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performanceopenglframebufferglreadpixels

glReadPixels() slow on reading GL_DEPTH_COMPONENT

My application is dependent on reading depth information back from the framebuffer. I've implemented this with glReadPixels(0, 0, width, height, GL_DEPTH_COMPONENT, GL_FLOAT, &depth_data)

However this runs unreasonable slow, it brings my application from a smooth 30fps to a laggy 3fps. If I try to other dimensions or data to read back it runs on an acceptable level.

To give an overview:

  • No glReadPixels -> 30 frames per second
  • glReadPixels(0, 0, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &depth_data); -> 20 frames per second, acceptable
  • glReadPixels(0, 0, width, height, GL_RED, GL_FLOAT, &depth_data); -> 20 frames per second, acceptable
  • glReadPixels(0, 0, width, height, GL_DEPTH_COMPONENT, GL_FLOAT, &depth_data); -> 3 frames per second, not acceptable

Why should the last one be so slow compared to the other calls? Is there any way to remedy it?

width x height is approximately 100 x 1000, the call gets increasingly slower as I increase the dimensions.

I've also tried to use pixel buffer objects but this has no significant effect on performance, it only delays the slowness till the glMapBuffer() call.

(I've tested this on a MacBook Air nVidia 320m graphics OS X 10.6, strangely enough my old MacBook Intel GMA x3100 got ~15 fps reading the depth buffer.)

UPDATE: leaving GLUT_MULTISAMPLE out of the glutInitDisplayMode options made a world of difference bringing the application back to a smooth 20fps again. I don't know what the option does in the first place, can anyone explain?

Solution

  • If your main framebuffer is MSAA-enabled (GLUT_MULTISAMPLE is present), then 2 actual framebuffers are created - one with MSAA and one regular.

    The first one is needed for you to fill. It contains front and back color surfaces, plus depth and stencil. The second one has to contain only color that is produced by resolving the corresponding MSAA surface.

    However, when you are trying to read depth using glReadPixels the driver is forced to resolve the MSAA-enabled depth surface too, which probably causes your slowdown.