% hist2D
% usage: H = hist2D(X,Y,m,n);
% find a two dimensional correlation histrogram of two matrix quantities 
% the returned nxm matrix H satisfies 
%
%         H(n-j+1,i) = # of X and Y values satifying  
%         x0 +(i-1)*dx <= X < x0 + i*dx 
%         y0 +(j-1)*dy <= Y < y0 + j*dy
% where
%          x0 = minX, dx = m/(maxX-minX) and  y0 = minY, dy = n/(maxY-minY)
% the resulting image of H will have the values of X and Y increasing in the standard directions  

function  H = hist2D(X,Y,m,n);

X = X(:);
Y = Y(:);
mX = min(X);
MX = max(X);
mY = min(Y);
MY = max(Y);
H = zeros(m,n);

X = ceil((m/(MX-mX))*(X-mX));
J = find(X<1);
X(J) = 1;
J = find(X>m);
X(J) = m;

Y = ceil((n/(MY-mY))*(Y-mY));
J = find(Y<1);
Y(J) = 1;
J = find(Y>n);
Y(J) = n;

for i=1:m
   for j=1:n
      H(n-j+1,i)= sum((X ==i)&(Y==j));
      end;
   end;