视频转码应用分析

在视频会议系统中还有日常拍摄处理视频时,需要利用视频转码技术对传统和新兴视频流进行格式转换。使用转码的意义在于可以在不损失任何视频质量的前提下节省高达50％的网络带宽。转码的分类按照媒体格式可分为音频转码、视频转码,视频转码比较出名的有暴风转码、格式工厂,音频转码用大众的千千静听就能满足使用需要。本文以转码应用为研究对象,进一步分析软件转码技术的应用。     

无线传输中对转码后视频的客观质量评价

构建了一个对于转码后的视频进行客观评价的构架.实验框架由主机端软件、特定传输带宽的无线网络、数字媒体适配器、YQM软件、主观视频质量评价系统等组成.原始视频信号经过主机端软件进行转码处理,通过一个特定带宽的无线网络传输后,到达数字媒体适配器,再通过视频收集卡将此时的视频采集到PC,在该PC上将此目标码流和原始码流输入VQM应用软件,同时配合一个主观视频质量评价系统对YQM的参数进行适当的调整,确定出带宽条件下的特定的评测参数,用此方法可以得到不同带宽下的评测参数.将此特定参数与特定带宽相结合,就可以衡量转码并经特定带宽的无线传输后的视频质量.实验以实际项目中应用的特定码流,由YQM软件测得多组数据,得出了适当的评测参数.     

无线传输中对转码后视频的客观质量评价

构建了一个对于转码后的视频进行客观评价的构架。实验框架由主机端软件，特定传输带宽的无线网络，数字媒体适配器，VQM软件，主观视频质量评价系统等组成。原始视频信号经过主机端软件进行转码处理，通过一个特定带宽的无线网络传输后，到达数字媒体适配器，再通过视频收集卡将此时的视频采集到PC，在该PC上将此目标码流和原始码流输入VQM应用软件，同时配合一个主观视频质量评价系统对VQM的参数进行适当的调整，确定出带宽条件下的特定的评测参数，用此方法可以得到不同带宽下的评测参数。将此特定参数与特定带宽相结合，就可以衡量转码并经特定带宽的无线传输后的视频质量。实验以实际项目中应用的特定码流，由VQM软件测得多组数据，得出了适当的评测参数。     

一种基于组播的视频转码系统的设计与实现

视频会议是一种支持人们远距离进行实时信息交流、开展协同工作的应用。基于组播骨干网(MBone)的视频应用已成为研究的热点。目前,在组播视频会议中,视频发送者基本上都是按事先设定的带宽进行数据发送。这种按固定速率发送视频的方式无法同时满足所有接收者对视频质量的要求。该文提出并实现了一个视频转码系统,该转码系统被配置在窄带网络和宽带网络的连接处,一方面接收来自宽带网络的组播视频数据,另一方面能够针对窄带网络的可用带宽对接收到的视频数据进行码率转换,实时降低输出视频的码率并将转码后的视频发送到另一个组播组,使窄带网络上的用户能通过该组播组参与视频会议。     

基于Spark Streaming的快速视频转码方法

针对单机视频转码方法转码速度较慢和面向批处理的并行转码方法效率提升有限的问题,基于Spark Streaming分布式流处理框架,提出了一种面向流处理的快速视频转码方法。首先,使用开源多媒体处理工具FFmpeg,构建了自动化的视频切片模型,提出编程算法;然后,针对并行视频转码的特点,对弹性分布式数据集（RDD）进行研究,构建了视频转码的流处理模型;最后,设计视频合并方案,将合并后的视频文件进行有效储存。根据所提出的快速视频转码方法设计与实现了基于Spark Streaming的快速视频转码系统。实验结果表明,与面向批处理Hadoop视频转码方法相比,所提方法转码效率提升了26.7%;与基于Hadoop平台的视频并行转码方法相比,该方法转码效率提升了20.1%。     

基于云计算框架的视频转码系统

随着视频服务和高清视频的普及,视频服务提供商正面临着越来越多的高清视频转码需求。但是由于高清视频分辨率高、码率大,其转码复杂度是标清视频的数倍,而常规的转码系统虽然利用了多核并发来提高转码速度,但终究受限于单个计算节点的物理条件和转码算法本身的并发能力,使得转码速度提升空间有限。提出一个利用云计算的"Map-Reduce"计算框架的转码系统,将单个转码任务并发至多个计算节点,每个计算节点对同一个视频文件的不同部分分别进行转码,再将转码后的视频文件合并,从而显著提升了转码速度。     

基于前后端交互的人脸识别系统

针对现有的人脸识别系统计算效率低和鲁棒性较差等问题,本文提出了一种基于前后端交互的人脸识别系统,系统包含客户端、数据库以及服务端.首先,在客户端提出了基于GrabCut的人脸兴趣区域(ROI)提取算法.其次,将提取到的ROI传输到服务端,并在服务端使用ResNet网络根据ROI提取人脸特征点.最后,将服务端中提取到的人脸特征点返回给客户端,在客户端将该信息与数据库中预存的特征点进行欧式距离匹配,得到人脸识别结果.实验在CeleA数据集与随机视频上进行测试,结果表明提出的ROI提取算法明显提升了人脸识别的精度和鲁棒性,并且系统的前后端交互结构相较于传统的非交互结构极大地提升了人脸识别的计算效率.     

