[Mesa-dev] Re mesa/st: add tgsi-lowering code for depth-clamp

[Ilia Mirkin]

On Fri, Aug 2, 2019 at 1:28 PM Gert Wollny <gert.wollny at 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