅ V_{local}

注意 矩阵运算的顺序是相反的（记住我们需要从右往左阅读矩阵的乘法）。

最后的顶点应该被赋值到顶点着色器中的gl_Position，OpenGL将会自动进行透视除法和裁剪。