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