of course this question is not directly related to opengl itself, but: why does my nvidia quadro fx 3500 card support only 24 bit depth buffer? having hundreds of MB onboard memory, is there no space for 8 more bits? would performance be affected if 32 bits were used? or does everybody consider 24 bits to be sufficient?
(it is not that i am too unhappy with 24 bit; but i am curious: everything gets bigger- why not the depth buffer?)
As you say, adding 8 bits to the depth buffer isn’t going to change much. In fact, I assume the depth buffer already is 32 bits to make the pixels properly aligned. Also heard it’s not uncommon to put the 8-bit stencil buffer in between the depth bits. So you really have an interleaved depth and stencil buffer.
But to your question; why not make depth buffer 32 bits (in the absence of a stencil buffer) then? I believe it’s about precision. If the internal calculations are performed using standard 32-bit IEEE floating point values, you have 23 bits mantissa with an implicit leading 1. That gives you a total of 24 significant bits. In that case, making the depth buffer 32 bits won’t change anything, because you only make the calculations wit 24 bits of precision.
WGL_EXT_depth_float
GL_NV_depth_buffer_float
Don’t know anything about the WGL extension, but NV is supported on GeForce 8 AFAIK.