Created attachment 96745 [details]
Example image (with mipmaps), the height of which triggers the problem
There seems to be a problem with some compressed textures. When the height of the image (more specifically, of the mipmap level used) is not a multiple of 4, it appears that the bottom part of the image – as rendered – contains random junk, apparently set at image load time.
The example image will show the problem at its original size. I've also played with its height (initially 570px): increasing it to a multiple of 4 would remove the problem if it's rendered using the full-size version, but since when reduced by one level the height is again not a multiple of 4, the problem re-appears. Setting the full-size height to 576px moves the problem down further.
(We're using http://crunch.googlecode.com/svn/trunk/ for image (de)compression.)
I don't know details of the problem here but I got interested and went to read the spec ..
EXT_texture_compression_s3tc specification states in "Implementation Note":
"If the width or height is not a multiple of four, there will be 4x4 blocks at the edge of the image that contain "extra" texels that are not part of the image."
I'm not sure though what should happen if one samples/reads such texel.
I'm not sure either, but I'd (naïvely) expect the image height not being a multiple of 4 not to matter.
Turns out that the bug also affects the right edges of textures.
(Evergreen, in case there's something hardware-specific about this.)
Are you able to reproduce this bug with simple program using a standard dds file? If this is possible, please attach such example.
Bug 86747 (on Intel GPU) could also be due to DXT texture mipmap levels not being multiple of 4.
please attach a test case
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