Commits on Oct 2, 2022

1. DDS: various improvements …

   - If a DDS pixel format header contains "this is a normal map" flag
     set, then handle that properly (#3525):
     - For a DXT5, this means it's a "DXT5nm" format. Normal X & Y
       components are in green and alpha channel respectively. Compute Z
       component from those, and the result is a 3-channel tangent space
       normal map.
     - For ATI2/BC5 format that also has a "normal map" flag set, compute
       the Z component automatically too, and result is a 3-channel image.
       However a bunch of ATI2/BC5 normal maps out there in the wild do
       not have that bit set, so also have a global "dds:bc5normal"
       attribute to optionally turn this on.
   - Fix support of R10G10B10A2 format - was producing garbage data due to
     code assumption that truncation down to 8 bits never needs to happen.
   - Fix bit expansion ("too dark") of very low-bit formats, e.g. R3G3B2.
     Code was simply shifting the bits, instead of properly repeating
     the high bits into new low bit positions (i.e. a 3-bit "111" needs
     to become an 8-bit "11111111" and not a "11100000").
   - Support "RXGB" (DXT5, with red & alpha swapped) as well as "BC4U"
     and "BC5U" formats (the latter two are just BC4 and BC5, produced by
     some older tools).
   - Support alpha-only (e.g. A8) formats.
   
   All this mostly prompted by doing DDS improvements in Blender, and
   wanting to get Cycles renderer (which uses OIIO) to the same feature
   parity. See https://developer.blender.org/T101405 and
   https://developer.blender.org/D16087.

   

   aras-p committed Oct 2, 2022
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