Best practices for generating GL VBO data

Question

I'm new to OpenGL ES and am using a VBO to speed up the rendering of a large number of objects. Currently I query a quadtree to determine which objects should appear in the viewport and use that data to update my VBO, like so:

glBufferData(
     GL_ARRAY_BUFFER,
     numObjects*sizeof(ObjectStruct),
     objects, 
     GL_STREAM_DRAW
); 

But is this the most efficient way of handling things? Should I instead avoid querying the quadtree and simply generate a static VBO containing information on all of my objects. If I do this, will GL be intelligent enough to cull objects which are positioned outside of the viewport?

Answer 1

GL won't know what's outside of the view frustum until vertex transformations have been executed. Objects outside of the view frustum will be culled to avoid redundant operations later in the pipeline. This does however mean that the driver and GPU have redundantly processed draws that will have no affect on the final image.

I'd recommend avoiding regular VBO updates, as each will either cause the graphics driver to stall or cause an additional memory allocations under the hood (enabling the GPU to continue rendering previous frames without interruption). I've written a blog post on the subject which may help: http://blog.imgtec.com/powervr/how-to-improve-your-renderer-on-powervr-based-platforms

The best approach depends on your use case. Ideally, you would batch a number of objects within a quadtree/octree node into a single draw. Doing so would enable your engine to reduce the amount of rendering work submitted without incurring the cost of VBO modifications