function [im_on, im_off] = retina(im0, wsize, S)

K = 3;
h = fspecial('gaussian', wsize, S) - fspecial('gaussian', wsize, K*S);
[ny, nx, nc] = size(im0);
h = wsize*h /norm(h);

%h = [[0 -0.25 0]; [-0.25 1 -0.25]; [0 -0.25 0]; ];
im_on = imfilter(im0, h, 'replicate');
im_on = im_on/max(reshape(im_on, 1, nx*ny));
%im_on = min(1, max(0, im_on));

im_off = imfilter(1-im0, h, 'replicate');
im_off = im_off/max(reshape(im_off, 1, nx*ny));
%im_off = min(1, max(0, im_off));