On Fri, Aug 2, 2019 at 1:28 PM Gert Wollny <gert.wol...@collabora.com> wrote: > > Am Donnerstag, den 01.08.2019, 07:22 -0400 schrieb Ilia Mirkin: > > Hey Gert, > > > > I'm looking at > > https://cgit.freedesktop.org/mesa/mesa/commit/?id=b048d8bf8f056759d1845a799d4ba2ac84bce30f > > , which attempts to implement depth clamping (rather than clipping) > > with shader tricks. > > > > You're forcing the final vertex stage's position's depth to 0, and > > then making up for it in the frag shader with an extra varying. > > However won't this screw up the barycentric coordinates for > > perspective interpolation? i.e. won't you effectively always just get > > noperspective interp everywhere as a result? > That is probably true, I was following @kumas lead [1], in fact > he implemented the initial version of this code, and I only fixed it up > for ARB_clip_control and GS and TES shaders. > > One fix I could think is maybe clamp the z value to the clip range [-1, > 1] or [0,1] depending on the clip control z accuracy, so that the error > only happens only for the clamped areas where the result is distorted > anyway, what do you think?
Unfortunately I can't think of a way to generically emulate it without implementing clipping in a geometry shader. (I believe Marek has done something like this in a compute shader, actually, to pre-clip geometry-heavy workloads.) Basically if depth is clipped, the triangle gets split into 2 -- one half which is shown, and one half which isn't. When depth is clamped, the half which isn't shown appears as a different polygon with the clamped depth for all of its vertices instead. An interesting question is whether it should be using the original z coords for its barycentric coords or not -- I have no idea, and the spec doesn't seem to explain it in a manner which is accessible to me. If it should be the original z coords, then perhaps you can just disable clipping entirely in that case, and then in the frag shader, just output clamp(gl_FragCoord.z, minz, maxz) or something [and those would be set based on the transform/halfz settings]. You may want to consult someone with a clearer understanding of depth than me though -- it has always been a concept just out of reach for me, unfortunately. Cheers, -ilia _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
