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

[Ilia Mirkin]

I've done the tests to satisfy my own curiousity... it all works
correctly as-is, it would seem. I need to go back and re-look at
barycentric coordinates generation.

Gert - your change should be all good from what I can tell
experimentally. Although I still don't understand why.

  -ilia

On Sat, Aug 3, 2019 at 12:11 AM Marek Olšák <maraeo at gmail.com> wrote:
>
> Z is 0 at the near plane and 1 at the far plane, but the planes can have an arbitrary distance from the viewer, therefore Z Is irrelevant.
>
> W is the real distance from the viewer. The greater W is, the smaller (farther away) the object is, because W is the divisor (which makes objects smaller as it increases).
>
> The W=0 plane intersects the viewer. It is exactly in the middle of the "eye ball".
>
> W < 0 means behind the viewer.
>
> Marek
>
> On Fri., Aug. 2, 2019, 21:22 Ilia Mirkin, <imirkin at alum.mit.edu> wrote:
>>
>> 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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