我们应该使用哪个CGImageAlphaInfo？ [英] Which CGImageAlphaInfo should we use?

问题描述

Quartz 2D编程指南定义了各种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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