(八)EBO和glDrawElements

EBO

EBO(Element Buffer Object)：元素缓冲对象，用于存储顶点绘制顺序索引号的GPU显存区域

cpp 复制代码

	unsigned int indices[] = {
		0, 1, 2,
		2, 1, 3
	};
	//EBO创建和绑定
	GLuint ebo = 0;
	glGenBuffers(1, &ebo);
	glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);
	glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

	//VAO创建和绑定
	GLuint vao = 0;
	glGenVertexArrays(1, &vao);
	glBindVertexArray(vao);

	//加入ebo到当前的vao
	glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);

绑定vao之后，GL状态机就认为后续操作针对的是当前这个vao，在当前vao状态下绑定任何vbo或者ebo，都会被记录到当前vao当中。

glDrawElements

cpp 复制代码

#include <glad/glad.h>//glad必须在glfw头文件之前包含
#include <GLFW/glfw3.h>
#include <iostream>

void frameBufferSizeCallbakc(GLFWwindow* window, int width, int height)
{
	glViewport(0, 0, width, height);
}
void glfwKeyCallback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
}

GLuint program = 0;
GLuint vao = 0;
void prepareVAO()
{
	//1 准备positions
	float positions[] = {
		-0.5f, -0.5f, 0.0f,
		0.5f, -0.5f, 0.0f,
		0.0f,  0.5f, 0.0f,
		0.5f,  0.5f, 0.0f,
	};

	unsigned int indices[] = {
		0, 1, 2,
		2, 1, 3
	};

	//2 VBO创建
	GLuint vbo = 0;
	glGenBuffers(1, &vbo);
	glBindBuffer(GL_ARRAY_BUFFER, vbo);
	glBufferData(GL_ARRAY_BUFFER, sizeof(positions), positions, GL_STATIC_DRAW);

	//3 EBO创建
	GLuint ebo = 0;
	glGenBuffers(1, &ebo);
	glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);
	glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

	//4 VAO创建
	vao = 0;
	glGenVertexArrays(1, &vao);
	glBindVertexArray(vao);

	//5 绑定vbo ebo 加入属性描述信息
	//5.1 加入位置属性描述信息
	glBindBuffer(GL_ARRAY_BUFFER, vbo);
	glEnableVertexAttribArray(0);
	glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);

	//5.2 加入ebo到当前的vao
	glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);

	glBindVertexArray(0);
}
void prepareShader() {
	//1 完成vs与fs的源代码，并且装入字符串
	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";


	//2 创建Shader程序（vs、fs）
	GLuint vertex, fragment;
	vertex = glCreateShader(GL_VERTEX_SHADER);
	fragment = glCreateShader(GL_FRAGMENT_SHADER);


	//3 为shader程序输入shader代码
	glShaderSource(vertex, 1, &vertexShaderSource, NULL);
	glShaderSource(fragment, 1, &fragmentShaderSource, NULL);

	int success = 0;
	char infoLog[1024];
	//4 执行shader代码编译 
	glCompileShader(vertex);
	//检查vertex编译结果
	glGetShaderiv(vertex, GL_COMPILE_STATUS, &success);
	if (!success) {
		glGetShaderInfoLog(vertex, 1024, NULL, infoLog);
		std::cout << "Error: SHADER COMPILE ERROR --VERTEX" << "\n" << infoLog << std::endl;
	}

	glCompileShader(fragment);
	//检查fragment编译结果
	glGetShaderiv(fragment, GL_COMPILE_STATUS, &success);
	if (!success) {
		glGetShaderInfoLog(fragment, 1024, NULL, infoLog);
		std::cout << "Error: SHADER COMPILE ERROR --FRAGMENT" << "\n" << infoLog << std::endl;
	}

	//5 创建一个Program壳子
	program = glCreateProgram();

	//6 将vs与fs编译好的结果放到program这个壳子里
	glAttachShader(program, vertex);
	glAttachShader(program, fragment);

	//7 执行program的链接操作，形成最终可执行shader程序
	glLinkProgram(program);

	//检查链接错误
	glGetProgramiv(program, GL_LINK_STATUS, &success);
	if (!success) {
		glGetProgramInfoLog(program, 1024, NULL, infoLog);
		std::cout << "Error: SHADER LINK ERROR " << "\n" << infoLog << std::endl;
	}

	//清理
	glDeleteShader(vertex);
	glDeleteShader(fragment);
}



void render()
{
	//执行opengl画布清理操作
	glClear(GL_COLOR_BUFFER_BIT);

	//1.绑定当前的program
	glUseProgram(program);
	//2 绑定当前的vao
	glBindVertexArray(vao);
	//3 发出绘制指令
	//glDrawArrays(GL_TRIANGLE_STRIP, 0, 6);
	glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, (void*)(3 * sizeof(int)));
}
int main()
{
	//初始化glfw环境
	glfwInit();
	//设置opengl主版本号
	glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
	//设置opengl次版本号
	glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
	//设置opengl启用核心模式
	glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

	//创建窗体对象
	GLFWwindow* window = glfwCreateWindow(800, 600, "lenarnOpenGL", nullptr, nullptr);
	//设置当前窗体对象为opengl的绘制舞台
	glfwMakeContextCurrent(window);
	//窗体大小回调
	glfwSetFramebufferSizeCallback(window, frameBufferSizeCallbakc);
	//键盘相应回调
	glfwSetKeyCallback(window, glfwKeyCallback);

	//使用glad加载所有当前版本opengl的函数
	if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
	{
		std::cout << "初始化glad失败" << std::endl;
		return -1;
	}
	;
	//设置opengl视口大小和清理颜色
	glViewport(0, 0, 800, 600);
	glClearColor(0.2f, 0.3f, 0.3f, 1.0f);

	//着色器
	prepareShader();
	//vao
	prepareVAO();

	//执行窗体循环
	while (!glfwWindowShouldClose(window))
	{
		//接受并分发窗体消息
		//检查消息队列是否有需要处理的鼠标、键盘等消息
		//如果有的话就将消息批量处理，清空队列
		glfwPollEvents();
		//渲染操作
		render();
		//切换双缓存
		glfwSwapBuffers(window);
	}

	//推出程序前做相关清理
	glfwTerminate();
	return 0;
}

cpp 复制代码

glDrawElements(GLenum mode, GLsizei count, GLenum type, const void *indices);

mode：绘制模式GL_TRIANGLES，GL_LINES

count：绘制索引数组中的几个点

type：索引的数据类型

indices：

如果使用了ebo，通常填写0；

如果使用了ebo，其不填写0，则表示索引内偏移

如果不使用ebo，可以直接传入索引数组