It looks like the RX 5700 XT with the latest driver (19.9.2, 19.9.3) do not react on texture LOD bias (GL_TEXTURE_LOD_BIAS) param. We have a number of GPUs, both AMD and Nvidia here in CI rack. Since we have got the RX 5700 XT it has produced slightly different renderings, so after tracking down the problem it is obvious that LOD bias has no effect on this GPU. Without setting LOD bias all GPUs including RX 5700 XT give the same results. With LOD bias set RX 5700 XT still samples textures on an unbiased mip level.
I wonder if it is a known issue or not yet. Before I start creating a repro case for dedicated bug report...
Hello. I was not able to reproduce our problem with LOD bias in an artificial repro example - LOD bias works as it should.
But there is still some significant difference in how textures are sampled on RX5700 compared to previous generations and Nvidia cards which breaks our CI apart. I understand that such changes may happen between GPUs generation, but it would be good to know if what I see is expected behavior or a bug, and a hint what could be tweaked to get results almost equal to reference as we have with all the other GPUs here. Let me demonstrate my findings.
Sample scene looks as follows:
First: mipmap level selection is pretty different from what I see on the other GPUs. Mipmap level queried via textureQueryLod(), encoded as a color: white for level <1.0, red for levels between 1.0 and 2.0, green for 2.0 <= level < 3.0, etc.
"All other" GPUs:
RX 5700 XT:
I understanda that mipmap level selection is not standardized, but combined with the next finding sampling in a different manner on a different mipmap level gives a good visible diff.
This diff between pre-RX5700 and RX5700 XT renderings shows some radial pattern, even in the areas where mipmap levels are equal betwenn RX5700 and non-RX5700:
Interresting that this difference is not present on the planes orthogonal to the camera view direction - look that there is almost no diff pixels on the far plane orthogonal to the camera view, whereas plane with a slope has significant differences texture sampling:
It would be great to have comments on if this behavior is expected or not.
Please let me know if you need more materials.