协助我了解OpenGL glGenBuffers() [英] Assist me to understand OpenGL glGenBuffers()

问题描述

这很重要:如果我将代码留给glGenBuffers(1, vertexBuffers)，则代码可以编译并运行.但是，我认为应该为2，因为vertexBuffers的大小为2.

Here is the deal: If I leave the code with glGenBuffers(1, vertexBuffers), the code compiles and works. But, I thought that it should be 2 since vertexBuffers is of size two.

我错过了什么吗?

以下代码:

-(void)drawRect:(NSRect)dirtyRect
{    
    // get program ID for shader program
    GLuint programID = [self loadShaders];

    // get new dimensions
    NSSize dim = [self frame].size;

    // clear the background with color
    glClearColor(0.0, 0.0, 0.0, 0.4);
    glDepthRange(0.1, 100.0);
    glViewport(0, 0, dim.width, dim.height);
    glClear(GL_COLOR_BUFFER_BIT);

    // vertex data
    GLfloat vertexPositionData[] = {-dim.width/2, -dim.height/2, 0.0, 5.0,
        dim.width/2, -dim.height/2, 0.0, 5.0,
        0.0, dim.height/2, 0.0, 5.0};

    GLfloat vertexColorData[] = {1.0, 0.0, 0.0, 1.0, 0.0, 1.0, 0.0, 0.5, 0.0, 0.0, 1.0, 0.5};

    GLfloat scaleMatrixData[] = {1/(dim.width/2), 0.0, 0.0, 0.0,
        0.0, 1/(dim.height/2), 0.0, 0.0,
        0.0, 0.0, 1.0, 0.0,
        0.0, 0.0, 0.0, 1.0};

    GLint scaleMatrixUniform = glGetUniformLocation(programID, "scaleMatrix");

    // generate a buffer for our triangle
    glGenBuffers(1, vertexBuffers);

    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffers[0]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertexPositionData), vertexPositionData, GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffers[1]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertexColorData), vertexColorData, GL_STATIC_DRAW);
    //glBindBuffer(GL_ARRAY_BUFFER, 0);

    glUseProgram(programID);

    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffers[0]);
    glEnableVertexAttribArray(VERTEX_POS_INDEX);
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffers[1]);
    glEnableVertexAttribArray(VERTEX_COLOR_INDEX);
    glVertexAttribPointer(VERTEX_POS_INDEX, VERTEX_POS_SIZE, GL_FLOAT, GL_FALSE, 0, vertexPositionData);
    glVertexAttribPointer(VERTEX_COLOR_INDEX, VERTEX_COLOR_SIZE, GL_FLOAT, GL_FALSE, 0, vertexColorData);

    glUniformMatrix4fv(scaleMatrixUniform, 1, GL_FALSE, scaleMatrixData);

    glDrawArrays(GL_TRIANGLES, 0, 3);

    glDisableVertexAttribArray(0);
    glDisableVertexAttribArray(1);
    glUseProgram(0);

    // flush buffer
    glFlush();
    [[self openGLContext] flushBuffer];
}

推荐答案

glGenBuffers不能像您期望的那样工作.当您调用glGenBuffers时，它实际上并没有创建任何东西.它只是返回一个当前不用作缓冲区名称的整数列表.

glGenBuffers doesn't work quite like what you expect. When you call glGenBuffers, it doesn't actually create anything. It just returns a list of integers that are not currently used as buffer names.

在调用glBindBuffer之前，不会创建实际的对象".因此，您只需组成您喜欢的任何整数，然后将其传递给glBindBuffer，就会在该索引处创建一个有效的缓冲区.实际上根本不需要glGenBuffers，它只是一个方便的功能，可以为您提供未使用的整数.

The actual 'object' is not created until you call glBindBuffer. So you can just make up any integer you like and pass it to glBindBuffer and a valid buffer will be created at that index. glGenBuffers is actually not required at all, it's just there as a convenience function to give you an unused integer.

因此，如果您只创建一个随机整数数组，只要它们之间没有重叠，就可以将其用作缓冲区列表，而无需调用glGenBuffers.这就是为什么无论告诉glGenBuffers创建1个还是2个缓冲区，代码都可以工作的原因.只要您有两个具有两个不同名称的缓冲区，整数的来源就无关紧要.

So if you just create an array of random integers, as long as none of them overlap, you can use that as your list of buffers without calling glGenBuffers. That's why your code works whether you tell glGenBuffers to create 1 or 2 buffers. As long as you have two buffers with two different names, it doesn't matter where the integer came from.

一些示范:

int buf;
glGenBuffers(1, &buf);
glIsBuffer(buf); //FALSE - buffer has not been created yet
glBindBuffer(GL_ARRAY_BUFFER, buf);
glIsBuffer(buf); //TRUE - buffer created on bind

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