[argyllcms] Re: negative primaries

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 07 Jun 2010 12:29:39 +1000

Elle Stone wrote:

Also, the ICC is coming around to realizing that negative primaries
are a good thing. Section 1.1.3, “PCS encoding range is limited to
[0,2) [sic]” of ICC Votable Proposal Submission, Floating-Point Device
Encoding Range, dated June 16, 2006, says "It is also possible that
some device values may have corresponding XYZ values that are
negative. Such values can result from digital camera color analysis
matrices, or chromatic adaptation transforms applied to extremely
saturated blue colors. In most cases, it is acceptable to clip
negative XYZ values to zero as such values do not correspond to real
colors. However in some cases this may be unacceptable, for example if
perfect round-tripping is desired."

If the colors are really outside the spectrum locus, then I guess
it's acceptable to clip them, since they must be erroneous.

But on the other hand if they are a result of a chromatic adaptation, or of
the metameric errors introduced by the different spectral
sensitivities of a sensor and the standard observer (non Luther
condition sensors), then these apparently imaginary colors may
actually correspond to real world colors, and shouldn't be clipped,
since they can be brought into the real world gamut during gamut mapping.

[Another possibly source of such "imaginary" colors may be
 the choice of the white point. If there is a highly saturated
 source in the scene that is brighter than the chosen white point,
 then it may well end up outside the spectrum locus if its Y value
 gets clipped to that of the white.]

It would be interesting to see if non-Luther condition cameras
routinely produce such imaginary XYZ values for highly saturated
source colors. If so, then the ICC's advice to clip such colors
is probably not so good...

Graeme Gill.

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