Motion-compensated matteing
Giles Thomas · 1994
Matteing and chroma-key are well-known techniques in frequent use in television production. When they are used, it is generally necessary to ensure that the camera remains stationary in order for the inserted picture material to remain in the correct position with respect to the foreground picture material. This places significant constraints on the use of these techniques. To allow matteing and keying to be used in the presence of camera motion in situations where mechanical sensors are inappropriate, a motion estimation algorithm is needed that can measure global image motion to a high degree of accuracy. The paper describes the development of such an algorithm, tailored to this application. If the algorithm is used in conjunction with a keying unit and a blue background, the background cannot be a uniform shade of blue, but must contain enough information to allow reliable motion estimation to be carried out. In such circumstances, the key signal can be used to indicate the areas of the image which correspond to background, so that the estimation process can ignore foreground areas. In applications where a blue background is not used, it is necessary to use other means to disregard areas having motion different from that to be tracked. Although the motion parameters can be used to drive a conventional DVE, this may be unsuitable for the generation of a full-screen background image, or when the image is being enlarged. Therefore, a specialised kind of image manipulator is being developed for this application.