5.11.4 Color Difference Metrics

(require 'color-space)

The low-level metric functions operate on lists of 3 numbers, lab1, lab2, lch1, or lch2.

(require 'color)

The wrapped functions operate on objects of type color, color1 and color2 in the function entries.

Function: L*a*b*:DE* lab1 lab2

Returns the Euclidean distance between lab1 and lab2.

Function: CIE:DE* color1 color2 white-point

Function: CIE:DE* color1 color2

Returns the Euclidean distance in L*a*b* space between color1 and color2.

Function: L*a*b*:DE*94 lab1 lab2 parametric-factors

Function: L*a*b*:DE*94 lab1 lab2

Function: CIE:DE*94 color1 color2 parametric-factors

Function: CIE:DE*94 color1 color2

Measures distance in the L*a*b* color-space. The three axes are individually scaled in their contributions to the total distance.

DE*94 is not symmetrical in its arguments. lab1 is the “reference” color and lab2 is the “sample” color.

The CIE has defined reference conditions under which the metric with default parameters can be expected to perform well. These are:

• The specimens are homogeneous in colour.
• The colour difference (CIELAB) is <= 5 units.
• They are placed in direct edge contact.
• Each specimen subtends an angle of >4 degrees to the assessor, whose colour vision is normal.
• They are illuminated at 1000 lux, and viewed against a background of uniform grey, with L* of 50, under illumination simulating D65.

The parametric-factors argument is a list of 3 quantities kL, kC and kH. parametric-factors independently adjust each colour-difference term to account for any deviations from the reference viewing conditions. Under the reference conditions explained above, the default is kL = kC = kH = 1.

The Color Measurement Committee of The Society of Dyers and Colorists in Great Britain created a more sophisticated color-distance function for use in judging the consistency of dye lots. With CMC:DE* it is possible to use a single value pass/fail tolerance for all shades.

Function: CMC-DE lch1 lch2 parametric-factors

Function: CMC-DE lch1 lch2 l c

Function: CMC-DE lch1 lch2 l

Function: CMC-DE lch1 lch2

Function: CMC:DE* color1 color2 l c

Function: CMC:DE* color1 color2

CMC:DE is a L*C*h metric. The parametric-factors argument is a list of 2 numbers l and c. l and c parameterize this metric. 1 and 1 are recommended for perceptibility; the default, 2 and 1, for acceptability.