Matching Point Features with Ordered Geometric, Rigidity, and Disparity Constraints

1041 and then the line invariant (6) was computed for each pair of line sets and compared using the metric (7). The sets of lines chosen are given in the following table (refer to Fig. I). The table in (9) shows the results. The only bad entry in this matnx is in the position (4, 4). This is because of the fact that the four lines chosen,contained three coplanar lines (lines B. D and E). This causes the values of the invariant to be indeterminate (that is (0, 0, O)), and shows that such instances must be detected and avoided. Once again, the four-line invariant is shown to be a useful discriminator between sets of four lines. ACKNOWLEDGMENT I am indebted to J. Mundy for introducing me to the subject of projective invariants, and for many enlightening conversations during the preparation of this correspondence.