[argyllcms] Re: Custom Illuminant

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 12 Jul 2014 12:44:32 +1000

robert@xxxxxxxxxxxxxxxxxx wrote:
> The second problem was that I was that I was using -pj in tiffgamut and -pl
> in iccgamut, so I wasn't comparing apples to apples.  With both set to -pj,
> an sRGB image which isn't clipped is within the sRGB gamut viewed with
> viewgam.

        yes, that is a trap. It would be nice to make the tools a  bit more
seamless in regard to that.

> If -g is not used, the Perceptual mapping is from the gamut of the source
> profile (which I will call SG) to the gamut of the destination profile
> (which I'll call DG).  The B2A0 table, which is responsible for the
> Perceptual mapping, effectively 'squeezes' the whole of SG into DG.  All of
> the colors that were present in SG will be remapped to new colors in DG,
> even if none of the colors in SG were outside of the DG gamut (the effect of
> the 'squeeze').
> The effect of using -g is to create a new gamut which is the union of DG and
> CG (which I will call UG).

No, effectively the Image Gamut (CG) becomes the Source Gamut (SG).

[There are some subtleties - an image gamut isn't expected to contain the source
 colorspace white and black points, so the effective source gamut is the Image 
 supplemented by some information from the Source Colorspace Gamut.]

The gamut mapping creation is then basically the same.

> - The table then does a Perceptual mapping from UG to DG.

No, the table does a Perceptual mapping from the Image Gamut (CG) to Destination
Colorspace Gamut (DG).

Graeme Gill.

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