Requantization transcoding for H.264/AVC video coding

In this paper, efficient solutions for requantization transcoding in H.264/AVC are presented. By requantizing residual coefficients in the bitstream, different error components can appear in the transcoded video stream. Firstly, a requantization error is present due to successive quantization in encoder and transcoder. In addition to the requantization error, the loss of information caused by coarser quantization will propagate due to dependencies in the bitstream. Because of the use of intra prediction and motion-compensated prediction in H.264/AVC, both spatial and temporal drift propagation arise in transcoded H.264/AVC video streams. The spatial drift in intra-predicted blocks results from mismatches in the surrounding prediction pixels as a consequence of requantization. In this paper, both spatial and temporal drift components are analyzed. As is shown, spatial drift has a determining impact on the visual quality of transcoded video streams in H.264/AVC. In particular, this type of drift results in serious distortion and disturbing artifacts in the transcoded video stream. In order to avoid the spatially propagating distortion, we introduce transcoding architectures based on spatial compensation techniques. By combining the individual temporal and spatial compensation approaches and applying different techniques based on the picture and/or macroblock type, overall architectures are obtained that provide a trade-off between computational complexity and rate-distortion performance. The complexity of the presented architectures is significantly reduced when compared to cascaded decoder–encoder solutions, which are typically used for H.264/AVC transcoding. The reduction in complexity is particularly large for the solution which uses spatial compensation only. When compared to traditional solutions without spatial compensation, both visual and objective quality results are highly improved.     

Pros and cons of fine granular scalability-based MPEG-2 compressed-domain processing

The pros and cons of FGS-based MPEG-2 video transcoding are examined. An existing solution for elastic storage of media (Barrau, 2002) is reviewed. Its shortfalls in terms of picture drift are identified and addressed by means of proposing a modified transcoding architecture, which is then compared to SNR-based MPEG-2 multilayer transcoding. It is shown that the FGS-based MPEG-2 video transcoder has the advantage of simplified transcoding and decoding architectures. However, the SNR-based MPEG-2 transcoder is shown to produce higher quality reconstructed images with superior rate-distortion performance.     

Video transcoding from H.264/AVC to MPEG-2 with reduced computational complexity

This paper addresses video transcoding from H.264/AVC into MPEG-2 with reduced complexity and high rate-distortion efficiency. While the overall concept is based on a cascaded decoder–encoder, the novel adaptation methods developed in this work have the advantage of providing very good performance in H.264/AVC to MPEG-2 transcoding. The proposed approach exploits the similarities between the coding tools used in both standards, with the objective of obtaining a computationally efficient transcoder without penalising the signal quality. Fast and efficient methods are devised for conversion of macroblock coding modes and translation of motion information in order to compute the MPEG-2 coding format with a reduced number of operations, by reusing the corresponding data embedded in the incoming H.264/AVC coded stream. In comparison with a cascaded decoder–encoder, the fast transcoder achieves computational complexity savings up to 60% with slightly better peak signal-to-noise ratio (PSNR) at the same bitrate.     

基于TMS320C6416的MPEG-2/MPEG-4 视频流转码器设计与实现

目前的视频流转码大致可以分为空域(像素域)转码和压缩域(DCT域)转码两种方法。针对目前最流行的MPEG-2和MPEG-4两个压缩编码标准,采用级联空域转码方案,并且基于TMS320C6416 DSP芯片,探讨了MPEG-2/MPEG-4视频流转码的硬件实现方法。在设计中,通过PCI消息机制,解决了DSP与PC机的数据传输“瓶颈”问题,同时合理分配存储器,并采取各种优化技术,近实时地实现了MPEG视频流转码。     

Efficient MPEG-4/H/263 video transcoder for interoperability of heterogeneous multimedia networks

As well as some similarities, there are a number of differences between MPEG-4 and the H.263 bitstream syntax. The inclusion of a video transcoder is proposed to achieve interoperability between the two video standards. This novel transcoding algorithm is proved to give highly improved service quality while reducing the complexity and the time delay of conventional cascaded decoding/re-encoding processes.     

On re-composition of motion compensated macroblocks for DCT-based video transcoding

To achieve portability between different kinds of encoding formats and network environments, heterogeneous video transcoding becomes a key technique for reducing the bitrate of a previously compressed video signal. A frame-skipping transcoder is often used to avoid an unacceptable picture quality when high transcoding ratio is required. Due to high computational complexity and quality degradation introduced by conventional frame-skipping transcoders, a DCT-based video frame-skipping transcoder has been proposed recently. However, the transcoding process of the motion compensated macroblocks in the DCT domain becomes the bottleneck since IDCT and DCT processes are required. In this paper, we propose a new architecture of the frame-skipping transcoder to reduce the computational complexity of motion compensated macroblocks in the frame-skipping process. The new architecture transcodes the dominant region of a motion compensated macroblock in the DCT domain by making use of the DCT coefficients of the incoming bistream and some pre-computed shift operators. By using a shifted version of the dominant vector, the re-encoding error introduced in the dominant region can be avoided. On the other hand, an adaptive transcoding architecture to transcode the boundary regions of MC marcoblocks and a way to perform error compensation are proposed. This architecture can further speed up the transcoding process of the motion compensated macroblocks. Half pixel accuracy related to our proposed frame skipping transcoder is also addressed. Experimental results show that, as compared to the conventional or DCT-based transocders, the new architecture is more robust to noise, gives rise to fewer requantization errors, and requires simple computational complexity.     

