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It's "documented" as in "tweeted out by an Apple engineer": https://twitter.com/bciechanowski/status/874345241666437120?...

One thing that is not documented is that the IOSurface must have been created with the 'BGRA' pixel format, or it will silently render as black instead.

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I hope that 'bgra' was at least a typo of 'rgba'

BGRA is a "weird" convention in the graphics world that goes back to at least OS/2 bitmaps, when there was no wrong answer, and has stuck since then. Since early GPUs targeted Windows (where BGRA is the native format for GDI/D2D), it makes sense that it made it this far :)

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