rning up will change. Other than that, very little in the scene changes. Without going into too much technical detail, what MPEG does is reuse those elements of the I frame that don't change-or that change infrequently-so that it does not have to recre- ate them, thus reducing overall compression time. In our fishing boat scene it should be fairly obvious that the background certainly won't change much (unless a bird flies into it), the immediate foreground won't change, and the color and shape of the boat are constant. This is a fairly predictable scene. As a result, the number of I frames that will be inter- spersed among the P and B frames is relatively small. If the scene were different-a fishing boat being tossed about on rough seas, for example -then the number of minimally uncompressed reference frames would be greater because of the constantly changing point of reference. MPEG looks backward to establish patterns of behavior of past frames, then looks at the reference frame, and finally predicts what future frames will probably look like based on past history. Ultimately, the sequence of frames is as follows: IBBPBBPBBIB There are 12 frames from I to I, and the ratio of P frames to B frames is based on experience. Sequentially, the frames would look like the sequence shown in Figure 2-23. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols Figure 2-22 A fishing boat -how much of the image must actually change as the result of MPEG compression? IBBPBBPBBPBBI Figure 2-23 The sequence of I, P, and B frames in an B B P B B P B B P B B MPEG- compressed sequence Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Chapter 2 The MPEG Standards Ther