glReadpixels slow for pbuffers ?

OpenGL OpenGL: Advanced Coding
1

Is there any reason why glReadPixels should be slow when reading from pbuffers ?

I’m experiencing massive slowdown when reading from pbuffer vs backbuffer.

[Edit]
1.I’m reading the stencil buffer.

  1. For standard pBuffers, when using RTT rather than copy/bind the glReadPixels is slow.

  2. For 32bit floating point pbuffers I would expect the readpixels to take 4 times as long, but I’m finding it’s about 30x slower.

Nvidia Quadro FX 42.86 drivers.

[This message has been edited by pocketmoon (edited 02-17-2003).]

2

What format are you using in your ReadPixels call?

3

glReadPixels( 0, 0, 256, 256, GL_STENCIL_INDEX , GL_BYTE, &pStencil);

4

Originally posted by pocketmoon:
glReadPixels( 0, 0, 256, 256, GL_STENCIL_INDEX , GL_BYTE, &pStencil);

Which is plenty fast enough if it’s a non floating point pbuffer but is 30x slower when it is floating point.