Rate-distortion optimal downsampling of H.264 compressed video using full-resolution information

This paper considers the problem of downsampling H.264 compressed video, where a full-resolution compressed video sequence conforming to H.264 is transcoded into another compressed video sequence conforming to H.264 at a lower target resolution. A transcoding framework that makes efficient use of full-resolution information is proposed. In this framework, residuals and motion vectors at the target resolution are first predicted from their full-resolution counterparts. These predicted residuals and motion vectors are then applied to optimize the actual rate-distortion (RD) performance. Experimental results show that, compared against the benchmark system, which cascades an H.264 decoder, a spatial downsampler, and an H.264 encoder, and is generally regarded having the best possible RD performance, the proposed framework, surprisingly, provides consistently superior RD performance with up to 0.7 dB gain. Furthermore, the framework has the desired feature of being configurable to strike the right balance between rate-distortion performance and computational complexity according to application requirements.