[argyllcms] Re: Custom Illuminant

  • From: <robert@xxxxxxxxxxxxxxxxxx>
  • To: <argyllcms@xxxxxxxxxxxxx>
  • Date: Mon, 7 Jul 2014 11:36:37 +0100

> - You say in relation to ICC V2 behaviour: "The main disadvantage is that
> the gamut mapping will only operate exactly as intended when the profile
> linked with the source profile it was setup for". (This only relates to
> Perceptual and Saturation as Colorimetric does not do a gamut mapping). I
> take it that for a print profile, that the source profile should normally
> the working space?

It is whatever space your source images are in. Typically the color space
is used as a proxy for the gamut the images occupy. With limited gamut
color spaces (i.e. sRGB) this is usually a good assumption, since images
are typically optimized to occupy much of the available gamut.

This assumption breaks down if images are stored in large/unlimited gamut
spaces such as L*a*b*, scRGB, ProPhoto etc., and alternative strategies
are needed to create an appropriate gamut mapping.

>  I had put the monitor profile in the -S flag in colprof,
> but this would seem to be incorrect.  Do I understand correctly that if
> profile is made with -S AdobeRGB1998.icc but the working space is ProPhoto
> RGB that the Perceptual and Saturation gamut mappings will not operate as
> intended (I was going to say 'wrong', but I'm sure you would tell me that
> there is no 'right' or 'wrong' in these matters :)).

ProPhoto is problematic, since it has a gamut much larger than
typical images, hence is a poor choice as a proxy for the gamut
of the images. Using it as the source gamut for gamut mapping
will typically result in a dull, desaturated result.

You can either use a small gamut, closer to the actual
gamut the images occupy as the source for gamut mapping
(although this then disregards much of the purpose of
storing images in a large gamut space), or you should
move to a more sophisticated workflow, where you
create the gamut mapping for each individual image,
or batch of images, where the source gamut is determined
by the images themselves.

I take it that the relationship ONLY breaks down in large/unlimited gamut
spaces IF the image gamut is larger than the destination gamut.  If I use
ProPhoto, say, but I make sure that my image is within the gamut of my print
profile, I should be OK for Colomimetric intents, surely? And also for
Perceptual or Saturation also, since no mapping is required?  The only issue
then would be resolution, presumably, but that should be OK in 16 bit, I
would have thought.

If that's right, then the only real risk of using large gamut spaces as
working spaces is having colors which are out-of-gamut for the destination

In relation to this I have another question.  Is there any reason why one
should not convert the image to the print color space prior to doing some
small final edits before printing?  The advantage is that these edits would
then be constrained to the print gamut, and also the gamut warning can be
set to the monitor profile to indicate which colors are not viewable
(clipped to the monitor profile).  The disadvantage is that the image is
'burnt' to the print profile, but that isn't a disadvantage for me because I
always use copies of the original for printing, one for each paper type,
size, resolution etc.

Thanks again ... I can't tell you how much I appreciate your time and effort
in answering what are mostly pretty dumb questions, not even directly
related to Argyll!


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