Before calculating these, we blur the
grayscale image to increase robustness against noise
and !ne detail. For the edge map, we use a 3 ⇥ 3 box
!lter and perform Canny edge detection [5] with hysteresis thresholds of 50 and 110. For the x-y gradients,
we use a 5 ⇥ 5 Gaussian !lter and apply the Scharr