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POProper way to convert CGColors between colorspaces
 

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  1. POProper way to convert CGColors between colorspaces
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    <p>There are several questions out there that ask about converting colors between colorspaces on Apple platforms. Unfortunately, the answers quite often involve <code>NSColor</code> or <code>UIColor</code> -- non portable Objective-C classes that cannot be interchangeably used on OS X and iOS.</p> <p>So I'd like to ask a very specific thing that I'm <strong>sure</strong> there must be a good answer to out there. I simply cannot believe that Apple would not foresee the need for this.</p> <p>How does one convert <code>CGColor</code> from one colorspace (for example, monochrome) to another one (for example, RGB) in a generic way, supporting all <code>CGColorSpace</code> types, while using solely the portable Core Graphics functions?</p> <hr> <p>Some context. I need to multiply the value provided by an online service with a value stored in <code>UIColor</code>. Correct way to extract the RGB components prior to iOS5, which finally introduced the method <code>-[UIColor getRed:green:blue:alpha:]</code>, is to use <code>CGColorGetComponents()</code>. I then multiply this color with the color fetched from the online service. This fails in case <code>+[UIColor grayColor]</code> was used to generate the <code>UIColor</code>. 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?</p> <p>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?</p> <hr> <p>Additional research:</p> <ul> <li><a href="http://web.archive.org/web/20090223205518/http://ofcodeandmen.poltras.com:80/" rel="nofollow noreferrer">This article</a> on color conversion, aside from <code>UIColor</code>'s undocumented/private method <code>-styleString</code>, more interestingly mentions also undocumented <code>CGColorTransform</code>s.</li> <li><a href="http://www.opensource.apple.com/source/WebCore/WebCore-737.5/platform/graphics/cg/ColorCG.cpp" rel="nofollow noreferrer">ColorCG.cpp from WebKit</a> suggests there is a header named <code>CoreGraphics/CGColorTransform.h</code>. Unfortunately, this header does not exist, at least not on Mountain Lion. Why would Apple hide these APIs?</li> <li>The only other sensible resource I found mentioning <code>CGColorTransform</code>s is FreeQuartz, a free/open source reimplementation of Core Graphics.</li> </ul> <p>Filed a radar with Apple, #12141580, to open up and document <code>CGColorTransform</code>. I'm not holding my breath, though, so if there are other sensible suggestions, I'm all ears.</p>
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    1. COI've been doing this for years either using vImage (Accelerate) or bit map contexts. The demand for such utilities as you suggest is low - why would Apple invest the time and energy to do them? Given pixels in RGB format, you can convert to other spaces using easy-to-find C code online, process the data, then convert back into RGB.
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      1. USDavid H
    2. CO@DavidH: Your argument is silly. Why does Apple provide `-[NSString pathExtension]`, then? People happily use substrings in other languages. If the demand is so low, why did Apple add `-[UIColor getRed:green:blue:alpha:]`? You talk about energy - why would thousands of developers invest energy to reinvent the wheel a thousand times if Apple could? I understand your point of view, but I politely disagree with it in belief that something can be done without creating a context or `NSImage`/`CGImage`. Using vImage sounds interesting, care to post an example as an answer?
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    3. COThe proper way to vent your frustration is to enter an enhancement bug on bug reporter.apple.com. I have entered something like 300 or so over the past 8 years. Or, you could choose to make a nice open source project to do this and put it on github (I have several such projects), and attain fame and fortune through it!
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