Which CGImageAlphaInfo should we use?

Question

The Quartz 2D programming guide defines the availability of the various alpha storage modes:

Which ones should we use for RGB contexts, and why?

For non-opaque contexts, kCGImageAlphaPremultipliedFirst or kCGImageAlphaPremultipliedLast?

For opaque contexts, kCGImageAlphaNoneSkipFirst or kCGImageAlphaNoneSkipLast?

Does the choice of value affect performance?

Typically, I see kCGImageAlphaPremultipliedFirst for non-opaque and kCGImageAlphaNoneSkipFirst for opaque contexts. Some state that these perform better but I haven't seen any hard evidence or documentation about this.

A quick GitHub search shows that developers favor kCGImageAlphaPremultipliedFirst over kCGImageAlphaPremultipliedLast and kCGImageAlphaNoneSkipLast over kCGImageAlphaNoneSkipFirst. Sadly, this is little more than anecdotal evidence.

Answer 1

Confirmed by an Apple engineer at WWDC 2014 that we should use kCGImageAlphaPremultipliedFirst or kCGImageAlphaNoneSkipFirst, and that it does affect performance.