Fig. 1. Schematic diagram of experiment 1

Fig. 2. Schematic diagram of experiment 2

Fig. 3. Flow chart of image pre-processing

Fig. 4. Flow chart of tracking algorithm

Fig. 5. Results of pre-processing. (a) Original image; (b) down-sampled image; (c) image filtered by Lee filter; (d) image filtered by Kuwahara filter; (e) image denoised by Gaussian filter; (f) edge detection result with Canny operator

Fig. 6. Tracking results of experiment 1. (a) Mark four corner points of marker in first frame manually; (b) ROI calculated by algorithm according to marked coordinates of corner points; (c) tracking result of 100^th frame with KLT algorithm

Fig. 7. Curves of actual displacement of body surface and pixel difference between ordinate frames of ROI corner points

Fig. 8. Tracking results from three perspectives

Fig. 9. Comparison of phantom motion curves and algorithm tracking curves under three breathing conditions. (a) Three breathing conditions; (b) phantom motion curves; (c) algorithm tracking curves

Fig. 10. Actual displacement of marker and pixel difference between feature points

Fig. 11. Error bar of tracking experimental results