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  1. PO
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    <p>This is actually a <em>lot</em> more tricky than you might expect. For many reasons.</p> <p>The primary issue is that RGB space is a device dependent space - RGB values that look similar on one device need not look similar on another. (If you think this sounds unlikely - there is a huge history of issues between getting good color correspondence between mac / win / SGI due to the different assumptions made by the OS about monitor gamma.)</p> <p>The second issue is that the perception of difference between two colors depends heavily on the surrounding colors / viewing environment. I.e. two colors may look different when viewed in a dark surround in a dark environment, but similar in a white surround in a light environment.</p> <p>With these provisos in mind Delta CIE 94 is a decent measure of color similarity.</p> <p>You'll calculate it by converting each color from RGB to XYZ and then to Lab. The RGB to XYZ transformation is device dependent - there are some simple standard transformations floating about for it, such as <a href="http://en.wikipedia.org/wiki/SRGB" rel="nofollow">sRGB</a>, but their applicability to real devices is questionable. The transformation from XYZ to Lab is complicated and can be found <a href="http://en.wikipedia.org/wiki/CIELAB" rel="nofollow">here</a> - for your reference whitepoint you'd use the white point of your device (or sRGB if you're using the standard).</p> <p>Finally you can use the formulae you have listed above to do the final transformations.</p> <p>Since this is all relatively painful, you may have better luck calling out to a proper color management tool like <a href="http://www.littlecms.com/" rel="nofollow">LCMS</a></p>
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    1. POHow would I compare a single RGB value with a collection to find the closest match using the CIE94 formula - in Ruby,
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    1. USthe Tin Man
    1. USMichael Anderson
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    1. POHow would I compare a single RGB value with a collection to find the closest match using the CIE94 formula - in Ruby,
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    1. COthat is a superb answer, exactly what I was looking for. Thank you for taking the time to put it together.
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      1. USRMcNairn
    2. CO+1 Excellent answer. I've yelled about some of your points for years, because amateur photographers don't understand the issues of color-spaces, their environments, and reference points.
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      1. USthe Tin Man
    3. COJust as a quick followup - i've got the conversions ticking over nicely and it seems to be matching quite nicely. thanks again.
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