Out-of-Gamut colors

[digitaldog]

According to the cited article, the amount of compression used is up to the maker of the profile, and in most cases the entire larger gamut (starting at its boundaries) is not simply smashed down to the size of the gamut of the printer.

Bill Janes
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With the perceptual rendering intent, the creator of the profile can use any method they wish. If you profile the same device using the same instrument and patches but build it using two packages, you'll almost always end up seeing difference in output using Perceptual.

In theory, Colorimetric should match but that never happens either. But Perceptual is all over the map.

[bjanes]

The compression will take place at the boundaries of the image gamut as you should see in the screen dump I placed in my last post. Hopefully you can see that there are colors that are not at the boundaries of the larger triangle. Some may be, some may not be but the area where a color lies in the chromaticity diagram is where the mapping starts. Does that answer the question?
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According to this post, the compression usually does NOT take place at the boundaries of the image gamut, because the CMM does not take into account the image gamut, nor does it take place at the boundaries of the larger color space.

[a href=\"http://www.steves-digicams.com/techcorner/July_2005.html:]http://www.steves-digicams.com/techcorner/July_2005.html:[/url]

"Some misinformation on the web would lead you to believe that because the CMM cannot account for  image gamut, that it simply compresses the entire gamut of the color space used by the image so that it fits inside the printer's gamut.  This is, however, also not true. "

[Hermie]

Unfortunately, you still have not answered my original question. If you are using ProphotoRGB with a moderately sized gamut image and printing to a relatively smaller gamut device using perceptual rendering with a table based profile, does the gamut compression occur for the entire ProphotoRGB gamut or only to the extent of bringing the out of gamut colors into gamut?

The latter instance would require a "smart algorithm" that actually looks at the contents of the file to determine the extent of compression required, whearas the former would simply squash down the whole ProphotoRGB gamut.

Bill Janes
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The whole idea of perceptual is to prevent gamut clipping and thus to maintain a distinction between out-of-gamut colors. This means that image colors are shifted towards the center of the xy chromaticity diagram. To make room for out-of-gamut colors, some in-gamut colors must move too.

The extend to which in-gamut colors are compressed depends on the algorithms used by the profiling software. As there are no icc specifications for the perceptual rendering intent, the implementation is vendor specific -> proprietary algorithms.

There a many methods of gamut mapping. The difficult thing with gamut mapping is to account for the differences in shape of source and destination spaces. An additional problem is: What does the profiling software know about the source space of an image? It has to make some assumptions regarding this source space/colors.

Perceptual rendering applies the same gamut compression to all images, even when the image contains no significant out-of-gamut colors. As Mike Chaney writes "No smart CMM" yet.

Herman

[bjanes]

The whole idea of perceptual is to prevent gamut clipping and thus to maintain a distinction between out-of-gamut colors. This means that image colors are shifted towards the center of the xy chromaticity diagram. To make room for out-of-gamut colors, some in-gamut colors must move too.

The extend to which in-gamut colors are compressed depends on the algorithms used by the profiling software. As there are no icc specifications for the perceptual rendering intent, the implementation is vendor specific -> proprietary algorithms.

There a many methods of gamut mapping. The difficult thing with gamut mapping is to account for the differences in shape of source and destination spaces. An additional problem is: What does the profiling software know about the source space of an image? It has to make some assumptions regarding this source space/colors.

Perceptual rendering applies the same gamut compression to all images, even when the image contains no significant out-of-gamut colors. As Mike Chaney writes "No smart CMM" yet.

Herman
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In the absence of a smart CMS, photographers will have to assume more responsibility for the rendering of out of gamut colors. Perceptual rendering may be helpful in many cases, but, since it does not take the image contents into account, may not give the intended results and manual tweaking may be needed.

In an old post, Bruce Fraser gives some advice:

[a href=\"http://www.creativepro.com/story/feature/8582.html]http://www.creativepro.com/story/feature/8582.html[/url]

He briefly discusses tweaking an out of gamut Dahlia in his Color Management book.

Here is another post explaining how to use Hue and Saturation in Photoshop together with the gamut warning view to tame out of gamut colors:

http://www.udel.edu/cookbook/scan-print/ga...mut-huesat.html

Bill Janes