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

[Ilia Mirkin]

Compare

generated_tests/spec/glsl-1.30/execution/interpolation/interpolation-smooth-other-smooth-none.shader_test
generated_tests/spec/glsl-1.30/execution/interpolation/interpolation-noperspective-other-smooth-none.shader_test

as you can see, the first one is perspective-interpolated and has a
lot more red and less blue (due to the depth). The second one is even,
since depth is ignored. gl_Position.w is the same in both cases, but
the "other" varying gets interpolated differently.

I think you're right that 1/w plays into it -- you're supposed to
divide the interpolated result by w in the frag shader. But I think
the ij coefficients themselves are different? Not sure, it just seems
like z has to be taken into account *somewhere* otherwise the whole
thing can't possibly work.

  -ilia

On Fri, Aug 2, 2019 at 8:11 PM Marek Olšák <maraeo at gmail.com> wrote:
>
> 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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