% demonstrate edge detection

% numbers of colors
sncols=128;  
ncols=32;

% get image from MATLAB image
load('trees');

% show original image
figure(1);
showgimg(real(X),sncols);
drawnow;
 

% construct convolution functions
[m,n] = size(X);
gs = [1 -1];  ge = [];
hs = [1 -1]; he = [];
g =  [gs,zeros(1,m-length(gs)-length(ge)),ge];
h =  [hs,zeros(1,n-length(hs)-length(he)),he];

% construct convolution matrices as sparse matrices
Y = spcnvmat(g);   
Z = spcnvmat(h);
Wg = Y*X;
Wh = X*Z';
  
% show transformed images
figure(2);
showgimg(Wg,ncols);
drawnow;

figure(3)
showgimg(Wh,ncols);
drawnow;

figure(4)
showgimg(abs(Wg)+abs(Wh),ncols);
drawnow;