Rangarajan Aravind

Motion-compensated video coding with adaptive perceptual quantization

The authors address the problem of adapting the Motion Picture Experts Group (MPEG) quantizer for scenes of different complexity (at bit rates around 1 Mb/s), such that the perceptual quality of the reconstructed video is optimized. Adaptive quantisation techniques conforming to the MPEG syntax can significantly improve the performance of the encoder. The authors concentrate on a one-pass causal scheme to limit the complexity of the encoder. The system employs prestored models for perceptual quality and a bit rate that have been experimentally derived. A framework is provided for determining these models as well as adapting them to locally varying scene characteristics. The variance of an 8*8 (luminance) block is basic to the techniques developed. Following standard practice, it is defined as the average of the square of the deviations of the pixels in the block from the mean pixel value. >

Video coding with motion-compensated interpolation for CD-ROM applications

Abstract This paper discusses the compression of digital video signals at bit-rates around 1 Mbps for interactive playback applications. The coding algorithm is required not only to provide good-quality reconstruction of complex material but also to facilitate interactivity with the bit-stream at the decoder. The algorithm proposed in the paper is based on the well-known technique of motion-compensated prediction and DCT-coding of the prediction error. This basic approach is considerably enhanced with motion-compensated interpolation of skipped video frames and selective coding of the interpolation errors. The interactivity requirements are met by partitioning the video sequence into segments each comprised of a small number of frames. Different ways of encoding a segment are examined. An arrangement is selected that has one intra-coded frame in the center of the segment and a symmetrical pattern of predicted and interpolated frames on the two sides of the intra-coded frame. Segments of length 9 and 15 frames are evaluated. While the shorter segment leads to faster interactivity and simpler decoder implementation, the associated picture quality is much inferior to that obtained with the longer segment. Finally, a rough design of the decoder, suitable for VLSI implementation, is outlined.

Adaptive frame/field motion compensated video coding

Abstract The second phase of the Motion Pictures Experts Group (MPEG-2) activity is in progress and is primarily aimed at coding of high resolution video with high quality at bit-rates of 4 to 9 Mbit/s. In addition, this phase is also required to address many issues including forward and backward compatibility with the first phase (MPEG-1) standard. For MPEG-2, an adaptive frame/field motion-compensated video coding scheme is proposed. This scheme builds on the proven framework of DCT and motion-compensation based techniques already optimized in MPEG-1 for coding of lower resolution video at low bit-rates. Various adaptations include techniques to improve efficiency of coding for interlaced video source as well as improving quality by better exploitation of characteristics of the video scenes. Statistics and subjective tests confirm that these adaptations provide significant improvement as compared to purely MPEG-1 based coding. We then discuss issues of compatibility with the MPEG-1 standard and of implementation complexity of the proposed scheme.

Comparing motion-interpolation structures for video coding

This paper considers the topic of video compression at bit-rates around 1 Mbps for digital storage/playback applications. The backbone algorithm employed is the well-known motion-compensated prediction-error coder. This approach is considerably enhanced by the incorporation of conditional motion-compensated interpolation (CMCI), where some of the video frames are encoded with prediction-error coding and the remainder are encoded with motion-compensated interpolation and interpolation-error coding. The advantage of this combined technique is that interpolation errors can be coded with much fewer bits than prediction errors for similar reconstructed-picture quality. We consider two different techniques for estimating motion in CMCI- coded frames. In order to facilitate direct interaction with the bit-stream, the input sequence is divided into groups such that one frame in each group is intra-coded. The arrangement of predictive- and CMCI-coded frames can be varied; a study of these arrangements is the major focus of this paper. Structures with one-, two- and four-frame interpolation are compared. Our results indicate that one-frame interpolation is inferior to two-frame interpolation on complex test material. Four-frame interpolation is comparable in performance to two-frame interpolation as long as the motion is compensable.

Adaptive perceptual quantization for video compression

International standards bodies snch as the ISO and CCITT have established various expert groups to standardize compressed digital representations of color image and video signals for different applications. Here we are concerned mainly with the work of the Motion Picture Experts Group (MPEG), whose charter is to specify a generic coded representation of video and audio. The MPEG work has been divided into three distinct phases, the first aimed at compressing low-resolution (1/4 CCIR-601) pictures at data-rates in 1-4 Mbits/sec, the second at compressing full CCIR-6O1 resolution pictures at data-rates in 4-10 Mbits/sec, and the third at compressing HDTV pictures at appropriately higher data-rates.