Drift compensation for reduced spatial resolution transcoding: a summary

This paper discusses the problem of reduced-resolution transcoding of compressed video bitstreams. An analysis of drift errors is provided to identify the sources of quality degradation when transcoding to a lower spatial resolution. Two types of drift error are considered: a reference picture error, which has been identified in previous works, and error due to the noncommutative property of motion compensation and down-sampling, which is unique to this work. To overcome these sources of error, four novel architectures are presented. One architecture attempts to compensate for the reference picture error in the reduced resolution, while another architecture attempts to do the same in the original resolution. We present a third architecture that attempts to eliminate the second type of drift error and a final architecture that relies on an intrablock refresh method to compensate for all types of errors. In all of these architectures, a variety of macroblock level conversions are required, such as motion vector mapping and texture down-sampling. These conversions are discussed in detail. Another important issue for the transcoder is rate control. This is especially important for the intra-refresh architecture since it must find a balance between number of intrablocks used to compensate for errors and the associated rate-distortion characteristics of the low-resolution signal. The complexity and quality of the architectures are compared. Based on the results, we find that the intra-refresh architecture offers the best tradeoff between quality and complexity and is also the most flexible.     

转码技术发展及在IPTV领域的应用研究

介绍了转码技术的分类、空间域和频率域的关键技术,分析对比了不同的转码体系结构及其优缺点.在此基础上,研究了转码系统的实现和在IPTV领域内的特点和应用.     

Efficient MPEG‐4 to H.264/AVC Transcoding with Spatial Downscaling

Efficient downscaling in a transcoder is important when the output should be converted to a lower resolution video. In this letter, we suggest an efficient algorithm for transcoding from MPEG‐4 SP (with simple profile) to H.264/AVC with spatial downscaling. First, target image blocks are classified into monotonous, complex, and very complex regions for fast mode decision. Second, adaptive search ranges are applied to these image classes for fast motion estimation in an H.264/AVC encoder with predicted motion vectors. Simulation results show that our transcoder considerably reduces transcoding time while video quality is kept almost optimal.     

Fast motion vector re-estimation for transcoding MPEG-1 into MPEG-4 with lower spatial resolution in DCT-domain

In this paper, we propose a fast motion vector re-estimation for transcoding MPEG-1 to MPEG-4 with lower spatial resolution. This task can be performed in the pixel-domain or in the discrete cosine transform (DCT) domain. In this paper, we concentrate on the DCT-domain approach, which requires lower delay and complexity than those in the pixel-domain. For the DCT-domain transcoding to lower spatial resolution pictures, DCT-domain down-sampling filter is applied and a base motion vector (BMV) for the down-sampled MPEG-4 macroblock is to be calculated from the input motion vectors operating on the higher spatial resolution image. Quality can be significantly improved by refining the BMV. Starting with the BMV, the motion vector refinement (MVR) scheme searches for a delta motion vector within a significantly reduced search area. We propose a fast MVR scheme for video down-sampling in the DCT-domain based on minimizing the number of required check points, and a computationally efficient method for extracting motion compensated DCT block. We also show an efficient scheme for selecting a coding mode for a macroblock in the lower resolution video from those of the corresponding higher resolution video.     

Transcoding of MPEG bitstreams

This paper discusses the problem of transcoding as it may occur in, for instance, the following situation. Suppose a satellite transmits an MPEG-compressed video signal at say 9 Mbit/s. This signal must be relayed at a cable head end. However, since the cable capacity is only limited, the cable head end will want to relay this incoming signal at a lower bit-rate of, say, 5 Mbit/s. The problem is how to convert a compressed video signal of a given bit-rate into a compressed video signal of a lower bit-rate. The specific transcoding problem discussed in this paper is referred to as bit-rate conversion. Basically, a transcoder used for such a purpose will consist of a cascaded decoder and encoder. It is shown in the paper that the complexity of this combination can be significantly reduced. The paper also investigates the loss of picture quality that may be expected when a transcoder is in the transmission chain. The loss of quality as compared to that resulting in the case of transmission without a transcoder is studied by means of computations using simplified models of the transmission chains and by means of using computer simulations of the complete transmission chain. It will be shown that the presence of two quantizers, i.e. cascaded quantization, in the transmission chain is the main cause of extra losses, and it will be shown that the losses in terms of SNR will be some 0.5 – 1.0 dB greater than in the case of a transmission chain without a transcoder.     

