There are several questions out there that ask about converting colors between colorspaces on Apple platforms. Unfortunately, the answers quite often involve NSColor or UIColor -- non portable Objective-C classes that cannot be interchangeably used on OS X and iOS.
So I'd like to ask a very specific thing that I'm sure there must be a good answer to out there. I simply cannot believe that Apple would not foresee the need for this.
How does one convert CGColor from one colorspace (for example, monochrome) to another one (for example, RGB) in a generic way, supporting all CGColorSpace types, while using solely the portable Core Graphics functions?
Some context. I need to multiply the value provided by an online service with a value stored in UIColor. Correct way to extract the RGB components prior to iOS5, which finally introduced the method -[UIColor getRed:green:blue:alpha:], is to use CGColorGetComponents(). I then multiply this color with the color fetched from the online service. This fails in case +[UIColor grayColor] was used to generate the UIColor. Meaning, I need to convert the color from the greyscale colorspace into RGB. In this case, it's easy. What about some other colorspace being provided? Or what if in a theoretical future scenario I just want to process a single pixel's color?
There is a suggestion somewhere that I paint a pixel into a bitmap context and then read this pixel. That's insane, and I hope it isn't the only way to do this. Obviously the drawing method can figure out how to perform the conversion; how can we leverage this without creating a bitmap context solely to draw a pixel in it?
Additional research:
UIColor's undocumented/private method -styleString, more interestingly mentions also undocumented CGColorTransforms.CoreGraphics/CGColorTransform.h. Unfortunately, this header does not exist, at least not on Mountain Lion. Why would Apple hide these APIs?CGColorTransforms is FreeQuartz, a free/open source reimplementation of Core Graphics.Filed a radar with Apple, #12141580, to open up and document CGColorTransform. I'm not holding my breath, though, so if there are other sensible suggestions, I'm all ears.
Apple added this API in iOS 9 and macOS 10.11:
CGColorCreateCopyByMatchingToColorSpace CGColor.converted(to:intent:options:)