"Once the geometry of the real and virtual spaces is correctly registered, the remaining problem to solve is the occlusion between real and virtual objects. To solve this problem, we have to know the geometric model and the position of a real object in MR space. It is easy when the real objects are static, however, the problem becomes drastically difficult when they start moving."
H. Tamura, “Overview and Final Results of the MR Project”, Proc. 2nd Int’l Symp. Mixed Reality (ISMR 2001), Virtual Reality Soc. Japan, Tokyo, 2001, pp. 97-104.