texconv: Added support for DXT5nm and RXGB DXT5 format variants #362
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walbourn
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Jun 12, 2023
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The "DXT5nm" format is a "swizzled" DXT5/BC3 variant supported by a number of tools. It's intended to encode normal maps using BC3 which is supported on all Direct3D Hardware Feature levels rather than BC4/BC5 which originally was an ATI only extension and then part of Direct3D Hardware Feature Level 10.0. It encodes the 'x' as the alpha channel, 'y' as the green channel, sets the red channel to 1.0, and the blue channel to 0 to get good quality compression of both components.
The "RXGA" format is a "swizzled" DXT5/BC3 variant supported by a number of tools and popularized by Doom 3. It swaps the 'r' and 'a' channels, leaving 'g' and 'b' in place. This is normally written as "DXT5", but if
-f RXGB -dx9is used, then it will write using the FourCC "RXGB" instead.The PR also adds support to the DirectXTex DDS codec for reading the numerous legacy FourCC's that are just 'swizzled' versions of BC3 or BC5. These are returned 'as is' so there's no logic to 'deswizzle' built-in. This should be handled by the client of the code, potentially using the
-swizzletexconv switch.