Commonly used evaluation functions are introduced, and an evaluation function based on boundary gra- dients is proposed to overcome the shortcomings of the existing evaluation functions. The evaluation value is computed by using the edge strengths of boundary pixels after the binary boundary is obtained by edge detection procedures. Since the computation of the new function takes more time, a coarse-to-fine focusing procedure is proposed to improve the overall running speed performance. Experimental results on real images show the effectiveness of the novel method. Compared with the existing evaluation functions, the new function can achieve higher accuracy with an acceptable execution speed.