Machine learning based fast H.264/AVC to HEVC transcoding exploiting block partition similarity

Video transcoding is to convert one compressed video stream to another. In this paper, a fast H.264/AVC to High Efficiency Video Coding (HEVC) transcoding method based on machine learning is proposed by considering the similarity between compressed streams, especially the block partition correlations, to reduce the computational complexity. This becomes possible by constructing three-level binary classifiers to predict quad-tree Coding Unit (CU) partition in HEVC. Then, we propose a feature selection algorithm to get representative features to improve predication accuracy of the classification. In addition, we propose an adaptive probability threshold determination scheme to achieve a good trade-off between low coding complexity and high compression efficiency during the CU depth prediction in HEVC. Extensive experimental results demonstrate the proposed transcoder achieves complexity reduction of 50.2% and 49.2% on average under lowdelay P main and random access configurations while the rate-distortion degradation is negligible. The proposed scheme is proved more effective as comparing with the state-of-the-art benchmarks.     

New architecture for dynamic frame-skipping transcoder

Transcoding is a key technique for reducing the bit rate of a previously compressed video signal. A high transcoding ratio may result in an unacceptable picture quality when the full frame rate of the incoming video bitstream is used. Frame skipping is often used as an efficient scheme to allocate more bits to the representative frames, so that an acceptable quality for each frame can be maintained. However, the skipped frame must be decompressed completely, which might act as a reference frame to nonskipped frames for reconstruction. The newly quantized discrete cosine transform (DCT) coefficients of the prediction errors need to be re-computed for the nonskipped frame with reference to the previous nonskipped frame; this can create undesirable complexity as well as introduce re-encoding errors. In this paper, we propose new algorithms and a novel architecture for frame-rate reduction to improve picture quality and to reduce complexity. The proposed architecture is mainly performed on the DCT domain to achieve a transcoder with low complexity. With the direct addition of DCT coefficients and an error compensation feedback loop, re-encoding errors are reduced significantly. Furthermore, we propose a frame-rate control scheme which can dynamically adjust the number of skipped frames according to the incoming motion vectors and re-encoding errors due to transcoding such that the decoded sequence can have a smooth motion as well as better transcoded pictures. Experimental results show that, as compared to the conventional transcoder, the new architecture for frame-skipping transcoder is more robust, produces fewer requantization errors, and has reduced computational complexity.     

Compressed-Domain Techniques for Error-Resilient Video Transcoding Using RPS

In video applications where video sequences are compressed and stored in a storage device for future delivery, the encoding process is typically carried out without enough prior knowledge about the channel characteristics of a network. Error-resilient transcoding plays an important role to provide an addition of resilience to the video data, where or whenever it is needed. Recently, a reference picture selection (RPS) scheme has been adopted in an error-resilient transcoder in order to reduce error effects for the already encoded video bitstream. In this approach, the transcoder learns through a feedback channel about the damaged parts of a previously coded frame and then decides to code the next P-frame not relative to the most recent, but to an older, reference picture, which is known to be error-free in the decoder. One straightforward approach of adopting RPS in error-resilient transcoding is to decode all the P-frames from the previously nearest I-frame to the current transmitted frame which is then re-encoded with a new reference frame; this can create undesirable complexity in the transcoder as well as introduce re-encoding errors. In this paper, some novel techniques are suggested for an effective implementation of RPS in the error-resilient transcoder with the minimum requirement on its complexity. All the proposed techniques will manipulate video data in the compressed domain such that the computational loading of the transcoder is greatly reduced. By utilizing these new compressed-domain techniques, we develop a new structure to handle various types of macroblocks in the transcoder which re-uses motion vectors and prediction errors from the encoded bitstream. Experimental results demonstrate that significant improvements in terms of transcoder complexity and quality of reconstructed video can be achieved by employing our compressed-domain techniques.     

On the methods and performances of rational downsizing video transcoding

Video transcoding is a popular technique for adapting the bit-rate or spatial/temporal resolution of a precoded video to suit better the constraints and requirements of different transmission networks and receiving devices. To minimize computational complexity, many fast methods have been proposed to obtain the motion vectors required for transcoding a precoded video through reducing its frame size by an integral factor. In this paper, we extend the existing work by developing and comparing several fast methods of downsizing precoded videos by a rational factor. Methods that outperform others under different conditions or with different computational requirements are identified, and an application scenario that can benefit from the proposed rational downsizing video transcoding is presented. An efficient scheme is also proposed to select the proper reduced frame size for sustaining the best possible video quality at a specified lower bit-rate. The superiority of the proposed transcoding approach in comparison with the existing integral downsizing video transcoding or cascaded video re-encoding methods is evident from the experimental results shown in this paper.     

