OpenGL 渲染一个三角形笔记

原教程地址

链接: link.

接上篇笔记

链接: link.

三个对象

顶点数组对象：Vertex Array Object，VAO
顶点缓冲对象：Vertex Buffer Object，VBO
索引缓冲对象：Element Buffer Object，EBO或Index Buffer Object，IBO

需要先了解渲染管线流程

创建、绑定缓冲对象和创建、传递顶点数据

		//创建顶点数据
		//因为这一章的教程里顶点着色器没有处理空间变换，所以这里的坐标都创建在-1.0到1.0，也就是在标准化设备坐标内的坐标
		float vertices[] = {
		    -0.5f, -0.5f, 0.0f,
		     0.5f, -0.5f, 0.0f,
 		     0.0f,  0.5f, 0.0f
		};
		//创建缓冲对象
		unsigned int VBO;
		glGenBuffers(1, &VBO);
		//绑定缓冲对象到GL_ARRAY_BUFFER，顶点缓冲对象的缓冲类型是GL_ARRAY_BUFFER
		glBindBuffer(GL_ARRAY_BUFFER, VBO);
		//传递数据到缓冲对象
		glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
		// 设置顶点属性指针
		glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
		glEnableVertexAttribArray(0);

顶点着色器、片段着色器、编译着色器、着色器程序

创建字符串，之后会用这个字符串创建着色器

• 顶点着色器：

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";

也就是

#version 330 core
		layout (location = 0) in vec3 aPos;

		void main()
		{
		    gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);
		}

aPos就是传进来的顶点数据
gl_Position是顶点着色器输出的数据（顶点）

• 片段着色器:

const char *fragmentShaderSource = "#version 330 core\n"
					          "out vec4 FragColor;\n"
					          "void main()\n"
					          "{\n"
					          "   FragColor = vec4(1.0f, 0f, 0.2f, 1.0f);\n"
					          "}\n\0";

也就是

		#version 330 core
		out vec4 FragColor;

		void main()
		{
		    FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);
		} 

FragColor就是输出的数据（颜色）

• 编译着色器

		   //创建顶点着色器对象
			int vertexShader = glCreateShader(GL_VERTEX_SHADER);
			//把源码附加到对象上
			glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
			//编译
			glCompileShader(vertexShader);
			// 失败返回错误信息
			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;
			}
			//创建片段着色器对象
			int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
			//把源码附加到对象上
			glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
			//编译
			glCompileShader(fragmentShader);
			// 失败返回错误信息
			glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);
			if (!success)
			{
				glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog);
				std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog << std::endl;
			}

• 创建着色器程序和链接着色器

   		//创建着色器程序对象
   		int shaderProgram = glCreateProgram();
   		//把着色器附加到程序上
   		glAttachShader(shaderProgram, vertexShader);
   		glAttachShader(shaderProgram, fragmentShader);
   		//链接着色器  当链接着色器至一个程序的时候，它会把每个着色器的输出链接到下个着色器的输入。当输出和输入不匹配的时候，你会得到一个连接错误。
   		glLinkProgram(shaderProgram);
   		// 失败返回错误信息
   		glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
   		if (!success) {
   			glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);
   			std::cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n" << infoLog << std::endl;
   		}
   		//程序链接完后，就不需要着色器对象了，删除
   		glDeleteShader(vertexShader);
   		glDeleteShader(fragmentShader);

绑定顶点数组对象VAO

		//在绑定了VAO后，再进行绑定VBO的一系列操作。VAO就有了顶点数据
		unsigned int VAO;
		glGenVertexArrays(1, &VAO);

在渲染循环中，每次都要传送顶点数据，所以每次都glBindVertexArray(VAO);就行了

代码

代码原文地址：link.

#include 
#include 

#include 

void framebuffer_size_callback(GLFWwindow* window, int width, int height);
void processInput(GLFWwindow *window);

// settings
const unsigned int SCR_WIDTH = 800;
const unsigned int SCR_HEIGHT = 600;

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";
const char *fragmentShaderSource = "#version 330 core\n"
    "out vec4 FragColor;\n"
    "void main()\n"
    "{\n"
    "   FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"
    "}\n\0";

int main()
{
    // glfw: initialize and configure
    // ------------------------------
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

#ifdef __APPLE__
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE); // uncomment this statement to fix compilation on OS X
#endif

    // glfw window creation
    // --------------------
    GLFWwindow* window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "LearnOpenGL", NULL, NULL);
    if (window == NULL)
    {
        std::cout << "Failed to create GLFW window" << std::endl;
        glfwTerminate();
        return -1;
    }
    glfwMakeContextCurrent(window);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    // glad: load all OpenGL function pointers
    // ---------------------------------------
    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        std::cout << "Failed to initialize GLAD" << std::endl;
        return -1;
    }


    // build and compile our shader program
    // ------------------------------------
    // vertex shader
    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
    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
    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);

    // set up vertex data (and buffer(s)) and configure vertex attributes
    // ------------------------------------------------------------------
    float vertices[] = {
        -0.5f, -0.5f, 0.0f, // left  
         0.5f, -0.5f, 0.0f, // right 
         0.0f,  0.5f, 0.0f  // top   
    }; 

    unsigned int VBO, VAO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    // bind the Vertex Array Object first, then bind and set vertex buffer(s), and then configure vertex attributes(s).
    glBindVertexArray(VAO);

    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);

    // 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); 

    // 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); 


    // uncomment this call to draw in wireframe polygons.
    //glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

    // render loop
    // -----------
    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, 3);
        // glBindVertexArray(0); // no need to unbind it every time 
 
        // 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);

    // glfw: terminate, clearing all previously allocated GLFW resources.
    // ------------------------------------------------------------------
    glfwTerminate();
    return 0;
}

// process all input: query GLFW whether relevant keys are pressed/released this frame and react accordingly
// ---------------------------------------------------------------------------------------------------------
void processInput(GLFWwindow *window)
{
    if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
        glfwSetWindowShouldClose(window, true);
}

// glfw: whenever the window size changed (by OS or user resize) this callback function executes
// ---------------------------------------------------------------------------------------------
void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
    // make sure the viewport matches the new window dimensions; note that width and 
    // height will be significantly larger than specified on retina displays.
    glViewport(0, 0, width, height);
}

附：索引缓冲对象

• EBO绑定到GL_ElEMENT_ARRAY_BUFFER
• EBO指定渲染顶点的顺序

//0代表第一个顶点以此类推
		unsigned int indices[] = { // 注意索引从0开始! 
		    0, 1, 3, // 第一个三角形
		    1, 2, 3  // 第二个三角形
		};
		unsigned int EBO;
		glGenBuffers(1, &EBO);
		glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
		glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

同样可以绑定到VAO中

在渲染时使用glDrawElements而不是glDrawArrays

glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0)