[Mesa-dev] Low interpolation precision for 8 bit textures using llvmpipe

[Ilia Mirkin]

llvmpipe takes a number of shortcuts in the interest of speed which
cause inaccurate texturing. Try running with

GALLIVM_DEBUG=no_rho_approx,no_brilinear,no_quad_lod

and see if the issue still occurs.

Cheers,

  -ilia



On Thu, Apr 11, 2019 at 8:30 AM Dominik Drees <dominik.drees at wwu.de> wrote:
>
> Hello, everyone!
>
> I have a question regarding the interpolation precision of llvmpipe.
> Feel free to redirect me to somewhere else if this is not the right
> place to ask. Consider the following scenario: In a fragment shader we
> are sampling from a 16x16, 8 bit texture with values between 0 and 3
> using linear interpolation. Then we write white to the screen if the
> sampled value is > 1/255 and black otherwise. The output looks very
> different when rendered with llvmpipe compared to the result produced by
> rendering hardware (for both intel (mesa i965) and nvidia (proprietary
> driver)).
>
> I've uploaded examplary output images here (https://imgur.com/a/D1udpez)
> and the corresponding fragment shader here (https://pastebin.com/pa808Req).
>
> My hypothesis is that llvmpipe (in contrast to hardware) only uses 8 bit
> for the interpolation computation when reading from 8 bit textures and
> thus loses precision in the lower bits. Is that correct? If so, does
> anyone know of a workaround?
>
> A little bit of background about the use case: We are trying to move the
> CI of Voreen (https://www.uni-muenster.de/Voreen/) to the Gitlab-CI
> running in docker without any hardware dependencies. Using llvmpipe for
> our regression tests works in principle, but shows significant
> differences in the raycasting rendering of an 8-bit-per-voxel dataset.
> (The effect is of course less visible than the constructed example case
> linked above, but still quite noticeable for a human.)
>
> Any help or pointers would be appreciated!
>
> Best,
> Dominik
>
> --
> Dominik Drees
>
> Department of Computer Science
> Westfaelische Wilhelms-Universitaet Muenster
>
> email: dominik.drees at wwu.de
> web: https://www.wwu.de/PRIA/personen/drees.shtml
> phone: +49 251 83 - 38448
>
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