Transcoding Method for Regions of Interest

When a bitstream of the higher bitrate is converted to the lower bitrate through the transcoding process, the ROI (Regions of Interest) can be considered. We classify the ROI as the groups of macro-blocks with non-zero motion vectors. For simple implementation, the motion vectors are not additionally estimated, but extracted from the coded bitstream of the higher bitrate before trascoding. Then, a proposed method assigns a specific quantization step-size differentially according to the ROI within a video. The picture quality can be increased by applying the quantization step-size as a small value relatively for the ROI compared with the non-ROI. We can find the result that the proposed method gives the improved picture quality by assigning the quantization step-size differentially.