The sample distribution is shown in Fig. 10. The particles generated into 3D space directly by pseudo-random sampling are scattered in a broad area, whereas those generated through back projection are aggregated. In the sampling step, the initial particles are generated by back-projection-based sampling. To cope with sudden changes in shape and motion and to avoid becoming stuck in local spurious minima, random particles at a rate of 10% are introduced at each iteration based on the back-projection-based sampling.