Page 51 - Computational Colour Science Using MATLAB

38 COMPUTING CIE TRISTIMULUS VALUES

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

a = 0.083;
n = length(P);
for i=2:n-1
cP(i) = -a*P(i-1) + (1 + 2*a)*P(i) - a*P(i+1);
end
cP(1) = (1 + a)*P(1) - a*P(2);
cP(n) = (1 + a)*P(n) - a*P(n-1);

The format for this function is

[cp] = cband(p)
where p is an n61or16n matrix. In cband the dimensions of p are checked to
ensure that only a single reflectance spectrum has been passed to the function.
The p matrix is then converted to a row matrix. The MATLAB command

p = p(:)

converts the matrix p into a column matrix and the transpose function is
added

p = p(:)’

to ensure that p is a row matrix.
This function operates on a single reflectance spectrum although it would be
relatively easy to modify the code so that it operates on an n6m matrix of m
reflectance spectra. The main purpose of this book is education rather than
producing the fastest and most efficient code, and therefore most of the functions
have been written to clearly demonstrate the computations involved. The following
modification of cband, however, demonstrates how the code would be changed to
allow more than one reflectance spectrum to be passed to the function: