Working color space

Started Apr 26, 2012 | Discussions thread
Chris Noble
Chris Noble Senior Member • Posts: 2,729
Some misconceptions on gamut vs. resolution

Simon said:
Chris, I think I know what you mean, but I don't follow the words.

I posted before my first cup of coffee -- sorry! I mixed comments on gamut and comments on resolution , which are two different and independent aspects of image coding.

ProPhoto RGB colour space could be 8-bit, 16-bit or any other.

Quite right. As could sRGB and aRGB.

However, with a very wide colour space, there's more danger of quantisation errors unless you use 16-bit or more. Hence, as you say, if you are using a wide colour space then you should work in 16-bit.

The higher working space resolution is more important than the wider gamut. The working color space can arguably have the same gamut as the output space (although a slightly wider one is probably useful), but it needs to have a much higher resolution (in this case 16 bits vs. 8 bits for the output) or else quantization error accumulates through the imaging pipeline.

So you could theoretically process an image intended for 8-bit sRGB output in 16-bit sRGB, as long as none of the pixels are right on the gamut edge. 16-bit ProPhoto RGB gives you the finer resolution that you always need, and a wider gamut than your output just in case you need that as well.

If you shoot JPEGs for the web that you don't plan to print aRGB, you should arguably shoot sRGB rather than aRGB, because the image is captured at the same resolution as the output. So you get a lower output resolution when you convert aRGB to sRGB than you would if you had shot sRGB to begin with. The aRGB coding squeezes a wider gamut into the same 8 bits as the narrower sRGB, hence sacrificing color resolution.

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