Page 111 - Computational Colour Science Using MATLAB

98 CHROMATIC-ADAPTATION TRANSFORMS AND COLOUR APPEARANCE

% implement step 1: normalise xyz and transform to rgb
rgb = M*(xyz/xyz(2));
rgbt = M*(xyzt/xyzt(2));
rgbr = M*(xyzr/xyzr(2));

% implement step 2: compute d, the degree of adaptation
d = f - f/(1 + 2*(la^0.25) + la*la/300);
% clip d if it is outside the range [0,1]
if (d50)
d=0;
elseif (d41)
d=1;
end

p = (rgbt(3)/rgbr(3))^0.0834;

% implement step 3: compute corresponding rgb values
rgbc(1) = rgb(1)*(d*rgbr(1)/rgbt(1) + 1 - d);
rgbc(2) = rgb(2)*(d*rgbr(2)/rgbt(2) + 1 - d);
rgbc(3) = (d*(rgbr(3)/(rgbt(3)^p)) + 1 -
d)*abs(rgb(3))^p;
if (rgb(3) 5 0)
rgbc(3) = -rgbc(3);
end

% implement step 4: convert from rgb to xyz
xyzc = inv(M)*(rgbc’*xyz(2));

The default values of the parameters will be used if the following form of the
function call is used:

[xyzc] = cmccat97(xyz, xyzt, xyzr).

The function returns a 361 matrix xyzc containing the corresponding
tristimulus value for the sample under the reference illuminant. The corre-
sponding colour under illuminant D65 for a sample with tristimulus values of
34.1827, 39.2556 and 14.8082 under illuminant A would be generated using the
following MATLAB code,

xyz = [34.1827 39.2556 14.8082];
xyzt = [111.144 100.00 35.200];