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.