% imagehist2D
% usage: imagehist2D(pic) or 
% read a jpeg image and split into three RGB components
% and show the 2-compoent histograms
% pic is the name of a jpg file given in 'filename.jpg' format
% typically the component images will be returned as matrices of byte-sized unsigned integers 
% allowing 256 levels for each colour, i.e., 24 bit colour
% Rmap, Gmap and Bmap are colormaps for showing the R,G,B components

function  imagehist2D(pic,numbins);

% load picture
[R,G,B,Rmap,Gmap,Bmap] = jpg2rgb(pic,'nopic');
Z = zeros(size(R));
mc = double(max([R(:);G(:);B(:)]));

% show images
figure(1)
clf
imshow(cat(3,R,G,B));
pause

figure(2)
clf
subplot(2,3,1)
imshow(cat(3,Z,G,B));
xlabel('GREEN-BLUE')
subplot(2,3,4)
hGB = hist2D(double(G(:)),double(B(:)),numbins,numbins);
imshow(hGB,[]);
xlabel ('green -->>')
ylabel ('blue -->>')

pause

subplot(2,3,2)
imshow(cat(3,R,Z,B));
xlabel('RED-BLUE')
subplot(2,3,5)
hRB = hist2D(double(R(:)),double(B(:)),numbins,numbins);
imshow(hRB,[]);
xlabel ('red -->>')
ylabel ('blue -->>')

pause

subplot(2,3,3)
imshow(cat(3,R,G,Z));
xlabel('RED-GREEN')
subplot(2,3,6)
hRG = hist2D(double(R(:)),double(G(:)),numbins,numbins);
imshow(hRG,[]);
xlabel ('red -->>')
ylabel ('green -->>')