[Bug 25871] New: nearest neighbour samples wrong texel (precision/rounding problem)

[Bug 25871] New: nearest neighbour samples wrong texel (precision/rounding problem)

[bugzilla-daemon]

http://bugs.freedesktop.org/show_bug.cgi?id=25871

           Summary: nearest neighbour samples wrong texel
                    (precision/rounding problem)
           Product: Mesa
           Version: git
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/R600
        AssignedTo: dri-devel@lists.sourceforge.net
        ReportedBy: pierre-bugzilla@ossman.eu


The following fragment program does the wrong thing on r600/r700 hardware:

TEX result.color, 0.498046860099, texture[0], 2D;

(the texture coord is as far as I got before I got bored. I'm not sure the
precision goes much further anyway)

The circumstances is that I have a 4x4 texture with nearest neighbour
interpolation. The above code should be sampling texel 1,1 but is instead
sampling texel 2,2. This causes problems for fragment programs that try to do
things based on texel coords (like bicubic interpolation).

This seems to be a hw bug as I've tried it with fglrx (albeit with a slightly
different card) and it exhibits the same bug. As such, I'm not sure it can be
solved but hopefully there is a workaround.


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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

[bugzilla-daemon]

http://bugs.freedesktop.org/show_bug.cgi?id=25871





--- Comment #1 from Pierre Ossman <pierre-bugzilla@ossman.eu>  2010-01-04 13:48:51 PST ---
Created an attachment (id=32446)
 --> (http://bugs.freedesktop.org/attachment.cgi?id=32446)
fp-tri.c

Test program to provoke the bug. This is a hacked version of fp-tri.c from
mesa/progs/fp that I was using to debug a problem with a bicubic filtering
fragment program, so it can be a bit messy. If you have trouble sorting it out,
then I can try to clean it up and remove non-essential bits.


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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

[bugzilla-daemon]

http://bugs.freedesktop.org/show_bug.cgi?id=25871





--- Comment #2 from Pierre Ossman <pierre-bugzilla@ossman.eu>  2010-01-04 13:49:22 PST ---
Created an attachment (id=32447)
 --> (http://bugs.freedesktop.org/attachment.cgi?id=32447)
fragment program

The fragment program in its full form.


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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #3 from Alex Deucher <agd5f@yahoo.com> 2011-02-28 18:27:18 PST ---
For point sampled textures, SQ_TEX_SAMPLER_WORD2_0.MC_COORD_TRUNCATE needs to
be set to 1.  The default behavior for texture addressing is to round unless
that bit is set.

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #4 from Alex Deucher <agd5f@yahoo.com> 2011-02-28 18:56:24 PST ---
Created an attachment (id=43951)
 View: https://bugs.freedesktop.org/attachment.cgi?id=43951
 Review: https://bugs.freedesktop.org/review?bug=25871&attachment=43951

possible fix

Does this patch help?

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

Andreas Boll <andreas.boll.dev@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|Drivers/DRI/R600            |Drivers/Gallium/r600

--- Comment #5 from Andreas Boll <andreas.boll.dev@gmail.com> 2012-09-11 11:32:01 UTC ---
(In reply to comment #4)
> Created attachment 43951 [details] [review]
> possible fix
> 
> Does this patch help?

this patch has been committed as

commit 1dc204d145dc8c0b19473a7814c201a8954b6274
Author: Alex Deucher <alexdeucher@gmail.com>
Date:   Mon Feb 28 21:52:19 2011 -0500

    r600g: truncate point sampled texture coordinates

    By default the hardware rounds texcoords.  However,
    for point sampled textures, the expected behavior is
    to truncate.  When we have point sampled textures,
    set the truncate bit in the sampler.

    Should fix:
    https://bugs.freedesktop.org/show_bug.cgi?id=25871

    Signed-off-by: Alex Deucher <alexdeucher@gmail.com>

and reverted in

commit 72c6a748b9ffdaa893f82faf911f22a241a5e4f5
Author: Marek Olšák <maraeo@gmail.com>
Date:   Mon May 2 01:10:19 2011 +0200

    Revert "r600g: truncate point sampled texture coordinates"

    This reverts commit 1dc204d145dc8c0b19473a7814c201a8954b6274.

    MC_COORD_TRUNCATE is for MPEG and produces quite an interesting behavior
    on regular textures. Anyway that commit broke filtering in demos/cubemap.


reassigning to r600g

is this still an issue with a newer mesa (e.g. 8.0.4 or git master)?

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

[bugzilla-daemon]

https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #6 from Pierre Ossman <pierre-bugzilla@ossman.eu> 2012-09-11 13:51:20 UTC ---
I don't have any updated systems right now, so I can't really test. Test
program should still be valid though.

