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