whether there is any branch pixel among the traced pixels in the exemplary case illustrated in fig2 the contour pixels 22 and 28 are found to be branch pixels at step 48 and at a next step s49 one of the branch pixels e g contour pixel 22 or 28 is selected at a next step s50 the branch pixel selected in step s49 is tested whether any one of the neighboring contour pixels thereof is an untraced contour pixel having the tracing index selected in step s42 if the number n of neighboring contour pixels conforming to the criterion specified in step s50 is larger than one one of such pixels is newly selected as a candidate pixel to trace at the next step s47 and the process returns to step s43 with the candidate pixel as the selected pixel when n 1 at step s50 the branch pixel mark on the branch pixel is removed at step s52 so that the pixel will not be considered as a branch pixel anymore and at the next step s47 the contour pixel counted at step s50 is newly chosen as a candidate pixel to trace and the process returns to step s43 with the candidate pixel as the selected pixel for the case n 0 at step s50 the branch pixel selected.