The Shape of Everything
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The Core Image format kCIFormatBGRA8 means the pixel data will be ordered BGRA in memory (alpha last). To match that with Core Graphics, you'll have to use kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little. Yes, it's alpha first for CG but last for CI.

Which doesn't make sense, until you stare at it for a long time. It's right there though. Alpha first (ARGB), but little endian for a 32 bit word. Not 16 bit, which at first glance you might assume to be the case. Which I did.