function out = safe_conv(I, f)
  w = floor(size(f)/2);
  Ipad = C = zeros(size(I,1)+2*w(1), size(I,2)+2*w(2));
  Ipad(w(1)+1:end-w(1), w(2)+1:end-w(2)) = I;
  C(w(1)+1:end-w(1), w(2)+1:end-w(2)) = 1;
  
  Ipad = conv2(Ipad, f, 'same');
  C = conv2(C, f, 'same');
  out = Ipad./C;
endfunction