问题描述
定义了各种alpha存储模式的可用性：
The Quartz 2D programming guide defines the availability of the various alpha storage modes:
我们应该将哪些用于RGB上下文，为什么？
Which ones should we use for RGB contexts, and why?
对于非不透明的上下文， kCGImageAlphaPremultipliedFirst 或 kCGImageAlphaPremultipliedLast ？
For non-opaque contexts, kCGImageAlphaPremultipliedFirst or kCGImageAlphaPremultipliedLast?
对于不透明的上下文， kCGImageAlphaNoneSkipFirst 或 kCGImageAlphaNoneSkipLast ？
For opaque contexts, kCGImageAlphaNoneSkipFirst or kCGImageAlphaNoneSkipLast?
价值的选择会影响效果吗？
通常，我看到<$对于不透明的上下文，c $ c> kCGImageAlphaPremultipliedFirst 用于非opaque和 kCGImageAlphaNoneSkipFirst 。有些人表示这些表现更好，但我没有看到任何关于此的确凿证据或文件。
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.
快速GitHub搜索显示开发者青睐 kCGImageAlphaPremultipliedFirst over kCGImageAlphaPremultipliedLast 和 kCGImageAlphaNoneSkipLast over kCGImageAlphaNoneSkipFirst 。可悲的是，这只不过是轶事证据。
A quick GitHub search shows that developers favor kCGImageAlphaPremultipliedFirst over kCGImageAlphaPremultipliedLast and kCGImageAlphaNoneSkipLast over kCGImageAlphaNoneSkipFirst. Sadly, this is little more than anecdotal evidence.
推荐答案
2014年WWDC的Apple工程师确认我们应该使用 kCGImageAlphaPremultipliedFirst 或 kCGImageAlphaNoneSkipFirst ，它确实会影响性能。
Confirmed by an Apple engineer at WWDC 2014 that we should use kCGImageAlphaPremultipliedFirst or kCGImageAlphaNoneSkipFirst, and that it does affect performance.
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