At present the foundations are being laid out for the implementation of three-dimensional television. Apart from the engineering problems to be solved there are a number of questions connected with human factors which have to be answered. These questions are related to fundamental requirements with regard to certain system features such as screen size, viewing distance and the spatio-temporal resolution of the image in the three dimensions of space. Since the implementation of data-compressing techniques is essential for the recording and transmission of 3DTV picture signals, it is necessary to show the possibilities of irrelevance reduction as well as to define the demands made on the quality of reconstructed images. The present paper summarizes the latest results of research and shows in which fields work must be carried out on hitherto unanswered questions.

3D-television: A survey of recent research results on subjective requirements

The draft international standard ITU-T H.263 is closely related to the well known and widely used ITU-T Recommendation H.261. However, H.263 does provide the same subjective image quality at less than half the bit-rate. In this paper we investigate to what extend single enhancements of H.263 contribute to this performance gain, and consider the trade-off quality vs. complexity. Based on typical test sequences, H.263 with its various modes is compared to H.261 on the basis of rate distortion curves at bit-rates up to 128 kbps. At 64 kbps, the performance gain of H.263 in its default mode compared to H.261 is approximately 2 dB. This improvement is achieved with only little increase of complexity, and is mainly due to more accurate motion compensation with half-pel accuracy. Considering the trade-off quality vs. complexity, the combination of the optional coding-modes "Advanced prediction mode" and "PB-frames mode" is a good compromise, resulting in an additional performance gain of 1.5 dB PSNR at 64 kbps. The "Syntax-based arithmetic coding mode" on the other hand, offers only a very small performance gain (0.1 dB at 64 kbps) for its increased computational complexity. Results from profiling an H.263 software codec are presented in order to support complexity considerations of the optional coding-modes.

Performance of the H.263 Video Compression Standard

A digital image sequence consists of a set of pixels each describing the scene intensity at a specific location on a specific instance in time. In a natural scene these pixels are spatially and temporally correlated with each other. An image coding scheme utilizes these correlations in order to represent the image sequence more efficiently in this way allowing for a more cost-effective storage or transmission. In this chapter spatio-temporal subsampling will be discussed as a data reduction technique. Subsampling is in use as a data reduction method for the standardized HDTV transmission systems MUSE [1] and HD-MAC [2] which are completely based on this technique. Recent proposals use this technique in a combination with transform coding [3][4],

Subsampling of Digital Image Sequences using Motion Information

The design of very low bit rate coders, below 64 kilo-bits per second, presents a number of new challenges. Such low bit rate coders are targeted to small size images, say 176/spl times/144, or below and are limited to head and shoulders scenes. The authors compare and rate the performance of several still image encoding methods such as DCT, subband and wavelets, using a new quality scale based on properties of human vision. These still image coders are embedded into video coders such as the H261 coder. The relative importance of intraframe and interframe quality and contributions to the total bit rate provide an overall design framework for these low bit rate video coders. The authors discuss and illustrate, for the encoding of low bit rate video, the still image and motion impairments that are perceptually most important. >

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Perceptual criteria and design alternatives for low bit rate video coding

In order to transmit high definition television signals economically some kind of data compression has to be utilized. We investigate the question how spatio-temporal properties of human vision can be exploited for data reduction. By means of pyramid coding we implement coding schemes which transmit spatial detail information with reduced temporal resolution. In moving areas with correlated motion we introduce motion-compensated interpolation. In the case of uncorrelated motion, where motion estimation often fails and the eye cannot follow the movement, high spatial frequencies are suppressed. The different systems are rated by means of subjective tests. Furthermore motion-compensated interpolation of the highpass signals of a subband codec using quadrature mirror filters will be analysed.

Ralph Schäfer

Papers

Considerations on the possibility to exchange temporal against spatial resolution in image coding