Am Samstag, den 03.08.2019, 13:01 -0400 schrieb Ilia Mirkin: > > Gert - your change should be all good from what I can tell > experimentally. Although I still don't understand why.
Thanks for testing, and I think Mareks explanation is spot on. Best, Gert > > -ilia > > On Sat, Aug 3, 2019 at 12:11 AM Marek Olšák <mar...@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, <imir...@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 <mar...@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 < > > > > imir...@alum.mit.edu> wrote: > > > > > On Fri, Aug 2, 2019 at 3:46 PM Gert Wollny < > > > > > gert.wol...@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.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. > > > > > > 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 > > > > > _______________________________________________ > > > > > mesa-dev mailing list > > > > > mesa-dev@lists.freedesktop.org > > > > > https://lists.freedesktop.org/mailman/listinfo/mesa-dev > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/mesa-dev _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
