Visually Lossless H.264 Compression of Natural Videos

We performed a systematic evaluation of ‘visually lossless’ (VL) threshold selection for H.264/AVC (Advanced Video Coding) compressed natural videos spanning a wide range of content and motion. A psychovisual study was conducted using a two alternative forced choice task design, where by a series of reference vs. compressed video pairs were displayed to the subjects, where bit rates were varied to achieve a spread in the amount of compression. A statistical analysis was conducted on these data to estimate the VL threshold. Based on the visual thresholds estimated from the observed human ratings, we learn a mapping from ‘perceptually relevant’statistical video features that capture visual lossless-ness, to statistically determined VL threshold. Using this VL threshold, we derive an H.264 compressibility index. This new Compressibility Index is shown to correlate well with human subjective judgments of VL thresholds. We have also made the code for compressibility index available online (Moorthy, A.K. and Bovik, A.C. (2010). H.264 Visually Lossless Compressibility Index (HVLCI), Software Release. http://live.ece.utexas.edu/research/quality/hvlci.zip.) for its use in practical applications and facilitate future research in this area.