Switching FBOs or Attachments Slow


I’m doing a one off project that requires blurring and rendering to a bunch of FBOs before compositing for the final frame. Final target size is 4000x1000. I’m noticing that the combination of the frame size and switching FBOs is really slow. My loop looks something like this:

for( passes = 0 to 4 ):

  • bind fbo1
  • blur shader using fbo0 as texture
  • unbind fbo1
  • bind fbo0
  • blur shader using fbo1 as texture
  • unbind fb0

It seems at the full target size, the frame rate drops from 60 to 30. If I half the target size, the frame rate jumps back up to 60. One might expect this. However if, I just don’t draw anything at all at the full target size and just have it bind and unbind, the frame rate is still 30.

I’m wondering if there is something I can do at full size to prevent the frame rate from just suddenly halving itself. My best guess is that it’s causing some kind of stall or sync issue with in the card itself.

Hardware is ATI 6779 on MacBookPro running BootCamp using whatever driver comes with BootCamp.

Any insights will be much appreciated!

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