hpique hpique - 2 months ago 11
iOS Question

Which CGImageAlphaInfo should we use?

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

enter image description here

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

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