Pedro A. Amado Assunção

A frequency-domain video transcoder for dynamic bit-rate reduction of MPEG-2 bit streams

Many of the forthcoming video services and multimedia applications are expected to use preencoded video for storage and transmission. Video transcoding is intended to provide transmission flexibility to preencoded bit streams by dynamically adjusting the bit rate of these bit streams according to new bandwidth constraints that were unknown at the time of encoding. In this paper, we propose a drift-free MPEG-2 video transcoder, working entirely in the frequency domain. The various modes of motion compensation (MC) defined in MPEG-2 are implemented in the discrete cosine transform (DCT) domain at reduced computational complexity. By using approximate matrices to compute the MC-DCT blocks, we show that computational complexity can be reduced by 81% compared with the pixel domain approach. Moreover, by using a Lagrangian rate-distortion optimization for bit reallocation, we show that optimal transcoding of high-quality bit streams can produce better picture quality than that obtained by directly encoding the uncompressed video at the same bit rates using a nonoptimized Test Model 5 (TM5) encoder.

Performance and Computational Complexity Assessment of High-Efficiency Video Encoders

This paper presents a performance evaluation study of coding efficiency versus computational complexity for the forthcoming High Efficiency Video Coding (HEVC) standard. A thorough experimental investigation was carried out to identify the tools that most affect the encoding efficiency and computational complexity of the HEVC encoder. A set of 16 different encoding configurations was created to investigate the impact of each tool, varying the encoding parameter set and comparing the results with a baseline encoder. This paper shows that, even though the computational complexity increases monotonically from the baseline to the most complex configuration, the encoding efficiency saturates at some point. Moreover, the results of this paper provide relevant information for implementation of complexity-constrained encoders by taking into account the tradeoff between complexity and coding efficiency. It is shown that low-complexity encoding configurations, defined by careful selection of coding tools, achieve coding efficiency comparable to that of high-complexity configurations.

Post-processing of MPEG2 coded video for transmission at lower bit rates

It is expected that most of the video services will be based on the MPEG2 standard and many of them using recorded streams. When compressed video is recorded, the characteristics of the channel through which it will be transmitted are assumed to be known beforehand. Therefore a great lack of flexibility arises in transmission of these streams when channels of diverse characteristics are used. If the same video programme is to be simultaneously distributed to several users through channels with different capacities, the service provider needs to keep several copies of that programme, each one encoded according to the corresponding channel characteristics. We show that it is only required to keep one copy of the coded data at its highest possible quality. Transcoding the main stream to lower rates is achieved with minimum delay. Therefore the transcoder is also capable of fast response to network demands to prevent packet loss.

Complexity control of high efficiency video encoders for power-constrained devices

The emerging High Efficiency Video Coding (HEVC) standard is expected to require much more processing power than its predecessors due to the higher algorithmic complexity of new coding tools and associated data structures. This paper proposes a novel complexity control method for the near future HEVC encoders running on power-constrained devices. The proposed method is based on a decision algorithm that dynamically adjusts the depth of the Coding Units (CU) defined by quad-tree structures. New evidence about the relationship between CU depth and coding complexity is used to selectively constrain the CU depth in order to not exceed a predefined complexity target. The experimental results show that the encoder computational complexity can be downscaled by up to 60% at the cost of negligible loss of rate-distortion (RD) performance. The proposed method finds application in the near future multimedia portable devices using HEVC codecs.

Fast HEVC Encoding Decisions Using Data Mining

The High Efficiency Video Coding standard provides improved compression ratio in comparison with its predecessors at the cost of large increases in the encoding computational complexity. An important share of this increase is due to the new flexible partitioning structures, namely the coding trees, the prediction units, and the residual quadtrees, with the best configurations decided through an exhaustive rate-distortion optimization (RDO) process. In this paper, we propose a set of procedures for deciding whether the partition structure optimization algorithm should be terminated early or run to the end of an exhaustive search for the best configuration. The proposed schemes are based on decision trees obtained through data mining techniques. By extracting intermediate data, such as encoding variables from a training set of video sequences, three sets of decision trees are built and implemented to avoid running the RDO algorithm to its full extent. When separately implemented, these schemes achieve average computational complexity reductions (CCRs) of up to 50% at a negligible cost of 0.56% in terms of Bjontegaard Delta (BD) rate increase. When the schemes are jointly implemented, an average CCR of up to 65% is achieved, with a small BD-rate increase of 1.36%. Extensive experiments and comparisons with similar works demonstrate that the proposed early termination schemes achieve the best rate-distortion-complexity tradeoffs among all the compared works.

Buffer analysis and control in CBR video transcoding

In video distribution services, various networks of different capacity may be linked together to form the branches of a multicasting tree. In order to efficiently use the available bandwidth of each subnetwork, a video transcoder capable of reducing the bit rate of the incoming bit stream must be used at those nodes whose output links have less bandwidth than the input one. In this paper, we analyze the buffering implications of inserting a video transcoder within the transmission path. For transcoders with either fixed or variable compression ratio, we show that the encoder buffer size can be maintained as if no transcoder existed while the decoder has to modify its own buffer size according to both the bit rate conversion ratio and transcoder buffer size. We derive the conditions that have to be met by both the encoder and transcoder buffers for preventing the decoder buffer from underflowing or overflowing. Furthermore, based on the characteristics of constant bit rate (CBR) MPEG coded video, we show that efficient bit rate control can be implemented in CBR video transcoders such that the picture quality of transcoded sequences is practically the same as those directly encoded from the original sequence at the same reduced bit rates.

Transcoding of MPEG-2 video in the frequency domain

Video transcoding techniques offer the possibility of matching coded video to transmission channels of lower capacity by reducing the bit rate of compressed bit streams. In this paper we propose a new frequency domain video transcoder for bit rate reduction of compressed bit streams. A motion compensation (MC) loop, operating in the frequency domain, is used for drift compensation at reduced computational complexity. We derive approximate matrices for fast computation of the MC blocks in the frequency domain. By using the Lagrangian optimisation in calculating the best quantiser scales for transcoding, we show that transcoded pictures from a high quality bit stream are better than those encoded from original frames at the same reduced bit rates.

Rate-reduction techniques for MPEG-2 video bit streams

Rate control is usually an intrinsic function of the video coding algorithm, responsible for matching the available bandwidth of the communication channel to the output bit stream. However, many of the forthcoming video services will use pre-encoded video, in compressed format, for storage and transmission purposes. In this case, the rate control function should work on the compressed bit stream by reducing its bit rate according to the network demand, which can be either static or dynamic. Bit rate reduction of compressed video is needed not only when pre-encoded video is transmitted, but also in video multicasting where several users receive the same bit stream through different channels. A similar problem results from the interactivity with the user in video on demand services, where each user is offered the possibility of choosing the amount of bandwidth assigned to its channel. In this paper we analyze rate reduction techniques for compressed video in terms of complexity and final picture quality. We propose a new scheme, working entirely in the frequency domain, for bit rate reduction of compressed video. It is a low delay and drift free video transcoder that outperforms a re-encoding system and performs very close to a normal encoder using uncompressed video.

Optimal transcoding of compressed video

Transcoding is regarded as a down conversion process, where the bit rate of a compressed video bit stream is reduced according to a given constraint. Based on a transcoding architecture, previously developed by the authors, an optimal transcoder in a rate-distortion sense is proposed. It is shown that the basic architecture provides the necessary support for measuring the global transcoding distortion to be minimised under the optimisation algorithm. The experimental results show that, optimally transcoded pictures have a better quality than those encoded from the original video material, using a standard encoder at the same bit rates. This paper shows that compressed video can be further compressed with a good efficiency.

Coding Tree Depth Estimation for Complexity Reduction of HEVC

The emerging HEVC standard introduces a number of tools which increase compression efficiency in comparison to its predecessors at the cost of greater computational complexity. This paper proposes a complexity control method for HEVC encoders based on dynamic adjustment of the newly proposed coding tree structures. The method improves a previous solution by adopting a strategy that takes into consideration both spatial and temporal correlation in order to decide the maximum coding tree depth allowed for each coding tree block. Complexity control capability is increased in comparison to a previous work, while compression losses are decreased by 70%. Experimental results show that the encoder computational complexity can be downscaled to 60% with an average bit rate increase around 1.3% and a PSNR decrease under 0.07 dB.