When to use glBufferData in OpenGL

Question

I have just started learning about vertex buffer objects in C++. I am reading a book about OpenGL that says that VBO rendering is more efficient than other forms of rendering because the data is stored on the GPU instead of on the heap. However, I am confused how this could be if you still have to load an array of data from the heap to the GPU. Every few seconds, I update the vertex data of my program, which means that I must then use glBufferData() to refresh the data to update to the new state. I don't see how this is more efficient than just rendering the array normally. I was wondering if I am calling glBufferData() more than is necessary, or if there is a better way to update the vertex data directly on the GPU.

Answer 1

Well, glBufferData (...) does more than you think. True it supplies data to a VBO, but the more important point is that it allocates memory on the server side (GPU for all intents and purposes) for vertex storage.

In your example, the number of vertices, and therefore size required to store them does not seem to change when you refresh your data. What you should actually be doing is calling glBufferSubData (...) to update the data without re-allocating space for it. Coupled with a correct usage flag (e.g. GL_DYNAMIC_DRAW) this can be much more efficient than copying from client to server everytime something is drawn.

Think of glBufferData (...) as a combination of malloc (...) and memcpy (...). glBufferSubData (...) on the other hand is memcpy (...). To this, end you can even do memory mapping of VBOs into your application's address space without having to allocate storage in both the client and server using glMapBuffer (...) and glUnmapBuffer (...), which are analogous to mmap (...) and munmap (...).

Answer 2

You should try to avoid modifying your vertex data every few frames. Vertex/fragment shaders are specifically there to allow you to modify your geometry on the fly, with some limitations of course.

However, in the simplest case (if you don't care about maximizing your performance), it is entirely possible to rewrite the buffer on every frame, and it should still beat calling glBegin..glEnd for every object.