帮助我理解OpenGL glGenBuffers

问题描述

这是交易。如果我离开代码与glGenBuffers（1，vertexBuffers），代码编译是工程。但是，我认为它应该是2，因为vertexBuffers是大小两。

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

我错过了什么？

代码如下：

-(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 不工作非常像你期望的。当调用 glGenBuffers 时，实际上并不会创建任何内容。

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.

实际的object直到你调用 glBindBuffer 。所以你可以组成任何你喜欢的整数，并将其传递给glBindBuffer和一个有效的缓冲区将创建在该索引。 glGenBuffers 实际上不是必需的，它只是作为一个方便的函数给你一个未使用的整数。

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.

所以如果你只是创建一个随机整数数组，只要没有重叠，你可以使用它作为你的缓冲区列表，而不调用 glGenBuffers 。这就是为什么你的代码工作，无论你告诉glGenBuffers创建1或2缓冲区。只要你有两个不同名字的缓冲区，整数来自哪里都没有关系。

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.

一个小示范：

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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