% DiffEdge: apply Phillips' differential edge detector
% 
% by Will Dwinnell
%
% Output = DiffEdge(Input,s)
%
% Output  = output image
% Input   = grayscale image
% s       = size of analysis template: odd integer >= 3
%
% Note: Output is same size as input, edges are padded with zeros
%
% Reference:
%   "Image Processing, Part 6: Advanced Edge Detection"
%   by Dwayne Phillips
%   "C/C++ Users Journal", Jan-1992
%
% Last modified: Mar-23-2007

function Output = DiffEdge(Input,s)

% initialize variable
Output = zeros(size(Input));

% useful calculation
r = floor(s / 2);   % radius of analysis template

% loop over opposing pixels
for i = -r:r-1,
    % top vs. bottom
    Output(r+1:end-r,r+1:end-r) = max(Output(r+1:end-r,r+1:end-r),abs(Input(r+1-r:end-r-r,r+1+i:end-r+i) - ...
        Input(r+1+r:end-r+r,r+1-i:end-r-i)));
    % left vs. right
    Output(r+1:end-r,r+1:end-r) = max(Output(r+1:end-r,r+1:end-r),abs(Input(r+1-i:end-r-i,r+1-r:end-r-r) - ...
        Input(r+1+i:end-r+i,r+1+r:end-r+r)));    
end


% EOF