%Extracts vectors from an image stack
function [X,R]=imstack2vectors(S,MASK)
[M,N,n]=size(S);
if nargin ==1
   MASK=true(M,N);
else
   MASK=MASK~=0;
end
[I,J]=find(MASK);
R=[I,J];
Q=M*N;
X=reshape(S,Q,n);
MASK=reshape(MASK,Q,1);
X=X(MASK,:);