Since Chapter 10, we have been devoting our discussion to motion analysis and motion-compensated coding. Following a general description in Chapter 10, three major techniques — block matching, pel recursion, and optical flow — are covered in Chapters 11, 12, and 13, respectively. In this chapter, before concluding this subject, we provide further discussion and a summary. A general characterization for 2-D motion estimation, thus for all three techniques, is given in Section . In Section , different classifications of various methods for 2-D motion analysis are given in a wider scope. Section is concerned with a performance comparison. | 14 Further Discussion and Summary on 2-D Motion Estimation Since Chapter 10 we have been devoting our discussion to motion analysis and motion-compensated coding. Following a general description in Chapter 10 three major techniques block matching pel recursion and optical flow are covered in Chapters 11 12 and 13 respectively. In this chapter before concluding this subject we provide further discussion and a summary. A general characterization for 2-D motion estimation thus for all three techniques is given in Section . In Section different classifications of various methods for 2-D motion analysis are given in a wider scope. Section is concerned with a performance comparison among the three major techniques. More-advanced techniques and new trends in motion analysis and motion compensation are introduced in Section . GENERAL CHARACTERIZATION A few common features characterizing all three major techniques are discussed in this section. Aperture Problem The aperture problem discussed in Chapter 13 describes phenomena that occur when observing motion through a small opening in a flat screen. That is one can only observe normal velocity. It is essentially a form of ill-posed problem since it is concerned with existence and uniqueness issues as illustrated in Figure a and b . This problem is inherent with the optical flow technique. We note however that the aperture problem also exists in block matching and pel recursive techniques. Consider an area in an image plane having strong intensity gradients. According to our discussion in Chapter 13 the aperture problem does exist in this area no matter what type of technique is applied to determine local motion. That is motion perpendicular to the gradient cannot be determined as long as only a local measure is utilized. It is noted that in fact the steepest descent method of the pel recursive technique only updates the estimate along the gradient direction Tekalp 1995 . Ill-Posed .
