What's 'glBufferData' for in OpenGL ES?

Question

I run across a sample OpenGL code that I ported to OpenGL ES 2.0 (there wasn't much to be done actually), but I cannot help wondering what the glBufferData function is for. The original source is like that:

glBufferData(GL_ARRAY_BUFFER, sizeof(GLfloat) * 2 * 6, quad, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(GLfloat) * 2, (void *) 0);

But I can successfully simplify it as:

glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(GLfloat) * 2, quad);

That is, I can omit the glBufferData function just by using a valid pointer to the quad array in glVertexAttribPointer.

So, could anyone explain what's the glBufferData function for? From what I'm doing it seems to be redundant but that must be because of my serious lack of knowledge of the API. As a matter of fact I tried reading the docs at khronos.org but this didn't help me understand its use.

Answer 1

If you are reusing the same data in multiple frames, using glBufferData as part of your setup/initialization will transfer data from the CPU to the GPU only once. Whereas glVertexAttribPointer must be called every frame, so using it to transfer data results in using a lot more bus bandwidth.

If you're updating the attribute array every frame, there's not much advantage one way or the other.