Alpha test disables Early Z?

Discussion created by BigHands on Sep 24, 2010
Latest reply on Sep 24, 2010 by pboudier

For a long time without giving it much tought, I've accepted that turning on alpha testing disables Early Z.  Emil Persson also mentions this in his "Depth In-depth" AMD white paper.

I have been thinking about this lately and, to me, it doesn't make sense why alpha testing would disable Early Z.  Why would Early Z care if the pixel is going to be accepted or rejected by the alpha test?  The only case I can think of is Early Z saying the pixel is visible, but then alpha test failing.  This would simply be a waste of processing time, but I would imagine that the average texture with alpha test information has more pixels that should be visible than not.

Thanks for sharing any thoughts.