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

[Marek Olšák]

IIRC, perspective interpolation is driven by W, not Z. Interpolating W and
then computing barycentric coordinates using 1/W is what causes the
perspective distortion.

Marek

On Fri, Aug 2, 2019 at 4:59 PM Ilia Mirkin <imirkin at alum.mit.edu> wrote:

> On Fri, Aug 2, 2019 at 3:46 PM Gert Wollny <gert.wollny at collabora.com>
> wrote:
> >
> > Am Freitag, den 02.08.2019, 15:09 -0400 schrieb 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.
> > Yeah, when I first looked into this, this is also what I thought of.
> >
> >
> >
> > > 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.
> > I think that the spec suggest to use the original z coords and the
> > clamping only happens when the depth test is executed:
> >
> > RESOLUTION:  ...   Eliminating far and near plane clipping and
> >       clamping *interpolated* depth values to the depth range is much
> >       simpler to specify.
> >
> > > If it should be the original z coords, then perhaps you can just
> > > disable clipping entirely in that case,
> > My guess is that hardware that disables clipping completely also
> > supports clamping the depth values. Our use case (virgl on top of e
>
> Yeah, that's how NVIDIA hardware works. There's a DEPTH_CLAMP_NEAR/FAR
> value which you set to a value between 0 and 1, based on the viewport
> transform, and that just clamps it prior to being supplied to the frag
> shader. (And separately, you disable clipping near/far planes.) Adreno
> hw works similarly.
>
> > GLES host that doesn't support EXT_depth_clamp) repesents the opposite:
> > a hardware that doesn't support disabling clipping because OpenGL (ES)
> > doesn't allow this and also not doesn't support depth clamping.
> >
> > I still have to think about whether the interpolation really goes
> > wrong. I think I need to write another piglit to get an idea.
>
> Should be easy - take one of the interpolation piglit tests, and
> enable depth clamp without doing anything else.
>
> Cheers,
>
>   -ilia
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