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Offscreen Texture speed

Hi everyone,

I'm currently writing a QT component that renders OpenGL to an offscreen GWorld. I'm having problems with the rendering speed (i expected that there would be some performance penalty for offscreen rendering) but I'm a little puzzled at what is going on.
I've tested the code on 3 different platforms:
Powerbook G4 500MHz
G4 Tower 400MHz
G4 Tower Dual 1GHz
also with increasing level of graphics card power
But I get some surprising results.
The rendering speed is almost exactly the same on all three.

I've narrowed it down to when I turn texturing off I get >30fps for all platforms (not too surprising), but with textures on I need to make the offscreen surface about 64x64 pixels to get almost 30fps, 128x128: 20fps, 256x256: 6 fps with the same results on all 3 platforms.
I would have thought the speed would have scaled with the processer or graphics card, but it doesn't seem to.
My scene isn't that complex, 2 objects, 2 textures.

Has anyone else tried this ( I wouldn't think it would be any different than any other Offscreen rendering app)?
Am I out of luck?

-Chris
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