View Full Version : gluPickMatrix and glReadPixels

Rong Yao
02-16-2007, 07:36 AM
I use glReadPixels to read the depth component of the pixel under my mouse point, then multiply the return value by 2^32-1 and get value v1. Then I use gluPickMatrix to get the same pixel's minimum Z buffer value v2 from the select buffer. I find that v1 is slightly different from v2(about 1% error). Is there any fundamental mistake of my method or the error is in the range of allowance? Thank you!

02-16-2007, 07:51 AM
In most implementations, selection buffer is done in software. So yes it is rasterized differently.

02-16-2007, 10:03 AM
Selection doesn't do any rasterization at all, so doesn't matter if it's hardware or not. Reading back a pixel relies on rasterization, but selection is purely a geometrical operation.

So the values are from different stages in the rendering pipe, so they are subject to different amount of calculations and roundings.

And then there's the issue that a primitive not perpendicular to the view vector has it's depth rasterized at the center of the pixel. Min/max depth as returned by selection will be at the edge of the pixel, which is different due to the fact that the depth varies over the primitive.