面向移动网络的实时视频转码系统

针对宽带IPTV视频源向移动网络和移动终端提供视频服务需要解决的视频转码问题,设计开发以软件方式实现的实时视频转码系统,可将MPEG-4格式的高码率、高分辨率节目源实时转换为适合移动网络和移动终端的低码率、低分辨率视频。在误差补偿转码框架的基础上,提出一种自适应的转码时延保证方法,解决了转码计算复杂性和转码质量均衡的关键技术问题,保证了转码系统的实时性。测试结果表明,实现的转码系统视频转换质量损失少、实时性高。     

基于随机森林的带宽预测算法研究与实现

为解决手机移动端带宽波动导致用户视频服务体验降低的问题,提出一种移动手机可用带宽预测算法(mobile available bandwidth prediction,MABP)。采集Android手机的LTE参数,使用随机森林预测手机带宽,当客户端发起视频服务请求时,同时向服务器反馈手机的当前可用带宽,服务器根据移动客户端提供的带宽信息进行自适应流分发(发送最优码率的视频),降低视频画面卡顿、画质模糊、切换时间过长等问题,提升用户体验(quality of experience,QoE)。在实际的网络环境下验证了所述算法的有效性。     

基于云计算平台的视频转码设计与应用

随着高清视频的普及和发展,视频服务提供商面临着高清晰度视频转码的需求日益增加。视频转码既是计算密集型工作又是数据密集型工作,需要大量消耗计算机资源,但是最终会因为单计算节点的物理条件和转码算法的并发能力受到限制,造成转码速度提升不大。该文依托云计算平台将单个转码任务同时发送到多个计算节点,以提高视频转码的速度。     

Video transcoding: an overview of various techniques and research issues

One of the fundamental challenges in deploying multimedia systems, such as telemedicine, education, space endeavors, marketing, crisis management, transportation, and military, is to deliver smooth and uninterruptible flow of audio-visual information, anytime and anywhere. A multimedia system may consist of various devices (PCs, laptops, PDAs, smart phones, etc.) interconnected via heterogeneous wireline and wireless networks. In such systems, multimedia content originally authored and compressed with a certain format may need bit rate adjustment and format conversion in order to allow access by receiving devices with diverse capabilities (display, memory, processing, decoder). Thus, a transcoding mechanism is required to make the content adaptive to the capabilities of diverse networks and client devices. A video transcoder can perform several additional functions. For example, if the bandwidth required for a particular video is fluctuating due to congestion or other causes, a transcoder can provide fine and dynamic adjustments in the bit rate of the video bitstream in the compressed domain without imposing additional functional requirements in the decoder. In addition, a video transcoder can change the coding parameters of the compressed video, adjust spatial and temporal resolution, and modify the video content and/or the coding standard used. This paper provides an overview of several video transcoding techniques and some of the related research issues. We introduce some of the basic concepts of video transcoding, and then review and contrast various approaches while highlighting critical research issues. We propose solutions to some of these research issues, and identify possible research directions.     

Efficient segment-based video transcoding proxy for mobile multimedia services

To support various bandwidth requirements for mobile multimedia services for future heterogeneous mobile environments, such as portable notebooks, personal digital assistants (PDAs), and 3G cellular phones, a transcoding video proxy is usually necessary to provide mobile clients with adapted video streams by not only transcoding videos to meet different needs on demand, but also caching them for later use. Traditional proxy technology is not applicable to a video proxy because it is less cost-effective to cache the complete videos to fit all kinds of clients in the proxy. Since transcoded video objects have inheritance dependency between different bit-rate versions, we can use this property to amortize the retransmission overhead from transcoding other objects cached in the proxy. In this paper, we propose the object relation graph (ORG) to manage the static relationships between video versions and an efficient replacement algorithm to dynamically manage video segments cached in the proxy. Specifically, we formulate a transcoding time constrained profit function to evaluate the profit from caching each version of an object. The profit function considers not only the sum of the costs of caching individual versions of an object, but also the transcoding relationship among these versions. In addition, an effective data structure, cached object relation tree (CORT), is designed to facilitate the management of multiple versions of different objects cached in the transcoding proxy. Experimental results show that the proposed algorithm outperforms companion schemes in terms of the byte-hit ratios and the startup latency.     

