OpenGL渲染管道，Shader，VAO&VBO&EBO

OpenGL渲染管线

(也就是)OpenGL渲染一帧图形的流程

• 以下列举最简单的，渲染一个三角形的流程，你可以将它视为 精简版OpenGL渲染管线

　　更复杂的流程也仅仅就是：在此基础上的各个流程中 添加额外的内容，而这些会在后续补充。

• • CPU将三角形的  [点坐标，及其索引信息]  传入GPU显存

• • CPU调用 gl函数编译链接挂载 shader

• • • shader是使用  类C语言： glsl 编写的程序

• • • GPU会执行shader中的内容，并根据shader程序进行绘图。

• • • GPU会   汇总shader提供的数据&CPU提供的数据，并根据他们进行绘图。

• • CPU调用 gl函数激活 GPU执行shader并绘制图

• • 以下是相关代码：

 // build and compile our shader program
 // ------------------------------------
    // vertex shader
    unsigned int vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);
    // check for shader compile errors
    int success;
    char infoLog[512];
    glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(vertexShader, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog << std::endl;
    }
    // fragment shader
    unsigned int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);
    // check for shader compile errors
    glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog << std::endl;
    }
    // link shaders
    unsigned int shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    glLinkProgram(shaderProgram);
    // check for linking errors
    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);
// -----------------------------以上：Shader编译，编译检查，链接，链接检查-----------------------------

// set up vertex data (and buffer(s)) and configure vertex attributes
// ------------------------------------------------------------------
   //三角形顶点，及其索引信息： 
　　float vertices[] = {
         0.5f,  0.5f, 0.0f,  // top right
         0.5f, -0.5f, 0.0f,  // bottom right
        -0.5f, -0.5f, 0.0f,  // bottom left
        -0.5f,  0.5f, 0.0f   // top left
    };
    unsigned int indices[] = {  // note that we start from 0!
        0, 1, 3,  // first Triangle
        1, 2, 3   // second Triangle
    };
    //向GPU传入相关数据
    unsigned int VBO, VAO, EBO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);

    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);

　　 glBindVertexArray(VAOID)
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    // note that this is allowed, the call to glVertexAttribPointer registered VBO as the vertex attribute's bound vertex buffer object so afterwards we can safely unbind
    glBindBuffer(GL_ARRAY_BUFFER, 0); 

    // remember: do NOT unbind the EBO while a VAO is active as the bound element buffer object IS stored in the VAO; keep the EBO bound.
    //glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

    // You can unbind the VAO afterwards so other VAO calls won't accidentally modify this VAO, but this rarely happens. Modifying other
    // VAOs requires a call to glBindVertexArray anyways so we generally don't unbind VAOs (nor VBOs) when it's not directly necessary.
    glBindVertexArray(0); 

　　 while (!glfwWindowShouldClose(window)) {
　　 // input 
　　 // ----- 
　　 　　processInput(window); 
　　 
　　 // render 
    // ------ 
　　 　　glClearColor(0.2f, 0.3f, 0.3f, 1.0f); 
　　 　　glClear(GL_COLOR_BUFFER_BIT); 
　　 
　　 // draw our first triangle 
　　 　　glUseProgram(shaderProgram); 
　　 　　glBindVertexArray(VAO); 

　　 // seeing as we only have a single VAO there's no need to bind it every time, but we'll do so to keep things a bit more organized 
　　 　　glDrawArrays(GL_TRIANGLES, 0, 6); 
　　 　　glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0); 

　　 // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.) 
　　 // ------------------------------------------------------------------------------- 
　　 　　glfwSwapBuffers(window); 
　　 　　glfwPollEvents(); 
　　} 
// optional: de-allocate all resources once they've outlived their purpose: 
// ------------------------------------------------------------------------ 
　　glDeleteVertexArrays(1, &VAO); 
　　glDeleteBuffers(1, &VBO); 
　　glDeleteBuffers(1, &EBO); 
　　glDeleteProgram(shaderProgram);

 

VAO&VBO&EBO

　　  1. VBO：一个抽象对象，OpenGL没有固定它的实现，

　　　  因此你不必了解它的实现，你只需知道VBO是  OpenGL用来管理  来自CPU数据的 一种工具：

    unsigned int VBOID, VAOID, EBOID;
    //申请三个整形值，用以存放 申请的VBO,VAB,EBO的 ID    

    glGenBuffers(1, &VBOID);
    glGenVertexArrays(1, &VAOID);
    glGenBuffers(1, &EBOID);
    //向OpenGL申请: 1*VBO, 1*VAO, 1*EBO  并将其ID传入各自的 地址
　  //[此地址可以是连续数组空间的首地址，也可以是单一的一个数据]

    glBindVertexArray(VAOID)
    //将
    glBindBuffer(GL_ARRAY_BUFFER, VBOID);
    //将VBOID指向的VBO 与flag:GL_ARRAY_BUFFER绑定。
    

　　  2.