[argyllcms] Re: Custom Illuminant

  • From: Brad Funkhouser <brad.funkhouser@xxxxxxxxxxx>
  • To: "argyllcms@xxxxxxxxxxxxx" <argyllcms@xxxxxxxxxxxxx>
  • Date: Mon, 07 Jul 2014 13:39:59 -0500

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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