sRGB vs. RGB Locked

Started Apr 22, 2013 | Discussions thread
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John King
John King Forum Pro • Posts: 14,941
Re: Why use sRGB for JPEGs...

Gidday Victor

Victor Engel wrote:

Bit depth has little to do with color space.

To a certain degree you are right. However, it does rather depend on how coarsely or finely you wish to display colour nuances. I have a test image that I have used extensively for testing what I have said here when printing. My infamous red headlight:

This prints very differently in 8 or 16 bit sRGB, but far less difference when using 16 bit aRGB or PPRGB.

There is a particular speck of sap above the bonnet join on the right of the image. Measuring just below this speck shows that the green channel value in 16 bit sRGB drops from 62 (in 8 bit colour numbers) in PPRGB to 2 in sRGB. This difference (not reflected in the R/B channels) changes the image from bright red to a sickly pinkish orange. The blacks also tend to grey, and the whites are wrong also.

I PP all of these images from the same RAW file, using the same settings. The only thing that I have changed is the bit depth and colour space (gamut). Printed from 16 bit PPRGB, this image is all but a perfect match for the original car paintwork.

I see the same effect in lots of other images, even when not a really saturated colour such as this. Many natural and man made colours are out of gamut for aRGB, and way out of gamut for sRGB.

A color space is usually a three dimensional volume. The bit depth comes into play when approximating the continuous space. The higher the bit depth, the closer neighboring colors are, but 6 bit, 8 bit, and 16 bit sRGB models all cover the same volume.

Yes. But that sRGB volume is both deficient and defective, as I said before. Higher bit depth gives one more editing latitude before one colour number becomes the same as another, but different colour number. Using a 16 bit colour space allows the reproduction of more finely nuanced prints and other images, without any form of posterisation.

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Regards, john from Melbourne, Australia.
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