iphone - How can I optimize the rendering of a large model in OpenGL ES 1.1?

Question

I just finished implementing VBO's in my 3D app and saw a roughly 5-10x speed increase in rendering. What used to render at 1-2 frames per second now renders at 10-11 frames per second.

My question is, are there any further improvements I can make to increase rendering speed? Will triangle strips make a big difference? Currently vertices are not being shared between faces, each faces vertices are unique but overlapping.

My Device Utilization is 100%, Tiler Utilization is 100%, Renderer Utilization is 11%, and resource bytes is 114819072. This is rendering 912,120 faces on a CAD model.

Any suggestions?

Answer 1

A Tiler Utilization of 100% indicates that your bottleneck is in the size of the geometry being sent to the GPU. Whatever you can do to shrink the geometry size can lead to an almost linear reduction in rendering time, in my experience. These tuning steps have worked for me in the past:

• If you're not already, you could look at using indexing, which might cut down on geometry by eliminating some redundant vertices. The PowerVR GPUs in the iOS devices are optimized for using indexed geometry, as well.

• Try using a smaller data type for your vertex information. I found that I could use GLshort instead of GLfloat for my vertices and normals without losing much precision in the rendering. This will significantly compact your geometry and lead to a nice speed boost in rendering.

• Bin similarly colored vertices and render them as one group at a set color, rather than supplying per-vertex color information. The overhead from the few extra draw calls this requires will be vastly outweighed by the speedup you get from not having to send all that color information. I saw a ~18% reduction in rendering time by binning the colors in one of my larger models.

• You're already using VBOs, so you've taken advantage of that optimization.

• Don't halt the rendering pipeline at any point. Cut out anything that reads the current state, like all glGet* calls, because they really mess with the flow of the PowerVR GPUs.

There are other things you can do that will lead to smaller performance improvements, like using interleaved vertex, normal, texture data in your VBOs, aligning your data to 4 byte boundaries, etc., but the ones above are what I've found to have the largest impact in the tuning of my own OpenGL ES 1.1 application.

Most of these points are covered well in the "Best Practices for Working with Vertex Data" section of Apple's OpenGL ES Programming Guide for iOS.