[Mesa-dev] [Bug 64170] [llvmpipe] piglit fbo-cubemap regression

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=64170

Roland Scheidegger <sroland at vmware.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #1 from Roland Scheidegger <sroland at vmware.com> ---
This is due to inaccuracies calculating rho (it goes away with no_rho_approx
set).

Examining this, the problem is at the mip level it's failing, there are 8x8
texels and 8x8 pixels, but the mapping to screen-aligned quad is such that at
the pixel it's failing there's only one pixel in the quad visible. And the
mapping to texel coords is done in such a way that this is in fact a texel
right at the corner of the cube map - the test is written in a way to use the
same face for all visible pixels, but I'm near certain the other 3 invisible
pixels map to different 2 faces. And the derivatives are not quite calculated
correctly in such a case (I believe this test really hits very close to worst
case, which is factor 2, hence one miplevel, which is exactly what the result
shows).
(Note that no_rho_opt does not fix the systematic error calculating derivatives
if the pixels don't lie on the same face, but the max error will drop to
sqrt(2) which is enough to make it pass.)

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