小白学opengl 第五课 之 索引缓冲对象



  • 索引缓冲对象(Element Buffer Objects,EBO)

    为什么使用EBO

    正常画一个立方体,需要8个顶点,合计6个面

    0_1526459951741_b44cc1f3-1cfb-4159-84ec-7621a7ce6414-image.png

    但实际我们是用了4*6=24个点来画的,因为4个点组成一个面,我们能看出来,有很多重复点,为了能节省内存空间,我们想只用8个点描画立方体,这需要用到EBO

    VertexData vc[] = {
            //正面
            {QVector3D(-0.5f, 0.0f, 0.5f), QVector3D(1.0f, 0.0f, 0.0f)},     //1
            {QVector3D(0.5f, 0.0f, 0.5f), QVector3D(0.0f, 1.0f, 0.0f)},      //2
            {QVector3D(0.5f, 1.0f, 0.5f), QVector3D(0.0f, 0.0f, 1.0f)},      //3
            {QVector3D(-0.5f, 1.0f, 0.5f), QVector3D(1.0f, 1.0f, 1.0f)},     //4
    
            //右面
            {QVector3D(0.5f, 0.0f, 0.5f),  QVector3D(0.0f, 1.0f, 0.0f)},     //2
            {QVector3D(0.5f, 0.0f, -0.5f), QVector3D(1.0f, 1.0f, 0.0f)},     //5
            {QVector3D(0.5f, 1.0f, -0.5f), QVector3D(0.0f, 1.0f, 1.0f)},     //6
            {QVector3D(0.5f, 1.0f, 0.5f),  QVector3D(0.0f, 0.0f, 1.0f)},     //3
    
            //左面
            {QVector3D(-0.5f, 0.0f, 0.5f), QVector3D(1.0f, 0.0f, 0.0f)},     //1
            {QVector3D(-0.5f, 0.0f, -0.5f),QVector3D(1.0f, 0.0f, 1.0f)},     //8
            {QVector3D(-0.5f, 1.0f, -0.5f),QVector3D(1.0f, 0.6f, 0.0f)},     //7
            {QVector3D(-0.5f, 1.0f, 0.5f), QVector3D(1.0f, 1.0f, 1.0f)},     //4
    
            //背面
            {QVector3D(0.5f, 0.0f, -0.5f),   QVector3D(1.0f, 1.0f, 0.0f)},   //5
            {QVector3D(0.5f, 1.0f, -0.5f),   QVector3D(0.0f, 1.0f, 1.0f)},   //6
            {QVector3D(-0.5f, 1.0f, -0.5f),  QVector3D(1.0f, 0.6f, 0.0f)},   //7
            {QVector3D(-0.5f, 0.0f, -0.5f),  QVector3D(1.0f, 0.0f, 1.0f)},   //8
    
            //顶面
            {QVector3D(0.5f, 1.0f, 0.5f),    QVector3D(0.0f, 0.0f, 1.0f)},   //3
            {QVector3D(0.5f, 1.0f, -0.5f),   QVector3D(0.0f, 1.0f, 1.0f)},   //6
            {QVector3D(-0.5f, 1.0f, -0.5f),  QVector3D(1.0f, 0.6f, 0.0f)},   //7
            {QVector3D(-0.5f, 1.0f, 0.5f),   QVector3D(1.0f, 1.0f, 1.0f)},   //4
    
            //底面
            {QVector3D(0.5f, 0.0f, 0.5f),    QVector3D(0.0f, 1.0f, 0.0f)},   //2
            {QVector3D(0.5f, 0.0f, -0.5f),   QVector3D(1.0f, 1.0f, 0.0f)},   //5
            {QVector3D(-0.5f, 0.0f, -0.5f),  QVector3D(1.0f, 0.0f, 1.0f)},   //8
            {QVector3D(-0.5f, 0.0f, 0.5f),   QVector3D(1.0f, 0.0f, 0.0f)},   //1
        };
    

