Alpha support about basis_universal HOT 6 CLOSED

The .basis format supports alpha channels already, but only a few transcoders can handle outputting alpha: ETC2 EAC and BC3, and BC5. The new ASTC transcoder (which will be released on the next milestone) also supports alpha channels. It also has optimizations for L and LA blocks (for XY normal maps) The next milestone also has transcoders for BC7 alpha (using mode 4) and PVRTC1 4bpp alpha (using a new transcoder).

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Alpha is supported by Basis, but not by every transcode target. It's explained in detail in (this section of the repo readme)[https://github.com/BinomialLLC/basis_universal#gpu-texture-format-support-details], but the short version is that ETC1, BC1, BC7, and PVRTC1 don't support alpha channels with Basis. (Yet. Some of them may support it in the future.) If those are the only formats available to your platform, you'll need to decode the alpha channel as a separate greyscale image (with the cDecodeFlagsTranscodeAlphaDataToOpaqueFormats flag) and sample it separately in your shaders.

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Just wanted to let y'all know that alpha support will be added for PVRTC and BC7 in the next release, which should be coming out in October if testing goes as planned. No contributions needed (it'd be too many cooks in the kitchen at this point, since it's almost done)-- look forward to release!

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Thanks @toji and @sehurlburt for the heads up.

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I hope that the alpha support will be at least of beta quality ;)

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As the system already supports alpha, I'm closing this bug. The next milestone (due in a few weeks) will have more transcoders supporting alpha.

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