it is proposed to use the posterior mean instead of MAP￣[4]
i.e.
to use square loss instead of 0-1 loss.The advantages are multiple:（1）much faster convergent speed.roughly 1/√ n versus 1/logn;（2）high precision in location of edges.especially for important corners
crossings and Y-shaped edges etc.;（3）more noise-resisting as indicated in[7].Both theoretical and experimental results are presented.
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