function plist = findEmpty(synthIM, w) 
    % try converting to binary first then xor the difference
    
    simsize = size(synthIM);
    
    dilate = imdilate(synthIM,ones(3));
    dilthresh = threshzero(dilate);
    synthresh = threshzero(synthIM);
    edges = xor ( dilthresh, synthresh );
    
    ptlst = find(edges == 1.0); 
    [is js] = ind2sub(simsize,ptlst);
    is = is - floor(w/2);  js = js - floor(w/2);
    plist = [is'; js'];
    
    %% delete out of bounds points
    %% plist(:,min(plist,[],1)<=0)=[];
end