    EBO原理

    //顶点有8个
    VertexData vcs[] = {
            //正面
            {QVector3D(-0.5f, 0.0f, 0.5f), QVector3D(1.0f, 0.0f, 0.0f)},     //1
            {QVector3D(0.5f, 0.0f, 0.5f), QVector3D(0.0f, 1.0f, 0.0f)},      //2
            {QVector3D(0.5f, 1.0f, 0.5f), QVector3D(0.0f, 0.0f, 1.0f)},      //3
            {QVector3D(-0.5f, 1.0f, 0.5f), QVector3D(1.0f, 1.0f, 1.0f)},     //4
    
            {QVector3D(0.5f, 0.0f, -0.5f), QVector3D(1.0f, 1.0f, 0.0f)},     //5
            {QVector3D(0.5f, 1.0f, -0.5f), QVector3D(0.0f, 1.0f, 1.0f)},     //6
            {QVector3D(-0.5f, 1.0f, -0.5f),QVector3D(1.0f, 0.6f, 0.0f)},     //7
            {QVector3D(-0.5f, 0.0f, -0.5f),QVector3D(1.0f, 0.0f, 1.0f)},     //8
        };
    
    //索引 对应8个面,每个面说明使用哪4个顶点
        GLuint indices[] = { // 起始于0!
            0, 1, 2, 3, // face 1
            1, 4, 5, 2,  // face 2
            0, 7, 6, 3,  // face 3
            4, 5, 6, 7,  // face 4
            2, 5, 6, 3,  // face 5
            1, 4, 7, 0,  // face 6
        };
    

    代码顺序

    创建

    //1 使用glGenBuffers函数生成一个缓冲ID    
        glGenVertexArrays(1, &VAO);
        glGenBuffers(1, &VBO);
        glGenBuffers(1, &EBO);
    

    绑定VAO

    //2 绑定vao
        glBindVertexArray(VAO);
    

    绑定VBO,EBO

    //3 使用glBindBuffer函数把新创建的缓冲绑定到GL_ARRAY_BUFFER缓冲类型上
        glBindBuffer(GL_ARRAY_BUFFER, VBO); //(绑定和解绑的顺序很重要,勿更改)
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    

    复制数据

     //4 把用户定的义数据复制到当前绑定缓冲的函数
        glBufferData(GL_ARRAY_BUFFER, sizeof(vcs), vcs, GL_STATIC_DRAW);
        glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
    

    链接属性

    //5 链接顶点属性
        //indx: 属性名
        //size: 顶点大小
        //type: 数据类型
        //normalized:数据被标准化
        //stride: 步长
        //ptr: 数据在缓冲中起始位置的偏移量
        glVertexAttribPointer(m_posAttr, 3, GL_FLOAT, GL_FALSE, sizeof(VertexData), (GLvoid*)0);
        glVertexAttribPointer(m_colAttr, 3, GL_FLOAT, GL_FALSE, sizeof(VertexData), (GLvoid*)sizeof(QVector3D));
    

    解绑 VBO

    //6 解绑缓存着色器(绑定和解绑的顺序很重要,勿更改)
        glBindBuffer(GL_ARRAY_BUFFER, 0);
    

    解绑VAO

    //7 解绑VAO
        glBindVertexArray(0);
    

    PS: 这里我颠倒了顺序,并没有出现问题,可能qt给优化过,不解绑也没事,另外EBO不用解绑,就算要解绑,也要在VAO解绑后

    绘制过程

    绑定vao

    //1 绑定vao
        glBindVertexArray(VAO);
    

    开启属性

    //2 开启顶点属性
        glEnableVertexAttribArray(0);
        //颜色值
        glEnableVertexAttribArray(1);
    

    绘制四边形

    //3 绘制四边形
        //24个索引值
        glDrawElements(GL_QUADS, 24, GL_UNSIGNED_INT, (GLvoid*)0);
    

    停用属性

    //4 停用对应的顶点属性数组
        glDisableVertexAttribArray(1);
        glDisableVertexAttribArray(0);
    

    解绑VAO

    //5 解绑VAO
        glBindVertexArray(0);
    

    源代码

    https://gitee.com/chen227/opengl_OpenGLFunctions5

    0_1526620699568_6cc1d02e-137c-4b7b-a0ce-4275e76ecfc0-image.png


 

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