Page 84 - Computational Colour Science Using MATLAB

IMPLEMENTATIONS AND EXAMPLES 71

% lab1 and lab2 must be 3 by 1 or 1 by 3 matrices
% and contain L*, a* and b* values
% p is +1 if the hue of the trial (lab2) is anticlockwise
% from the standard (lab1) and -1 otherwise

[c1, h1] = car2pol([lab1(2) lab1(3)]);
[c2, h2] = car2pol([lab2(2) lab2(3)]);
p = (h2-h1);
if (p==0)
p=1;
else
if (p>180)
p = p - 360;
end
p = p/abs(p);
end

The function cmcde operates in a similar way to cielabde but computes the
CMC(l:c) colour difference. A typical function call would be

[de, dl, dc, dh] = cmcde(lab1, lab2, paral, parac);

where the variables paral and parac represent the parametric values l and c.
Default values of 1 are used for both l and c if the number of arguments to the
function is less than four. Note that the component delta values that are also
returned are the CMC delta values rather than the CIELAB delta values. Note
also that the MATLAB trigonometric functions expect input in radians and
therefore the hue angle in degree must be multiplied by the factor p/180 when
computing the parameter T in Equation (5.14). The dimensions of the CMC
tolerance ellipsoids are computed based upon the CIELAB values of the first of
the triplets lab1 which is assumed to be the standard.

Box 12: cmcde.m

function [de,dl,dc,dh] = cmcde(lab1,lab2,paral,parac)

% function [de,dl,dc,dh] = cmcde(lab1,lab2,paral,parac)
% computes colour difference from CIELAB values