microsoft/DirectXTex

DDS 8.8 Alpha Luminance - incorrect conversion

Trava715 opened this issue · 3 comments

Trava715 commented

When trying to convert textures saved as 8.8AL to other formats using texconv the channels are converted incorrectly.

Sample texture, L8A8
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L8A8 to 8888RGBA
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L8A8 to BC3_UNORM
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L8A8 to BC7_UNORM (opened in Nvidia Texture Tools, channels look the same as when converting to BC3)
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Textures saved as 8 Luminance and 8 Alpha convert correctly.
Attaching test 88AL texture below
88AL_sample.zip

walbourn commented

TL;DR: Use the -xlum texconv switch.

The DirectXTex library 'default conversions' for legacy DDS formats matches those from DDSTextureLoader. These mappings are based on 'no-conversion' behavior so that the data is directly loaded into memory.

In this case, this means D3DFMT_A8L8 maps to the only DXGI format that exactly matches its bit and channel count DXGI_FORMAT_R8G8_UNORM. Therefore L8 channel is going to map to the RED channel your target format, and the A8 is going to map to the GREEN channel in the target format. That is why you end up with a "Yellow" shape with an all opaque-alpha channel.

Semantically, these are quite different of course, so there are a number of other ways to map them. The -xlum switch in texconv (which enables the DDS loader flag DDS_FLAGS_EXPAND_LUMINANCE) will instead map the L8 channel to RED, GREEN, and BLUE and the A8 channel will end up in the alpha channel of the target format.

If you were trying to render classic D3DFMT_A8L8 content directly loaded into memory, you'd have the shader do the conversion from DXGI_FORMAT_R8G8_UNORM.

Trava715 commented

I see, very sorry, not sure how I missed that in the documentation. Thanks for the detailed explanation.