While I was deriving expressions for perspective-correct linear interpolation on triangles, I reached the conclusion that the interpolation may be kept linear only if view-space $Z$ is available. However, computing view-space $Z$ requires nonlinear interpolation (it is a rational function based on my results). Is nonlinear interpolation performed in hardware to get perspective correct $Z$, or equivalently linear interpolation is performed on $\frac{1}{Z}$ but a division is required to get view-space $Z$ anyways? As far as I get it there is no way around that - a division is required per-pixel to get perspective-correct Z values - is that correct?
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Yes, that's correct. Perspective-correct interpolation works by (for some quantity $u$ to be interpolated) calculating $u/z$ and $1/z$ at each vertex, linearly interpolating those values in screen space, then calculating $u = (u/z) / (1/z)$ at each sample point. This is done by the GPU hardware behind the scenes.
answered Jul 11, 2020 at 17:56
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$\begingroup$ This seems to require a division/reciprocal for the interpolation of every element of every attribute. Isn't this more expensive at the end of the day, than doing it just for $z$ and then linearly interpolating (without division) the attributes without using any division/reciprocal? $\endgroup$ Commented Jul 12, 2020 at 5:41
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$\begingroup$ Sure, it's more expensive. But without it, you don't get perspective-correct interpolation, and anything with a texture on it will look all kinds of wrong. It's a necessary expense to pay. $\endgroup$ Commented Jul 12, 2020 at 5:57
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$\begingroup$ I meant that performing the division per pixel attribute is more expensive. You can linearly interpolate $u$ if you know view-space $z$ to get perspective-correct interpolation. Only for view-space $z$ do you require a division. $\endgroup$ Commented Jul 12, 2020 at 6:38
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$\begingroup$ But you can't linearly interpolate $u$ itself; you must interpolate $u/z$, then cancel out the $z$ per pixel. Or it is not perspective-correct. For example, textures will appear to shift and bend over a surface as you move the camera around, if the texture coordinates are not interpolated perspective-correctly. All vertex attributes require perspective correction if the geometry is to look correct. $\endgroup$ Commented Jul 12, 2020 at 19:43
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1$\begingroup$ Hmm, maybe I see what you mean now. You could calculate $(1-s)z_1^{-1}/((1-s)z_1^{-1} + sz_2^{-1})$ once per pixel (and another one for the other screen-space coordinate), and use those scalars to interpolate each of the attributes. Yeah, that sounds like it works, so maybe that's what they actually do under the hood. I'm not sure how it's implemented exactly. $\endgroup$ Commented Jul 12, 2020 at 20:01