[osg-users] State changes causing huge drop in framerate
osgforum at tevs.eu
Wed Apr 6 06:14:40 PDT 2011
I have a very large scene graph where I have many geodes.
Each of this geode has a stateset with a GPU program defined on it, contianing a vertex and a fragment shader.
The GPU program instance is unique for my whole application - I am not recreating the program over and over again for every geode. This would be an easy, silly mistake.
I have done performance tests and having a StateSet assigned to each Geode causes a MASSIVE performance drop. For a given model, containing thousands of geodes, the framerate goes from 64 to 30 fps just when you call getOrCreateStateSet on every geode, causing state to be created on each individual geode.
An OpenGL trace shows that OSG is clever enough to avoid calling glUseProgram(id) / glUseProgram(0) in between every geode being rendered. Declaring the GPU program once, on the topmost graph node, and not on every geode, results in the same GL trace, with no performance degradation. This suggests OSG is very heavily using the CPU to 'diff' the state changes across geodes.
Is there a way, maybe by fiddling with OVERRIDE/PROTECTED flags, to improve the way OSG handles state changes, in order to keep good performance?
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