sRGB vs. RGB Locked

Started Apr 22, 2013 | Discussions thread
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Victor Engel
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Re: Why use sRGB for JPEGs...
In reply to John King, Apr 24, 2013

John King wrote:

Gidday Victor

Victor Engel wrote:

Bit depth has little to do with color space.

To a certain degree you are right.

Notice I didn't say it had nothing to do with it....

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:

Looks silver to me (with blue reflections of the sky, apparently). 

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

You must be doing something wrong, then. Adobe RGB and sRGB are very similar at the red corner of the color space.

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.

If the whites and blacks are significantly different, you have not set things up properly. White and black should be identical in most color spaces.

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.

I have a particular rose whose flowers have the most saturated red I think I've ever seen, except, perhaps from LEDs. I don't think it's out of gamut in Prophoto RGB, but it is out of gamut in sRGB and Adobe RGB. It's one of those colors that almost hurts to look at.

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.

How is it defective?

Higher bit depth gives one more editing latitude before one colour number becomes the same as another, but different colour number.

Huh? If it's the same, how can it be different?

Using a 16 bit colour space allows the reproduction of more finely nuanced prints and other images, without any form of posterisation.

Sure, but that doesn't have anything to do with gamut, except that if you use a wider gamut, consecutive numbers will be farther apart. You are more likely to see posterization in an 8-bit ProPhoto RGB image, for example, than an 8-bit sRGB image.

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Victor Engel

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