View Full Version : FBO vs PBuffer mipmap speeds

Lord crc
05-15-2007, 01:28 AM
Hi, I've tried to make a small FBO application, which seems to work. However the rendering speeds I get doesn't quite make much sense to me.

I render the (simple) scene from a different POV to a texture, then render this alternate view in a quad along with the scene from the original POV.

I can toggle between using the FBO or a PBuffer for rendering to the texture.

Now, if I don't enable mipmapping, the FBO is about twice as fast as the PBuffer. But if I enable mipmapping the FBO is about 2/3 the speed of the PBuffer.

I use glGenerateMipmapEXT with the FBO and glCopyTexSubImage2D and GL_SGIS_generate_mipmap for the PBuffer.

Any ideas as to why this is so? I was at least expecting them to be about same in speed. Am I doing something wrong?

I'm using a 7800GT with 93.71 drivers, the FBO is rendered using a 32bit RGBA color texture, and a 24bit depth RBO.

05-15-2007, 02:05 AM
Hm, could you try glGenerateMipmapEXT with the Pbuffer approach too? I mean, after you copy the texture instead of the SGIS flag...

Lord crc
05-15-2007, 02:14 AM
Hmm didn't think of that. However, speeds are the same, FBO slow, PBuffer fast.

Thanks for the suggestion though :)

05-15-2007, 02:30 AM
As far as I know, SGIS flag actually does quite the same, except for glGenerateMipmapEXT has more control over max lod.
So the problem is not here I hope, but, maybe, somewhere in your code...

Lord crc
05-16-2007, 12:01 AM
Just talked to someone else who experiences the same with his own code... He hadn't found any solution to it either... :(