ng of the human eye. The eye- brain team is quite sensitive in some ways and quite tolerant in others. For instance, the eye adjusts very well to variations in relative and absolute brightness, but not to lack of focus. It is sensitive to distortions in familiar images (e.g., people's faces), but can tolerate minor flaws in motion handling as long as the overall movement looks realistic. Viewers want the parts of the picture that convey critical information-numbers, letters, and the eyes of other people-to be clearly discernible, but blurring in a picture's background is not distracting. Techniques that sacrifice video quality for reduced bandwidth fall into two categories. One method is to approximate instead of sending the actual information. DCT approximates luminance. Motion compensation uses motion vectors to approximate how an object will move from one frame to the next. These techniques reduce quality, but the approximations are good enough and, since most humans cannot perceive the loss, trivial. Another method of quality reduction works on picture resolution. Resolution depends on pixels, which are the conveyers of video information. The more pixels, the more detail. Detail is, for some applications, critical. The more detail in an X-ray, the better. Other applications require less detail. Seeing the pattern on the tie worn by a person in a videoconference is not important. Capturing the messages conveyed through body language and gesture is important. As long as quality-reduction does not impair the conveyance of important information, we overlook it. Most participants in a videoconference become accustomed to the reduced quality image and cease to notice it-particularly if the audio quality is good. Different video formats define different picture resolutions. For instance, color VGA video specifies a graphics mode that is 640 pixels wide by 480 pixels tall. Super-VGA is 1,024-by-768 pixels. In both formats, a colored pixel is defined using 24-bits. The CCIR (an international standards body that is par