The schematic flow of registration. At STEP 1, the template models were updated until the convergence criterion or termination criterion was met. The convergence criterion was that the average mean bidirectional difference between after the end of the k – 1th loop and after the end of the kth loop in the most recent 10 iterations was <0.001 mm. Otherwise, the iteration process was terminated when the optimization calculation completed the 3000th iteration. When the convergence criterion or termination criterion was met, the iteration process for STEP 1 was terminated, and the iteration process for STEP 2 was then initiated. These processes were repeated until the convergence criterion or termination criterion was met in the last STEP. Once registration was completed for one target model, the iteration process was initiated for the next target model.