基于C++的OpenGL 01 之Hello Triangle
本文基于C++语言，描述OpenGL核心模式(Core-profile)的绘制流程
1. 引言
本文基于C++语言，描述OpenGL的绘制流程，这里描述的是OpenGL的核心模式(Core-profile)
本文基于Ubuntu 20.04.3 LTS系统，使用CMake构建程序，OpenGL环境搭建可参考：
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. 流程综述
OpenGL的绘制流程（图形渲染管线，Graphics Pipeline）如下：
- 顶点着色(vertex shader)阶段将CPU传入的数据进行一定的变换处理
- 图元装配(shape assembly)阶段的就是上阶段的顶点数据处理成图元（如，三角形）
- 几何着色(geometry shader)阶段是根据一定规则将输入的图元变更或输出更多的图元（可选）
- 光栅化(rasterization)阶段的是将上阶段的图元进行计算得到图元占据的屏幕像素列表
- 片元着色(fragment)阶段是将上阶段生成的片元进行着色处理后
- 测试与混合阶段计算片元的深度、颜色等从而进行舍弃或保留
绘制流程繁琐，然而，我们能配置的只有三个蓝色的着色器部分。几何着色器可选，一般配置顶点着色器和片段着色器即可，即，以下步骤就是配置顶点着色器和片段着色器
3. 生成顶点数据
生成顶点缓冲对象(Vertex Buffer Objects, VBO)并加载数据：
float vertices[] = {
0.0f, 0.5f, 0.0f,
-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 0.0f
};
unsigned int VBO;
glGenBuffers(1, &VBO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
4. 链接属性数据
顶点数组对象(Vertex Array Object, VAO)与VBO绑定，用于保存属性数据（先绑定VAO，再创建VBO就会绑定到VAO上）：
unsigned int VAO;
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);
float vertices[] = {
0.0f, 0.5f, 0.0f,
-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 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, 3 * sizeof(float), (void *)0);
glEnableVertexAttribArray(0);
5. 创建顶点着色器
创建顶点着色器(Vertex Shader)并编译：
const char *vertexShaderSource =
"#version 330 core\n"
"layout (location = 0) in vec3 aPos;\n"
"void main()\n"
"{\n"
"gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);\n"
"}\0";
unsigned int vertexShader = glCreateShader(GL_VERTEX_SHADER);
glShaderSource(vertexShader, 1, &vertexShaderSource, nullptr);
glCompileShader(vertexShader);
6. 创建片段着色器
创建片段着色器(Fragment Shader)并编译：
const char *fragmentShaderSource =
"#version 330 core\n"
"out vec4 FragColor;"
"void main()\n"
"{\n"
"FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"
"}\0";
unsigned int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(fragmentShader, 1, &fragmentShaderSource, nullptr);
glCompileShader(fragmentShader);
7. 链接着色器
着色器程序对象(Shader Program Object)是多个着色器合并之后并最终链接完成的版本：
unsigned int shaderProgram = glCreateProgram();
glAttachShader(shaderProgram, vertexShader);
glAttachShader(shaderProgram, fragmentShader);
glLinkProgram(shaderProgram);
8. 绘制
开始（循环）绘制：
glClearColor(0.2, 0.3, 0.3, 1.0);
glClear(GL_COLOR_BUFFER_BIT);
glUseProgram(shaderProgram);
glBindVertexArray(VAO);
glDrawArrays(GL_TRIANGLES, 0, 3);
9. 完整代码
基于GLFW与GLAD创建OpenGL开发环境，完整代码如下：
#include <glad/glad.h>
#include <GLFW/glfw3.h>
#include <iostream>
void framebuffer_size_callback(GLFWwindow *window, int width, int height);
void process_input(GLFWwindow *window);
unsigned int *renderInit();
void render(unsigned int shaderProgram, unsigned int VAO);
bool checkCompile(unsigned int shader);
bool checkProgram(unsigned int shaderProgram);
int main()
{
glfwInit();
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
GLFWwindow *window = glfwCreateWindow(800, 600, "hello triangle", 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);
unsigned int *arr = renderInit();
while (!glfwWindowShouldClose(window))
{
process_input(window);
// render
std::cout << arr[0] << " " << arr[1] << " " << arr[2] << std::endl;
render(arr[0], arr[1]);
glfwSwapBuffers(window);
glfwPollEvents();
}
glDeleteProgram(arr[0]);
glDeleteVertexArrays(1, &arr[1]);
glDeleteBuffers(1, &arr[2]);
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);
}
}
unsigned int *renderInit()
{
unsigned int VAO;
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);
float vertices[] = {
0.0f, 0.5f, 0.0f,
-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 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, 3 * sizeof(float), (void *)0);
glEnableVertexAttribArray(0);
const char *vertexShaderSource =
"#version 330 core\n"
"layout (location = 0) in vec3 aPos;\n"
"void main()\n"
"{\n"
"gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);\n"
"}\0";
unsigned int vertexShader = glCreateShader(GL_VERTEX_SHADER);
glShaderSource(vertexShader, 1, &vertexShaderSource, nullptr);
glCompileShader(vertexShader);
checkCompile(vertexShader);
const char *fragmentShaderSource =
"#version 330 core\n"
"out vec4 FragColor;"
"void main()\n"
"{\n"
"FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"
"}\0";
unsigned int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(fragmentShader, 1, &fragmentShaderSource, nullptr);
glCompileShader(fragmentShader);
checkCompile(fragmentShader);
unsigned int shaderProgram = glCreateProgram();
glAttachShader(shaderProgram, vertexShader);
glAttachShader(shaderProgram, fragmentShader);
glLinkProgram(shaderProgram);
checkProgram(shaderProgram);
glDeleteShader(vertexShader);
glDeleteShader(fragmentShader);
return new unsigned int[3]{shaderProgram, VAO, VBO};
}
void render(unsigned int shaderProgram, unsigned int VAO)
{
glClearColor(0.2, 0.3, 0.3, 1.0);
glClear(GL_COLOR_BUFFER_BIT);
glUseProgram(shaderProgram);
glBindVertexArray(VAO);
glDrawArrays(GL_TRIANGLES, 0, 3);
}
bool checkCompile(unsigned int shader)
{
int success;
char infoLog[512];
glGetShaderiv(shader, GL_COMPILE_STATUS, &success);
if (!success)
{
glGetShaderInfoLog(shader, 512, NULL, infoLog);
std::cout << "ERROR::SHADER::COMPILATION_FAILED\n"
<< infoLog << std::endl;
return false;
}
return true;
}
bool checkProgram(unsigned int shaderProgram)
{
int success;
char infoLog[512];
glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
if (!success)
{
glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);
return false;
}
return true;
}
代码基于CMake构建，CMakeLists.txt如下：
cmake_minimum_required(VERSION 3.3)
set(CMAKE_C_STANDARD 11)
set(CMAKE_CXX_STANDARD 14)
project(HelloTriangle)
find_package(glfw3 REQUIRED)
find_package( OpenGL REQUIRED )
include_directories( ${OPENGL_INCLUDE_DIRS} )
file(GLOB project_file glad.c main.cpp)
add_executable(${PROJECT_NAME} ${project_file})
target_link_libraries(${PROJECT_NAME} ${OPENGL_LIBRARIES} glfw)
main.cpp是上述代码的文件名
使用CMake构建并编译运行：
$ cmake build .
$ make
$ ./HelloTriangle
运行结果如下：
10. 参考资料
[1]你好，三角形 - LearnOpenGL CN (learnopengl-cn.github.io)
