Tài liệu Image and Videl Comoression P13 doc

14

© 2000 by CRC Press LLC

Further Discussion

and Summary on

2-D Motion Estimation

Since Chapter 10, we have been devoting our discussion to motion analysis and motion-compen￾sated 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 14.1. In Section 14.2, different classifications of various methods for 2-D motion analysis

are given in a wider scope. Section 14.3 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 14.4.

14.1 GENERAL CHARACTERIZATION

A few common features characterizing all three major techniques are discussed in this section.

14.1.1 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 13.2(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).

14.1.2 ILL-POSED INVERSE PROBLEM

In Chapter 13, when we discuss the optical flow technique, a few fundamental issues are raised. It

is stated that optical flow computation from image sequences is an inverse problem, which is usually

ill-posed. Specifically, there are three problems: nonexistence, nonuniqueness, and instability. That

is, the solution may not exist; if it exists, it may not be unique. The solution may not be stable in

the sense that a small perturbation in the image data may cause a huge error in the solution.

Now we can extend our discussion to both block matching and pel recursion. This is because

both block matching and pel recursive techniques are intended for determining 2-D motion from

image sequences, and are therefore inverse problems.