Transrating-assisted MPEG-4 video resizing with rate-distortion optimization

A transrating-assisted spatial resolution conversion method for compressed videos is proposed. By utilizing the property of equivalence between downsampling of the video spatial resolution and bitrate reducing of the video streams in frequency domain, it implements video resizing through transrating. That is, it decodes the compressed video stream and then reencodes it to a new bitrate corresponding to the target video spatial resolution instead of direct changing the video spatial resolution before the conversion like the traditional spatial transcoding, which facilitates efficient using of motion vectors and macroblock modes. To alleviate mismatching between the original bitrate allocation mechanism and the newly-changed distribution of the discrete cosine transform(DCT) coefficients, a tactic which only proactively sets the high frequency DCT coefficients of P frames to zeros is used in the transrating process to implicitly implement subsampling of the video. Experimental results show that it can significantly improve the rate-distortion performance of the obtained video streams.