Is using Vertex Buffer Object's for very dynamic data a good idea performance-wise?

Question

I have many particles who's vertices change every frame. The vertices are currently being drawn using a vertex array in 'client' memory. What performance characteristics can I expect if I use a vertex buffer object?

Since I have to use a number of glBuffersubData's to update the particle vertices, I am therefore transferring the vertices to video memory every frame anyway right(like i would if i use a regular vertex array)? Is there any benefit to VBO's in this case?

This is for iOS devices. The actual draw call: glDrawElements(GL_POINTS,num_particles,GL_UNSIGNED_SHORT,pindices);

Should I use GL_STREAM_DRAW or GL_DYNAMIC_DRAW?

Answer 1

Apple's documentation appears to recommend VBOs in all situations. If you're using ES 2.x then the GL_STREAM_DRAW vertex buffer type is explicitly for "when your application needs to create transient geometry that is rendered a small number of times and then discarded. This is most useful when your application must dynamically change vertex data every frame in a way that cannot be performed in a vertex shader." Use of glBufferSubData is then directly advocated.

Logically, I guess the only difference between supplying the data completely afresh and sending it to an existing GL_STREAM_DRAW or GL_DYNAMIC_DRAW buffer is that your space in the memory map (GPU or CPU, depending on the chip — MBXs don't really do VBOs but Apple supports them for other performance reasons) can be allocated once rather than allocated and released every frame.

Using the alignment and packing tips given in that document is likely to give a better improvement than a switch to VBOs, since otherwise the CPU just has to unpack and repack data upon glDrawElements. Though quite probably you're already aware of that and I appreciate that it isn't directly part of the question — I mainly throw it in as a comparative guess about performance benefits.

Answer 2

By setting VBOs properly, you are using optimal way of transferring data to the GPU. By doing so, you might skip some driver processing. The only way to see how much you get of improvement you get is to measure. It is different from card to card.

For VBO how-to, see this : VBO tutorial

EDIT Forgot to answer the question : yes, it is a good idea. But first measure.