As mentioned in Chapter 10, optical flow is one of three major techniques that can be used to estimate displacement vectors from successive image frames. As opposed to the other two displacement estimation techniques discussed in Chapters 11 and 12, block matching and pel recursive method, however, the optical flow technique was developed primarily for 3-D motion estimation in the computer vision community. Although it provides a relatively more accurate displacement estimation than the other two techniques, as we shall see in this and the next chapter, optical flow has not yet found wide applications for motion-compensated video coding. This. | 13 Optical Flow As mentioned in Chapter 10 optical flow is one of three major techniques that can be used to estimate displacement vectors from successive image frames. As opposed to the other two displacement estimation techniques discussed in Chapters 11 and 12 block matching and pel recursive method however the optical flow technique was developed primarily for 3-D motion estimation in the computer vision community. Although it provides a relatively more accurate displacement estimation than the other two techniques as we shall see in this and the next chapter optical flow has not yet found wide applications for motion-compensated video coding. This is mainly due to the fact that there are a large number of motion vectors one vector per pixel involved hence the more side information that needs to be encoded and transmitted. As emphasized in Chapter 11 we should not forget the ultimate goal in motion-compensated video coding to encode video data with a total bit rate as low as possible while maintaining a satisfactory quality of reconstructed video frames at the receiving end. If the extra bits required for encoding a large amount of optical flow vectors counterbalance the bits saved in encoding the prediction error as a result of more accurate motion estimation then the usage of optical flow in motion-compensated coding is not worthwhile. Besides more computation is required in optical flow determination. These factors have prevented optical flow from being practically utilized in motion-compensated video coding. With the continued advance in technologies however we believe this problem may be resolved in the near future. In fact an initial successful attempt has been made Shi et al. 1998 . On the other hand in theory the optical flow technique is of great importance in understanding the fundamental issues in 2-D motion determination such as the aperture problem the conservation and neighborhood constraints and the distinction and relationship between 2-D motion
