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

[Dominik Drees]

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

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 5572 bytes
Desc: S/MIME Cryptographic Signature
URL: <https://lists.freedesktop.org/archives/mesa-dev/attachments/20190411/906b385b/attachment.bin>