In such a setup, the captured video contains writings or user gesture (foreground) along with projected contents. If we simply send the image for display on the other end, there could be a feedback loop that will distort the projected image. After a few frames, some part of the image becomes saturated and some part of the real writing has ghosting effect. Analogous to audio echo in telephone communication, we call this effect visual echo, i.e., the appearance of the projected contents viewed by the camera. Our approach to visual echo cancellation includes an off-line calibration procedure that records the geometric and photometric transfer between the projector and the camera in a look-up table. During run-time, projected contents in the captured video are identified using the calibration information and suppressed, therefore achieving the goal of canceling visual echo.
See more on this video at www.microsoft.com/en-us/research/video/projector-whiteboard-camera-system-remote-collaboration/