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #7 from Andreas Boll <andreas.boll.dev@gmail.com> 2012-09-11 17:12:41 UTC ---
Created attachment 66984
  --> https://bugs.freedesktop.org/attachment.cgi?id=66984
captured shader

I have tested your modified fp-tri-c from attachment 32446
with r600g on my rv770 .
I got the following output with mesa git master
e81ee67b51651e99e7e8e52c1ccafc66835d57cd
and mesa 8.0.4:

$ ./fp-tri -fps foo2.arb
!!ARBfp1.0

TEX result.color, 0.498046860099, texture[0], 2D;

END

GL_RENDERER = Gallium 0.4 on AMD RV770
glGetError = 0x0
12850 frames in 5.0 seconds = 2570.000 FPS

Additionally I have attached a screen shot from the shader.

What is the expected behavior?

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #8 from Alex Deucher <agd5f@yahoo.com> 2012-09-11 17:23:43 UTC ---
might be fixed with this commit:
http://cgit.freedesktop.org/mesa/mesa/commit/?id=f44bda17f515c411071ca8744ebd96039d9c583b

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #9 from Andreas Boll <andreas.boll.dev@gmail.com> 2012-09-12 12:34:21 UTC ---
(In reply to comment #8)
> might be fixed with this commit:
> http://cgit.freedesktop.org/mesa/mesa/commit/?id=f44bda17f515c411071ca8744ebd96039d9c583b


I can't see any difference between mesa git master and mesa 8.0.4 .

What is the expected behavior?

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #10 from Pierre Ossman <pierre-bugzilla@ossman.eu> 2012-09-12 13:03:19 UTC ---
(In reply to comment #7)
> 
> What is the expected behavior?

It's so long ago I don't remember the exact details. You can easily figure it
out though. The test program creates a 4x4 checker board texture and then uses
a single point on it to draw the triangles. The range [0.25,0.5[ should all be
the same color. The bug was that as you approached 0.5, it started sampling the
next pixel prematurely.

So change the fragment program to use coordinate 0.4 or something like that.
That colour should be the same as 0.4999... . Looking at the code, white seems
to be the correct colour.

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #11 from Andreas Boll <andreas.boll.dev@gmail.com> 2012-09-12 13:56:46 UTC ---
(In reply to comment #10)
> 
> It's so long ago I don't remember the exact details. You can easily figure it
> out though. The test program creates a 4x4 checker board texture and then uses
> a single point on it to draw the triangles. The range [0.25,0.5[ should all be
> the same color. The bug was that as you approached 0.5, it started sampling the
> next pixel prematurely.
> 
> So change the fragment program to use coordinate 0.4 or something like that.
> That colour should be the same as 0.4999... . Looking at the code, white seems
> to be the correct colour.

Ok I've got it.

With coordinate 0.4 I get a white color and with 0.4999 I get black.

Additionally I checked the other end of the range:
Between the range [0.2480468676,0.498046860099] the color is white.
If I change the fragment program to use coordinate 0.2480468675 then the color
is black. With 0.4980468601 I get also black.

So the issue persists in git mesa master 9.1-devel (git-e81ee67)

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

Marek Olšák <maraeo@gmail.com> changed:

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

--- Comment #12 from Marek Olšák <maraeo@gmail.com> ---
I think this is a normal behavior and it's not a precision issue. The
coordinates are rounded to 0.5, because the filter is NEAREST (and 0.5 is the
nearest pixel). Closing as NOTABUG.

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #13 from Pierre Ossman <pierre-bugzilla@ossman.eu> ---
Hardly NOTABUG. There is no pixel at 0.5. The pixels are at 0.125, 0.375, 0.625
and 0.875. 0.49999 should get rounded to the 0.375 pixel. That's the closest,
not the 0.625 one.

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #14 from Marek Olšák <maraeo@gmail.com> ---
Thanks for the feedback. So it is a precision issue after all. If fglrx
exhibits the same behavior, there is nothing I can do. I don't know of any
hardware state which controls precision of texture addressing.

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #15 from Erik Faye-Lund <kusmabite@gmail.com> ---
(In reply to comment #14)
> Thanks for the feedback. So it is a precision issue after all. If fglrx
> exhibits the same behavior, there is nothing I can do. I don't know of any
> hardware state which controls precision of texture addressing.

AMD's Windows drivers at least seemingly consistently performs
nearest-filtering with round-off-point off by a 512th of a texel. So I think
this is somehow intended (even if really unfortunate for some non-trivial
usecases).

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[Bug 25871] nearest neighbour samples wrong texel (precision/rounding problem)

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https://bugs.freedesktop.org/show_bug.cgi?id=25871

--- Comment #16 from Pierre Ossman <pierre-bugzilla@ossman.eu> ---
(In reply to comment #14)
> Thanks for the feedback. So it is a precision issue after all. If fglrx
> exhibits the same behavior, there is nothing I can do. I don't know of any
> hardware state which controls precision of texture addressing.

I would guess it needs input from the AMD folks. So I guess getting their
attention is one thing that could be done. Maybe a microcode update could
solve.

I also haven't tested this in ages. Might not be an issue for modern chips...

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