FITD: Fast Intra Transcoding from H.264/AVC to high efficiency video coding based on DCT coefficients and prediction modes

In the literatures, the designs of H.264 to High Efficiency Video Coding (HEVC) transcoders mostly focus on inter transcoding. In this paper, a fast intra transcoding system from H.264 to HEVC based on discrete cosine transform (DCT) coefficients and intra prediction modes, called FITD, is proposed by using the intra information retrieved from an H.264 decoder for transcoding. To design effective transcoding strategies, FITD not only refers block size of intra prediction and intra prediction modes, but also effectively uses the DCT coefficients to help a transcoder to predict the complexity of the blocks. We successfully use DCT coefficients as well as intra prediction information embedded in H.264 bitstreams to predict the coding depth map for depth limitation and early termination to simplify HEVC re-encoding process. After a HEVC encoder gets the prediction of a certain CU size from depth map, if it reaches the predicted depth, the HEVC encoder will stop the next CU branch. As a result, the numbers of CU branches and predictions in HEVC re-encoder will be substantially reduced to achieve fast and precise intra transcoding. The experimental results show that the FITD is 1.7–2.5 times faster than the original HEVC in encoding intra frames, while the bitrate is only increased to 3% or less and the PSNR degradation is also controlled within 0.1 dB. Compared to the previous H.264 to HEVC transcoding approaches, FITD clearly maintains the better trade-off between re-encoding speed and video quality.     

Adaptive error-resilience transcoding using prioritized intra-refresh for video multicast over wireless networks

In this paper, we propose a two-pass error-resilience transcoding scheme based on adaptive intra-refresh for inserting error-resilience features to a compressed video at the intermediate transcoder of a three-tier streaming system. The proposed transcoder adaptively adjusts the intra-refresh rate according to the video content and the channel's packet-loss rate to protect the most important macroblocks against packet loss. In this work, we consider the problem of multicast of video to multiple clients having disparate channel-loss profiles. We propose a MINMAX loss rate estimation scheme to determine a single intra-refresh rate for all the clients in a multicast group. For the scenario that a quality variation constraint is imposed on the users, we also propose a grouping method to partition a multicast group of heterogeneous users into a minimal number of subgroups to minimize the channel bandwidth consumption while meeting the quality variation constraint. Experimental results show that the proposed method can effectively mitigate the error propagation due to packet loss as well as achieve fairness among clients in a multicast.     

Cost-efficient HEVC-based quadtree splitting (HEQUS) for VVC Video Transcoding

The release of the latest video coding standard, known as Versatile Video Coding (VVC), has created the need to convert current High Efficiency Video Coding (HEVC) content to this new standard. However, the traditional cascade transcoding pipeline is not effective due to the exorbitant computational complexity of VVC. With this in mind, this paper proposes a fast HEVC-VVC transcoder that implements a probabilistic classifier based on Naïve-Bayes at the first partitioning level (128 × 128 pixels). This model uses features extracted from the 128 × 128 size blocks of the residual and reconstructed frames in the HEVC bitstream, and their correlation with the block partitioning structure. For the subsequent VVC coding depth levels, partitioning decisions are derived from the HEVC structure. The results achieve a 57.08% transcoding time reduction with a BD-rate penalty of 2.40%, compared with a traditional transcoding approach for the random access encoding configuration.     

Transcoding of single-layer MPEG video into lower rates

Forthcoming interactive video services such as video on demand will use pre-encoded bit streams for transmission. A great lack of flexibility arises when heterogeneous networks are used or when the user is allowed to use a bandwidth of his/her own choice. In the paper, a mechanism capable of decoupling video encoders from transmission-network constraints and user demands is proposed. The authors devise a low-cost, low-delay video transcoder capable of providing transmission flexibility to pre-encoded bit streams, by reducing their bit rates according to either channel capacity or user demand. Simple techniques such as open-loop coarse requantisation or drop of high-frequency transform coefficients are shown to be inefficient, because of the drift introduced in the transcoded pictures. It is shown that transcoded pictures are drift-free and their quality, on average, is only about 1 dB worse than those directly encoded at the same bit rate. The proposed transcoder is far less complex than a cascade of decoder-encoder, while the picture quality is shown to be better for almost all frames     

码率控制算法对广电网络视频图像损伤的性能分析

广电网络视频转码中码率控制主要有恒定码率控制(CBR)、自适应码率控制(ABR)等形式,前者主要用于广电DVB播出分发、后者用于广电IP播出分发等应用场景。本文对采用SDK架构的转码器码率控制算法以及码率控制对图像损伤程度开展性能分析,并给出相应的测试数据和实验选型结论。