Page 79 - Computational Colour Science Using MATLAB

66 COMPUTING COLOUR DIFFERENCE

Box 8: car2pol.m

function [c,h] = car2pol(ab)

% function [c,h] = cartopol(ab)
% converts a*b* or u*v* into the polar coordinates
% of Chroma C and Hue H
% ab must be a row or column matrix 2 by 1 or 1 by 2
% see also pol2car

dim = size(ab);
if (dim(1) == 1) | (dim(2) == 1)
ab = ab(:)’; % force to be a row matrix
else
disp(’ab must be a row matrix’);
return;
end
if (dim(2) *=2)
disp(’ab must be 2 by 1 or 1 by 2’);
return;
end

% compute the distance from the centre
c = sqrt(ab(1)*ab(1) + ab(2)*ab(2));

% compute the angular term
if (ab(1) == 0) & (ab(2) > 0)
h = 90;
elseif (ab(1) == 0) & (ab(2) < 0)
h = 270;
elseif (ab(1) < 0) & (ab(2) == 0)
h = 180;
elseif (ab(1) > 0) & (ab(2) == 0)
h=0;
elseif (ab(1) == 0) & (ab(2) == 0)
h=0;
else
h = atan(abs(ab(2))/abs(ab(1)));
h = 180*h/pi; % convert from radians to degrees
if ((ab(1) > 0) & (ab(2) > 0))
h = h; % first quadrant