positioning said edge detection filter at an origin of a first direction and {TAIL ~ an origin} of a second direction within {HEAD ~ said image} 
positioning said edge detection filter at an origin of a first direction and { TAIL ~ an origin } of a second direction within { HEAD ~ said image }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD