Steffen Wittmann

Transmission of Post-Filter Hints for Video Coding Schemes

In the field of video coding, it is common to apply post-filtering in order to enhance the quality of a decoded video signal. An optimal post-filter can be designed by minimizing the error between the original signal and the decoded filtered signal. This approach requires information about the statistics of the decoded image and the original signal, which is available on encoder-side only. In order to provide this necessary information to a post-filter on the decoder-side, this paper presents a technique to transmit so-called post-filter hints in the bit-stream of a video coding scheme. In H.264/AVC, mean picture quality improvements of 0.4 dB are achieved with this technique compared to H.264/AVC without post-filtering. This corresponds to a mean bit-rate reduction of 8.5% at same picture quality. The mean improvements are calculated as an average over all sequences and bit-rates that are specified in the common test conditions of JVT for professional applications as well as over additional HD sequences.

New Standardized Extensions of MPEG4-AVC/H.264 for Professional-Quality Video Applications

To support high quality video applications, the Joint Video Team (JVT) has recently added five new profiles, two new supplemental enhancement information (SEI) messages, and two new extended gamut color space indicators to the MPEG4-AVC/H.264 video coding standard. The new profiles include substantial feature enhancements for high-quality video applications, including improved-efficiency 4:4:4 video format coding, improved-efficiency lossless macroblock coding, coding 4:4:4 video pictures using three separately-coded color planes, and support of bit depths up to 14 bits per sample. The new features were developed to support a wide range of applications where high quality video compression is demanded, including professional and semi-professional scenarios in particular. They also anticipate the introduction of higher fidelity displays. In this paper, the new extensions are presented along with quantitaive estimates of the benefits of the new features and a discussion o the target aplication environments.

Video Coding Using a Simplified Block Structure and Advanced Coding Techniques

This paper describes a new video coding scheme based on a simplified block structure that significantly outperforms the coding efficiency of the ISO/IEC 14496-10 ITU-T H.264 advanced video coding (AVC) standard. Its conceptual design is similar to a typical block-based hybrid coder applying prediction and subsequent prediction error coding. The basic coding unit is an 8 × 8 block for inter, and an 8 × 8 or a 16 × 16 block for intra, instead of the usual 16 × 16 macroblock. No larger block sizes are considered for prediction and transform. Based on this simplified block structure, the coding scheme uses simple and fundamental coding tools with optimized encoding algorithms. In particular, the motion representation is based on a minimum partitioning with blocks sharing motion borders. In addition, compared to AVC, the new and improved coding techniques include: block-based intensity compensation, motion vector competition, adaptive motion vector resolution, adaptive interpolation filters, edge-based intra prediction and enhanced chrominance prediction, intra template matching, larger trans forms and adaptive switchable transforms selection for intra and inter blocks, and nonlinear and frame-adaptive de-noising loop filters. Finally, the entropy coder uses a generic flexible zero tree representation applied to both motion and texture data. Attention has also been given to algorithm designs that facilitate parallelization. Compared to AVC, the new coding scheme offers clear benefits in terms of subjective video quality at the same bit rate. Objective quality improvements are equally significant. At the same quality, an average bit-rate reduction of 31% compared to AVC is reported.

Separable adaptive interpolation filter for video coding

Motion-compensated prediction using fractional-pel motion vectors followed by transform coding of the resulting prediction error is used in hybrid video coding. In the case of fractional-pel motion, pixels at fractional-pel positions have to be determined by interpolation. For this purpose, fixed interpolation filters are applied in H.264/AVC. By using fixed interpolation filters, time varying effects such as aliasing, quantization errors, errors from inaccurate motion estimation, camera noise, etc cannot be considered accurately. Thus, the coding efficiency of the motion compensated prediction is limited. The concept of adaptive interpolation filtering addresses these effects resulting in an increased coding efficiency. Since a non-separable adaptive filter is used in prior art, it is associated with a significantly increased computational expense at encoder and decoder. In order to reduce the computational expense, a separable adaptive interpolation filtering is proposed in this paper that achieves the same coding efficiency than the non-separable adaptive filter. With this separable interpolation filter, the computational expense of the filtering is reduced by 24% in case of 4times4 motion-compensated blocks, 36% in case of 8times8 motion-compensated blocks, and 42% in case of 16times16 motion-compensated blocks compared to a non-separable filter, whereas the computational expense is measured by the number of calculation operations.

Quantization offsets for video coding

In this paper we propose a modification in the H.264/AVC quantization scheme for transform coefficients. This modification is based on a simple offset that provides a more accurate adjustment of the quantizer, especially of the dead-zone size, to the signal statistics. Compared to a deadzone adjustment that is H.264/AVC compliant, the proposed modification improves the coding efficiency and the subjective picture quality. Although there are several other applications that would benefit from the proposed quantization offset, the experimental results in this paper are restricted to coding of HD movies that contain film grain. A quantizer modification for such high quality consumer applications leads to significantly improved subjective picture quality.

Frequency Selective Update for Video Coding

In this paper the so-called frequency selective update (FSU) technique is presented. FSU is based on a hybrid video coding scheme, where the prediction error is coded and used to correct the prediction signal. This correction of the prediction signal is called update. Standardized video coding schemes such as H.264/AVC apply a transformation, quantization and entropy coding to code the prediction error. In order to reduce the bitrate while keeping the same subjective picture quality, so-called quantization matrices are applied to adapt the quantization to the human visual system (HVS). FSU further enhances the current exploitation of the HVS by selectively updating only few frequency parts. Furthermore, the entropy-coding scheme is adapted to the selected frequency parts and the selection is alternated on frame-by-frame basis. Compared to H.264/AVC high profile, which includes quantization matrices, bitrate reductions of 12-25% are obtained at very similar subjective picture quality.

Advances in hybrid video coding

Hybrid video coding is known for a long time and is applied in all video coding standards such as MPEG-x or H.26x. This paper shows that there is still enough potential for further coding efficiency improvements. The paper starts with an overview of state-of-the-art hybrid video coding schemes such as H.264/AVC. Thereafter, our advances on main building blocks of H.264/AVC are presented that significantly improve the coding efficiency. For instance, intra prediction is improved by changing scan directions and thus providing better reference pixels for specific prediction directions. Adaptive filtering and high precision motion compensation improves the motion compensated prediction. Furthermore, the combination of transformation, quantization, and entropy coding of the prediction error is improved using an advanced frequency selective coding technique. With the transmission of post-filter hints it is possible to design a Wiener post-filter that significantly enhances the picture quality. Finally, texture synthesis techniques are used to improve the subjective quality for specific textures with homogenous statistical characteristics. This paper presents our above-mentioned techniques in detail. Depending on the input sequence and the bit rate, the objective and/or subjective gains compared to H.264/AVC are quite significant.