[argyllcms] Re: Custom Illuminant

  • From: Roger Breton <graxx@xxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 07 Jul 2014 16:13:23 -0400

Also, what I like about eciRGB is the fact there is never any issues with
chromatic adaption since everything happens on a pure-D50 scale, from PCS to
RGB to XYZ to "infinite and beyond!" -- sorry, got carried away...

/ Roger

-----Original Message-----
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of Brad Funkhouser
Sent: 7 juillet 2014 14:40
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Custom Illuminant


When AdobeRGB is a little too small, and ProPhotoRGB is just too big, try
Bruce Lindbloom's BetaRGB, it might be just right.

- Brad


> On Jul 7, 2014, at 9:18 AM, <robert@xxxxxxxxxxxxxxxxxx> wrote:
> 
> << How does the gamut mapping know that no mapping is required ? If 
> you tell it the source gamut is ProPhoto, then it will think that lots 
> of gamut mapping is needed, since the ProPhoto gamut is so much larger 
> than a printer !>>
> 
> Well that has finally clarified that point for me!  So with Perceptual 
> the whole source gamut is squashed down to the destination gamut, even 
> if all the colors are within the destination gamut.  This means that 
> in a Perceptual mapping from ProPhoto to print, colors will be 
> compressed resulting in desaturation of the image, particularly of the 
> more saturated colors nearer the print gamut boundary.
> 
> So the following strategy might make sense for a Relative intent
conversion:
> - Going from ProPhoto to print, make sure the colors are more or less 
> within the destination space to avoid too much clipping.
> 
> And the following for Perceptual:
> - Do a Relative conversion from ProPhoto to AdobeRGB (making sure the 
> colors are more or less within the AdobeRGB space before the 
> conversion to avoid too much clipping).
> - Do a Perceptual mapping from AdobeRGB to print.
> 
> I did a comparison of a 1-step ProPhoto [Perceptual to Print 
> conversion] with a 2-step [ProPhoto to AdobeRGB conversion, followed 
> by an AdobeRGB Perceptual to print conversion]:
> - If the original image colors are all within the AdobeRGB gamut there 
> is quite a difference if the print gamut is much smaller than 
> AdobeRGB, but none if it is of similar size.
> - If the original image colors are outside the AdobeRGB gamut, the 
> 2-step approach is significantly better even if the print gamut is 
> very close to the AdobeRGB gamut.
> This is a bit puzzling because I would have expected a difference in 
> all cases (if the full ProPhoto gamut is squashed down in a Perceptual 
> conversion).
> 
> What would be nice would be to be able to make the smaller 
> intermediate working color space using tiffgamut/colprof (from a range 
> of typical images), but I don't see how that could be done.
> 
> Robert
> 
> 


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