arrayfire looks more reasonable on Lena than on the car 'inpt.jpg' image
ok, gradients look reasonable on Lena now that I took the absolute value. less intensity than my opencv ref impl, because arrayfire likes to divide the result by 2
all right, the results match visually for Lena, using ref impl and gradient_arrayfire
results match for arrayfire and reference on car.jpg.
including arrayfire results too