OpenGL 2.1: glMapBuffer and usage hints

Question

I've been using glBufferData, and it makes sense to me that you'd have to specify usage hints (e.g. GL_DYNAMIC_DRAW).

However, it was recently suggested to me on Stack Overflow that I use glMapBuffer or glMapBufferRange to modify non-contiguous blocks of vertex data.

When using glMapBuffer, there does not seem to be any point at which you specify a usage hint. So, my questions are as follows:

• Is it valid to use glMapBuffer on a given VBO if you've never called glBufferData on that VBO?
• If so, how does OpenGL guess the usage, since it hasn't been given a hint?
• What are the advantages/disadvantages of glMapBuffer vs glBufferData? (I know they don't do exactly the same thing. But it seems that by getting a pointer with glMapBuffer and then writing to that address, you can do the same thing glBufferData does.)

Answer 1

• Is it valid to use glMapBuffer on a given VBO if you've never called glBufferData on that VBO?

No, because to map some memory, it must be allocated first.

• If so, how does OpenGL guess the usage, since it hasn't been given a hint?

It doesn't. You must call glBufferData at least once to initialize the buffer object. If you don't want to actually upload data (because you're going to use glMapBuffer), just pass a null pointer for the data pointer. This works just like with glTexImage, where a buffer/texture object is created, to be filled with either glBufferSubData/glTexSubImage, or in the case of a buffer object as well as through a memory mapping.

• What are the advantages/disadvantages of glMapBuffer vs glBufferData? (I know they don't do exactly the same thing. But it seems that by getting a pointer with glMapBuffer and then writing to that address, you can do the same thing glBufferData does.)

glMapBuffer allows you to write to the buffer asynchronously from another thread. And for some implementations it may be possible, that the OpenGL driver gives your process direct access to DMA memory of the GPU or even better to the memory of the GPU itself. For example on SoC architectures with integrated graphics.