[Mesa-dev] [Bug 65376] New: [softpipe] img_filter_3d_linear returns incorrect results due to texture cache aliasing

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Tue Jun 4 13:36:46 PDT 2013


          Priority: medium
            Bug ID: 65376
          Assignee: mesa-dev at lists.freedesktop.org
           Summary: [softpipe] img_filter_3d_linear returns incorrect
                    results due to texture cache aliasing
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: arseny.kapoulkine at gmail.com
          Hardware: All
            Status: NEW
           Version: 9.1
         Component: Mesa core
           Product: Mesa

Created attachment 80309
  --> https://bugs.freedesktop.org/attachment.cgi?id=80309&action=edit
Bug screenshot

img_filter_3d_linear calls get_texel_3d 8 times for each of the 8 neighbor
pixels; get_texel_3d returns a pointer to the texel data.

In some cases the data referenced by one the pointers changes in a subsequent
call to get_texel_3d; as a result, filtering works on wrong values and produces
bogus results.

Screenshot of the bug is attached; note that the lighting data (gradients) is
sampled from a 3D texture, and the texel size is the same as the size of one
brick-textured cell on the screenshot; i.e. the edges in the gradients are
caused by filtering errors.

Here's what happens:
1. The 3D texture is 256x256x64; it has a 'wrap' filtering applied - the
texture X axis is roughly horizontal on the screen
2. A shader requests a texture fetch which results in the following 2x2x2 texel
[255 11 0] - [0 12 1]

(the region wraps around along X)

3. Mesa computes the texture tile indices for each sample; for one corner of
the sampled region this is [3 0 0]; for the other it's [0 0 1]
4. The texture cache is addressed based on the hash of the tile index, which is
computed by tex_cache_pos; it returns 3 for both tiles

As a result, when the second group of texels (that corresponds to Z=1) is
fetched, a tile for Z=0 is overwritten by the tile for Z=1. Since
img_filter_3d_linear does not copy the data, the pointers tx00, tx01, tx02,
tx03 now point to the tile for Z=1 instead of Z=0.

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