I am doing an edge detection pass, creating a stencil mask by means of the texkill instruction (clip). Then in the following passes I use the stencil buffer to only process edge pixels. The curious thing is that I am not getting any performance gain from stenciling, but a performance drop.
After reading this:
Quote:As a result this impacts other functionality that kills
fragments such as alpha test and texkill (called “clip” in HLSL and “discard” in GLSL). If Early Z
would be left on and the alpha test kills a fragment, the depth- and/or stencil-buffer would have been
incorrectly updated for the killed fragments. Therefore, Early Z is disabled for these cases. However, if
depth and stencil writes are disabled there are no updates to the depth-stencil buffer anyway, so in this
case Early Z will be enabled. On the Radeon HD 2000 series, Early Z works in all cases.
(from http://developer.amd.com/media/gpu_assets/Depth_in-depth.pdf)
It seems to me that using the clip operation is disabling Early Stencil, so the stencil operation is being executed after the pixel shader has already been executed. I supose Xbox is older than a Radeon HD 2000.
Some opinions about this? Can someone confirm this information? There is a workaround for this? Basically I want to perform edge detection of the color buffer, filling the stencil buffer so that it can be used later for optimizations purposes.
Thanks!
