【OpenGL】三角形

步骤

1. 初始化顶点数组对象VAO
2. 分配顶点缓冲对象VBO
3. 将顶点数据载入缓冲对象中 glBufferData()
4. 链接顶点属性 glVertexAttribPointer（指定了顶点着色器的变量与我们存储在缓冲对象VBO中数据的关系）

---

顶点数组

GLfloat vertices[] = {
    -0.5f, -0.5f, 0.0f,
    0.5f, -0.5f, 0.0f,
    0.0f, 0.5f, 0.0f
};

因为是3D坐标，所以有三个轴(x,y,z)，三个轴范围在-1~1之间。这里统一将z设为0，这样三个点的深度为0，所以看起来就是2D平面上的三角形。

顶点缓冲对象VBO

GLuint VBO;
glGenBuffers(1, &VBO);
 	
glBindBuffer(GL_ARRAY_BUFFER, VBO);  //把缓冲绑定到GL_ARRAY_BUFFER上
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW); //复制顶点数据到内存

glBufferData是一个用来把用户定义数据复制到当前绑定缓冲的函数。它的第一个参数是我们希望把数据复制到上面的缓冲类型：顶点缓冲对象当前绑定到GL_ARRAY_BUFFER目标上。第二个参数指定我们希望传递给缓冲的数据的大小（字节）；用一个简单的sizeof计算出顶点数据就行。第三个参数是我们希望发送的真实数据。

第四个参数指定了我们希望显卡如何管理给定的数据。有三种形式：

• GL_STATIC_DRAW：数据不会或几乎不会改变。
• GL_DYNAMIC_DRAW：数据会被改变很多。
• GL_STREAM_DRAW：数据每次绘制时都会改变。

顶点着色器

#version 330 core   //版本声明，opengl3.3
 
layout (location = 0) in vec3 position;
 
void main()
{
    gl_Position = vec4(position.x, position.y, position.z, 1.0);
}

编译

GLuint vertexShader;
vertexShader = glCreateShader(GL_VERTEX_SHADER);
glShaderSource(vertexShader, 1, &vertexShaderSource, null);
glCompileShader(vertexShader);

//错误检查
GLint success;
GLchar 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;
}

像素着色器

#version 330 core

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

编译

GLuint fragmentShader;
fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(fragmentShader, 1, &fragmentShaderSource, null);
glCompileShader(fragmentShader);

着色器程序

GLuint shaderProgram;
shaderProgram = glCreateProgram();
glAttachShader(shaderProgram, vertexShader);
glAttachShader(shaderProgram, fragmentShader);
glLinkProgram(shaderProgram);
glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
 
if(!success) {
    glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);
    ...

 	
glUseProgram(shaderProgram); //将着色器对象链接到程序
//删除着色器对象
glDeleteShader(vertexShader);
glDeleteShader(fragmentShader);
}

VAO

// ..:: 初始化代码 (一次完成 (除非你的物体频繁改变)) :: ..
 
// 1. 绑定VAO
glBindVertexArray(VAO);
 
// 2. 把顶点数组复制到缓冲中提供给OpenGL使用
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
 
// 3. 设置顶点属性指针
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), (GLvoid * )0);
glEnableVertexAttribArray(0);
 
//4. 解绑VAO
glBindVertexArray(0);
 
[...]
 
// ..:: 绘制代码 (in Game loop) :: ..
 
// 5. 绘制物体
glUseProgram(shaderProgram);
glBindVertexArray(VAO);
someOpenGLFunctionThatDrawsOurTriangle();
glBindVertexArray(0);

绘制

glUseProgram(shaderProgram);
glBindVertexArray(VAO);
glDrawArrays(GL_TRIANGLES, 0, 3);
glBindVertexArray(0);

所有代码

#include<iostream>
// Include GLEW
#include <GL/glew.h>
// Include GLFW
#include <glfw3.h>
GLFWwindow* window;

// Function prototypes
void key_callback(GLFWwindow* window, int key, int scancode, int action, int mode);

// Window dimensions
const GLuint WIDTH = 800, HEIGHT = 600;

// Shaders
const GLchar* vertexShaderSource = "#version 330 core\n"
"layout (location = 0) in vec3 position;\n"
"void main()\n"
"{\n"
"gl_Position = vec4(position.x, position.y, position.z, 1.0);\n"
"}\0";
const GLchar* fragmentShaderSource = "#version 330 core\n"
"out vec4 color;\n"
"void main()\n"
"{\n"
"color = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"
"}\n\0";

// The MAIN function, from here we start the application and run the game loop
int main()
{
	// Init GLFW
	glfwInit();
	// Set all the required options for GLFW
	glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
	glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
	glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
	glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);

	// Create a GLFWwindow object that we can use for GLFW's functions
	GLFWwindow* window = glfwCreateWindow(WIDTH, HEIGHT, "LearnOpenGL", nullptr, nullptr);
	glfwMakeContextCurrent(window);

	// Set the required callback functions
	glfwSetKeyCallback(window, key_callback);

	// Set this to true so GLEW knows to use a modern approach to retrieving function pointers and extensions
	glewExperimental = GL_TRUE;
	// Initialize GLEW to setup the OpenGL Function pointers
	glewInit();

	// Define the viewport dimensions
	int width, height;
	glfwGetFramebufferSize(window, &width, &height);
	glViewport(0, 0, width, height);


	// Build and compile our shader program
	// Vertex shader
	GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
	glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
	glCompileShader(vertexShader);
	// Check for compile time errors
	GLint success;
	GLchar 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
	GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
	glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
	glCompileShader(fragmentShader);
	// Check for compile time 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
	GLuint 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 attribute pointers
	GLfloat vertices[] = {
		-0.5f, -0.5f, 0.0f, // Left  
		0.5f, -0.5f, 0.0f, // Right 
		0.0f, 0.5f, 0.0f  // Top   
	};
	GLuint VBO, VAO;
	glGenVertexArrays(1, &VAO);
	glGenBuffers(1, &VBO);
	// Bind the Vertex Array Object first, then bind and set vertex buffer(s) and attribute pointer(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(GLfloat), (GLvoid*)0);
	glEnableVertexAttribArray(0);

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

	glBindVertexArray(0); // Unbind VAO (it's always a good thing to unbind any buffer/array to prevent strange bugs)

	// Game loop
	while (!glfwWindowShouldClose(window))
	{
		// Check if any events have been activiated (key pressed, mouse moved etc.) and call corresponding response functions
		glfwPollEvents();

		// Render
		// Clear the colorbuffer
		glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
		glClear(GL_COLOR_BUFFER_BIT);

		// Draw our first triangle
		glUseProgram(shaderProgram);
		glBindVertexArray(VAO);
		glDrawArrays(GL_TRIANGLES, 0, 3);
		glBindVertexArray(0);

		// Swap the screen buffers
		glfwSwapBuffers(window);
	}
	// Properly de-allocate all resources once they've outlived their purpose
	glDeleteVertexArrays(1, &VAO);
	glDeleteBuffers(1, &VBO);
	// Terminate GLFW, clearing any resources allocated by GLFW.
	glfwTerminate();
	return 0;
}

// Is called whenever a key is pressed/released via GLFW
void key_callback(GLFWwindow* window, int key, int scancode, int action, int mode)
{
	if (key == GLFW_KEY_ESCAPE && action == GLFW_PRESS)
		glfwSetWindowShouldClose(window, GL_TRUE);
}