KTX2 BGRA8 sampling is wrong

[dragonbane0]

Describe the bug
At least when viewing KTX2 B8G8R8A8 textures, the tool still seems to treat the B channel as R and R as B causing the colors to be inverted.

To Reproduce
Steps to reproduce the behavior:

1. Open B8G8R8A8 KTX2 texture and find the colors are wrong. Unsure if other formats or DDS is affected too

Expected behavior
For the BGRA format to be correctly sampled.

Desktop:

• OS: Windows 11
• Version 3.8

Additional context
Thumbnail generator is also affected and needs the same fix.

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Excellent work on the tool, compatibility is very high otherwise

[kopaka1822]

Hi, it would help me debugging if you could attach the KTX2 file

[dragonbane0]

Thanks for the fast reply. And sure thing, here is the KTX2 file (in B8G8R8A8_SRGB):
https://www.dropbox.com/scl/fi/sasghenkfwlizqign33vu/Diffuse_Roughness.ktx2?rlkey=5q2ol16v0popltf37kby27ipg&st=dtp152pk&dl=0

I confirmed DDS seems to be affected too. Here is the same file with a DDS container (vertically flipped, in B8G8R8A8_UNORM_SRGB):
https://www.dropbox.com/scl/fi/9qv5vm9ztd5x3jf9fwmn3/Diffuse_Roughness.dds?rlkey=26rn6yl2tmxmvl3ecqtp6gsij&st=vzz5zmcr&dl=0

Screenshot of the intended look:

[kopaka1822]

I have confirmed that the BGR swizzle is not properly applied in the viewer. I'll need some time to come up with a solution, that will properly work on all formats. As im currently working on another task, it will probably take some time until I can fix that.

In the meantime, you can apply a BGR swizzle via the image equations. You can replace the I0 with rgb(b(I0),g(I0),r(I0)) for the RGB field to achieve that. Sorry for the inconvenience.

[dragonbane0]

Thank you. And no problem. Honestly I tried so many tools now for KTX2 and they all have way bigger quirks or ignoring parts of the spec one way or another. And they are all closed source, so it's hard or impossible to get these even looked at. So I'm glad your tool exists and this is a minor issue comparatively. An engine I'm doing modding work on pretty much uses every Vulkan format under the sun including all sRGB formats, IBL cubemaps, runtime swizzling and bottom-left origin and without conversion KTX2 is the best format to cover all this directly in the container. But it also means I'm kinda hitting every possible edge case there is