[argyllcms] Re: Characterizing a scanner using Argyll

  • From: Gerhard Fuernkranz <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 02 Feb 2009 23:42:24 +0100

Klaus Karcher wrote:

> I inspected the XYZ CLUT profile and noticed that the A2B tag contains
> the identity matrix.
> Could XYZ CLUT profiles with "real" matrices be an option to improve
> scanner profiles?

The model of a "lut16Type" for the A2B tables is RGB -> matrix -> shaper
-> CLUT -> shaper -> PCS. So I'm not sure, whether using this matrix
makes sense for non-linear RGB. If we assume linear (gamma 1.0) RGB
(e.g. raw data from the sensors), and if we populate this matrix with an
approximate RGB -> XYZ transformation, then it would likely shrink the
coverage of the CLUT, resulting in the need for a CLUT with an even
higher grid resolution in order to achieve the same accuracy. More
appropriate would be IMO rather a matrix, which maximizes the CLUT
utilization for the set of all valid RGB triples which can be returned
by the scanner, while still producing non-negative numbers after
applying the matrix for all valid RGB triples which can be returned by
the scanner.

BTW: The ICC spec also sais that the matrix shall be the identity matrix
unless the input color space is XYZ. So w/o violating the ICC spec,
using the matrix seems not to be an option at all, though I guess that
most CMMs won't care and will honor a non-identity matrix also for input
color spaces different from XYZ.


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