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

Thu Apr 11 18:18:27 UTC 2019

What version of mesa are you using?
The debug flags were changed a while ago (so that those perf tweaks can
be disabled on release builds too), it needs to be either:
GALLIVM_PERF=no_rho_approx,no_brilinear,no_quad_lod
or easier
GALLIVM_PERF=no_filter_hacks (which disables these 3 things above together)

Although all of that only really affects filtering with mipmaps (not
sure if you do?).

(more below)

Am 11.04.19 um 18:00 schrieb Dominik Drees:
> Running with the suggested flags in the environment does not change the
> result for the test case I described below. The results with and without
> the environment variables set are pixel-wise equal.
> 
> By the way, and if this of interest: For GL_NEAREST sampling the results
> from hardware and llvmpipe are equal as well.
> 
> Best,
> Dominik
> 
> On 4/11/19 4:36 PM, Ilia Mirkin wrote:
>> 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://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fimgur.com%2Fa%2FD1udpez&data=02%7C01%7Csroland%40vmware.com%7Cbdef52eb504c4078f9f808d6be96da17%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636905952501149697&sdata=vymggYHZTDLwKNh7RpcM1eSyhVA2L%2BfHNchvYS8yQPQ%3D&reserved=0)
>>>
>>> and the corresponding fragment shader here
>>> (https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpastebin.com%2Fpa808Req&data=02%7C01%7Csroland%40vmware.com%7Cbdef52eb504c4078f9f808d6be96da17%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636905952501149697&sdata=%2FqKVJCXFS4UswynKeSoqCKivTHAb2o%2FZwVE1nwNms3M%3D&reserved=0).
The shader looks iffy to me, how do you use that vec4 in the if clause?

>>>
>>> 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?

So, in theory it is indeed possible the results are less accurate with
llvmpipe (I believe all recent hw does rgba8 filtering with more than 8
bit precision).
For formats fitting into rgba8, we have a fast path in llvmpipe
(gallivm) for the lerp, which unpacks the 8bit values into 16bit values,
does the lerp with that and packs back to 8 bit. The result is
accurately rounded there (to 8 bit) but only for 1 lerp step - for a 2d
texture there are 3 of those (one per direction, and a final one
combining the result). And yes this means the filtered result only has 8
bits.

I do believe you should not rely on implementations having more accuracy
- as far as I know the filtering we do is conformant there (it is tricky
to do better using the fast path).

There would be code to actually do filtering with full float precision,
although there's no way to reach it with rgba8 formats unless you change
the code (if you want to try out the theory, look at
lp_bld_sample_soa.c, lp_build_sample_soa_code() determines whether to
use the fast (aos) filtering path (use_aos, determined mostly by
util_format_fits_8unorm()). If you set this to false it will use the
full float filtering path. (FWIW I was actually thinking a while ago we
should force this path when there's only 1 channel, albeit I never got
around to test (benchmark) it - this is because the AoS filtering path
is really optimized for rgba8 formats, and if you only have 1 channel
it's quite possible float filtering is actually faster, since this
handles the channels individually.)
I guess though if the full float precision filtering is useful in
general, we could add that to GALLIVM_PERF.

Roland

>>>
>>> A little bit of background about the use case: We are trying to move the
>>> CI of Voreen
>>> (https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.uni-muenster.de%2FVoreen%2F&data=02%7C01%7Csroland%40vmware.com%7Cbdef52eb504c4078f9f808d6be96da17%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636905952501149697&sdata=tZf1sxXpC0rDhAAzqXNp9UQnRmrnZceKCerfJKcMdmk%3D&reserved=0)
>>> 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://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.wwu.de%2FPRIA%2Fpersonen%2Fdrees.shtml&data=02%7C01%7Csroland%40vmware.com%7Cbdef52eb504c4078f9f808d6be96da17%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636905952501159687&sdata=tZeO2bZCQzdIz8ifZnNRbQ8tM46CCTDrDFgTeXbVWUU%3D&reserved=0
>>>
>>> phone: +49 251 83 - 38448
>>>
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