Colour transformation method

Claim 1

A method for transforming color image data of first media so as to be reproduced by second media, the color reproducing capability of which is smaller than that of the first media, comprising: obtaining a first colour range or gamut on a colourimetric system on the basis of a plurality of color image data of the first media beforehand, the colorimetric system being one of the CIE standard L*u*v* and L*a*b* color systems; obtaining a second colour range or gamut on the colourimetric system on the basis of a plurality of color image data of the second media beforehand; calculating the first lightness difference between the maximum and minimum values of the lightness on the lightness axis of the first color gamut, the second lightness difference between them of the seond color gamut, and the lightness defference ratio between the first and second lightness differences in advance; inputting image data of the first media; converting the L* value of a color image data "T" of the first media on the basis of the lightness difference ratio; comparing the chroma "rT" of the color image data "T" of the first media with a threshold value which is "a" (a < 1.0) times of the maximum chroma "r2max" of the second color gamut at the lightness "LT" and the hue " ϑ T" of the color image data "T" ; and in the case that the chroma "rT" is not larger than the threshold value, using the u* and v* values or the a* and b* values of the color image data "T" without any change, or in the case that the chroma "rT" is larger than the threshold value, calculating the first chroma difference between the maximum chroma "r1max" of the first color gamut at the lightness "LT" and the hue "ϑ T" and the threshold value, the second chroma difference between the maximum chroma "r2max" of the second color gamut and the threshold value, and the chroma difference ratio between the first and second chroma differences, converting the chroma "rT" on the basis of the chroma difference ratio, and converting the u* and v* values or the a* and b* values of the color image data "T" so as to obtain the converted chroma without changing the hue " ϑ T".

Claim 2

A method of transforming color image data of first media so as to be reproduced by second media, the color reproducing capability of which is smaller than that of the first media, comprising: obtaining a first colour range or gamut on a colourimetric system on the basis of a plurality of color image data of the first media beforehand, the colorimetric system being one of the CIE standard L*u*v* and L*a*b* colorimetric systems; obtaining a second color gamut on the colorimetric system on the basis of a plurality of color image data of the second media beforehand; calculating the first lightness difference between the maximum and minimum values of the lightness on the lightness axis of the first color gamut, the second lightness difference between them of the seond color gamut, and the lightness defference ratio between the first and second lightness differences in advance; inputting image data of the first media; converting the L* value of a color image data "T" of the first media on the basis of the lightness difference ratio; assuming a line passing the lightness "LT" and the chroma "rT" of the color image data "T" while keeping the hue "ϑ T" of the color image data "T"; obtaining the maximum chroma "r1max" on the line of the first color gamut and the maximum chroma "r2max" on the line of the second color gamut; comparing the chroma "rT" of the color image data "T" with a threshold value which is "a" (a < 1.0) times of the maximum chroma "r2max"; and in the case that the chroma "rT" is not larger than the threshold value, using the u* and v* values or the a* and b* values of the color image data "T" without any change, or in the case that the chroma "rT" is larger than the threshold value, calculating the first chroma difference between the maximum chroma "r1max" of the first color gamut and the threshold value, the second chroma difference between the maximum chroma "r2max" of the second color gamut and the threshold value, and the chroma difference ratio between the first and second chroma differences, converting the chroma "rT" on the basis of the chroma difference ratio, converting the u* and v* values or the a* and b* values of the color image data "T" so as to obtain the converted chroma without changing the hue "ϑ T", and converting further the L* value of the color image data "T" in accordance with the conversion amount of the chroma so as to lower the lightness in accordance with the lowering of the chroma at least in a high lightness region.

Claim 3

A method for transforming color image data of first media so as to be reproduced by second media, the color reproducing capability of which is smaller than that of the first media, comprising: obtaining a first color gamut on a colorimetric system on the basis of a plurality of color image data of the first media beforehand, the colorimetric system being one of the CIE standard L*u*v* and L*a*b* colorimetric systems; obtaining a second color gamut on the colorimetric system on the basis of a plurality of color image data of the second media beforehand; calculating the first lightness difference between the maximum and minimum values of the lightness on the lightness axis of the first color gamut, the second lightness difference between them of the seond color gamut, and the lightness defference ratio between the first and second lightness differences in advance; inputting image data of the first media; converting the L* value of a color image data "T" of the first media on the basis of the lightness difference ratio; comparing the chroma "rT" of the color image data "T" of the first media with a threshold value which is "a" (a < 1.0) times of the maximum chroma "r2max" of the second color gamut at the lightness "LT" and the hue "ϑ T" of the color image data "T"; and in the case that the chroma "rT" is not larger than the threshold value, using the u* and v* values or the a* and b* values of the color image data "T" without any change, or in the case that the chroma "rT" is larger than the threshold value, assuming a first region enclosed between the inner gamut surface which comprises a value which is "a" times of the maximum chroma value at the each hue and lightness of the second color gamut and the outer surface of the first color gamut which comprises the maximum chroma value at each of hue and lightness of the first color gamut, and a second region enclosed between the inner gamut surface abovementioned and the outer surface of the second color gamut which comprises the maximum chroma value at each of hue and lightness of the second color gamut; converting the u* and v* values or the a* and b* values of the color image data "T" so as to make the first region in continuous adaption with the second region.