said orientation field is computed by deriving {HEAD ~ a gradient field} through {TAIL ~ 2D convolution} on said image and then extracting a principal component of the gradient field over a sliding window of fixed size using principal component analysis 
through
said orientation field is computed by deriving { HEAD ~ a gradient field } through { TAIL ~ 2D convolution } on said image and then extracting a principal component of the gradient field over a sliding window of fixed size using principal component analysis
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH