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View Full Version : FBO + MRT = SOFT?



Dmitri
01-08-2006, 01:52 AM
GL_VENDOR:ATI Technologies Inc.
GL_RENDERER:RADEON 9700 PRO x86/MMX/3DNow!
GL_VERSION:2.0.5525 WinXP Release (Catalyst 5.13)

FBO with 4 RGB8 ColorAttachments.

int colorBuffers;
unsigned int bufs[4]={
GL_COLOR_ATTACHMENT0_EXT,
GL_COLOR_ATTACHMENT1_EXT,
GL_COLOR_ATTACHMENT2_EXT,
GL_COLOR_ATTACHMENT3_EXT,
};

glDrawBuffersARB(colorBuffers,bufs);
DrawScene();
glDrawBuffersARB(1,bufs);

colorBuffers=1 -> 60 fps (GPU limited)
colorBuffers=2 -> 23 fps (CPU limited)
colorBuffers=3 -> 23 fps (CPU limited)
colorBuffers=4 -> 23 fps (CPU limited)

FBO + MRT not supported in hardware? yet?

Thanks

ChiefWiggum
01-13-2006, 06:25 PM
Not sure how you deduced that 23fps is CPU limited... but dont forget if you're using MRT your shaders are outputting several times the data they normally do, this has to be written somewhere! You're effectively doubling (or in your case using 4x) the memory-write bandwidth of your card!

So it'll definitly incurr a slowdown... the fact that its running at 23fps for all of them could simply mean you've flooded some bus somewhere, and it cant do any better...