電機工程學系 ; © 2003 Institute of Electrical and Electronics Engineers-This paper presents a region of interest transcoding scheme for multipoint video conferencing to enhance visual quality. In a multipoint video conference, usually there are only one or two active conferees at one time, which are the regions of interest to the other conferees involved. We propose a dynamic sub-window skipping scheme to firstly identify the active participants from the multiple incoming encoded video streams by calculating the motion activity of each sub-window and then dynamically reduce the frame rates of the motion inactive participants by skipping these less-important sub-windows. The bits saved from the skipping operation are reallocated to the active sub-windows to enhance the regions of interest. We also propose a low-complexity scheme to compose, as well as trace, the unavailable motion vectors with a good accuracy in the dropped inactive sub-windows after performing sub-window skipping. Simulation results show that the proposed methods not only significantly improve the visual quality of the active sub-windows without introducing serious visual quality degradation in the inactive ones, but also reduce the computational complexity and avoid whole-frame skipping. Moreover, the proposed algorithm is fully compatible with the H.263 video coding standard.