 already known) is present in a scene only when motion occurs. Moreover, information is present only in the part of the frame that contains motion. CR compression techniques transmit only the changes in the part of the frame that comprise motion. It works well when the motion in a scene does not exceed 15% between frame intervals. Sine of angle ACB = AB/BC Figure 6-3. Sine (Michel Bayard). In 1972, two engineers recognized that, in a moving sequence, there is less correlation between pixels that occupy the same place in subsequent frames than there is between pixels that are close to each other but shifted to a different position. After reflection, it became clear that motion vectors play a significant role in pixel prediction. As objects move, pixels shift left, right, up, or down. If a codec were prepared to recognize the direction and rate of pixel motion, it would do a much better job of predicting where each pixel might end up in successive frames. This discovery led to another form of interframe compression called motion compensation. It improved the image compression factor by 50-100% when compared to CR. The basis of motion compensation is the observation that moving objects are best described by re-mapping their original position to a new position. For instance, a pointer used to call attention to a figure on a whiteboard moves in a predictable Coding and Compression fashion. Only the motion vector requires retransmission because the receiving codec already possesses a mathematical representation of original object. In other words, in motion vector prediction, the codec must send information about the position of a subject, but not about the subject itself. Intraframe encoding came along in the mid-1970s. It is compute-intensive; and uses more sophisticated algorithms than all but a few interframe codecs. Two different intraframe coding techniques are typical used today. One, which is common to almost every standards-based video encoding technique today, is the discrete cosine transform, better known as 