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