PTC: Proxies that Transcode and Cache in Heterogeneous Web Client Environments

Advances in computing and communication technologies have resulted in a wide variety of networked mobile devices that access data over the Internet. In this paper, we argue that servers by themselves may not be able to handle this diversity in client characteristics and so intermediaries, such as proxies, should be employed to handle the mismatch between the server-supplied data and the client capabilities. Since existing proxies are primarily designed to handle traditional wired hosts, such proxy architectures will need to be enhanced to handle mobile devices. We propose such an enhanced proxy architecture that is capable of handling the heterogeneity in client needs—specifically the variations in client bandwidth and display capabilities. Our architecture combines transcoding (which is used to match the fidelity of the requested object to client capabilities) and caching (which is used to reduce the latency for accessing popular objects). Proxies that Transcode and Cache, PTCs, intelligently adapt to prevailing system conditions using learning techniques to decide whether to transcode locally or fetch an appropriate version from the server. Our experimental results indicate that the use of PTCs produces significant improvements in the client response times. We show that such results hold true for a variety of data content types like images and video data. Further, we find that even simple learning techniques can lead to significant performance improvements.     

Reducing bandwidth requirement for delivering video over wide area networks with proxy server

Due to the high bandwidth requirement and rate variability of compressed video, delivering video across wide area networks (WANs) is a challenging issue. Proxy servers have been used to reduce network congestion and improve client access time on the Internet by caching passing data. We investigate ways to store or stage partial video in proxy servers to reduce the network bandwidth requirement over WAN. A client needs to access a portion of the video from a proxy server over a local area network (LAN) and the rest from a central server across a WAN. Therefore, client buffer requirement and video synchronization are to be considered. We study the tradeoffs between client buffer, storage requirement on the proxy server, and bandwidth requirement over WAN. Given a video delivery rate for the WAN, we propose several frame staging selection algorithms to determine the video frames to be stored in the proxy server. A scheme called chunk algorithm, which partitions a video into different segments (chunks of frames) with alternating chunks stored in the proxy server, is shown to offer the best tradeoff. We also investigate an efficient way to utilize client buffer when the combination of video streams from WAN and LAN is considered.     

一种基于视频转码与IP组播的媒体推送系统设计

为了减轻源节点负载,提高带宽利用率,提出了一种新型媒体推送系统设计.所提系统结合了视频转码与IP组播技术特点,一方面继承IP组播的带宽优势,另一方面实现了多种格式及码率用户间的数据共享.在所提系统的具体组织形式上,提出了顺序模式与上传模式,并对两种组织模式进行了建模分析.在仿真实验中,对所提系统和现行系统进行了仿真实现和比较,结果显示,所提系统在带宽消耗和源节点负载性能上具有一定的优势.     

基于视点预测的全景视频码率自适应策略

为解决全景视频传输中存在的视频卡顿多、用户体验质量（quality of experience,QoE）低等问题,研究当前主流的视点自适应传输方案,提出一种基于视点预测的码率自适应策略（VPBAS）。首先,构建了一种基于长短期记忆网络和全卷积网络的视点预测模型,模型将视点数据和视频显著性信息进行特征融合,实现不同模态数据的相互补充和修正,提高视点预测的准确率;然后,客户端采用随机森林算法预测当前的可用带宽,并根据视点预测结果和可用带宽信息为视频分块选择码率。最后,客户端把选择的码率信息定期发送给服务器,服务器根据反馈的信息向客户端推送最佳码率的全景视频流,这种交互过程在视频播放期间不断地重复,直至客户端观看完毕。实验结果表明,与现有传输方案相比,VPBAS能有效提高带宽受限情况下的视频观看体验。     

A platform for transcoding heterogeneous markup documents using ontology-based metadata

As new standards, markup languages, protocols, and client devices continue to emerge, the main problem of existing transcoding systems is the lack of intelligence to cope with the heterogeneous effects, including various transcoding policies, markup documents, device constraints, and server platforms. This study proposes a new approach, called hybrid transcoding, to combine the traditional transcoding technologies based on ontology-based metadata to improve these heterogeneous problems. Additionally, the heterogeneous markup document transcoding (HMDT) platform, based on the proposed hybrid transcoding and web services technologies, is also presented to serve as a transcoding service broker to facilitate interoperability between distributed heterogeneous transcoders. To demonstrate the feasibility of HMDT platform, an application scenario of hybrid transcoding is implemented to convert HTML forms into various client devices.