运动图像国际压缩标准MPEG.ppt

MPEG-4STEP1STEP2STEP3STEP41999年推出ISO/IEC标准MPEG-4。MPEG-4是目前视频压缩技术的最新发展水平。数字化电视、交互式图形应用(如PC游戏、虚拟环境)及WWW（万维网）这三个领域的成功促进了MPEG-4的诞生。MPEG-4旨在为视音频数据的通信、存取与管理提供一个灵活的框架与一套开放的编码工具。这些工具将支持大量的应用功能(新的和传统的)。MPEG-4提供的多种视音频(自然的与合成的)的编码模式使图像或视音频中对象的存取大为便利,称作基于内容的存取。MPEG1与MPEG2最主要的目标是通过数据压缩技术，实现数字音频、视频数据的有效存储和传输。因此，所处理的是音频及基于“矩形帧”的视频信息，而其交互功能也仅仅是局限在音频及矩形帧层次上。MPEG-4标准支持基于内容的交互功能，以音视频对象AVO(AudiovisualObject）的形式对AV场景进行描述，这些AVO在空间及时间上有一定的关联，分析后，可对AV场景进行分层描述。因此，MPEG-4提供了一种崭新的交互方式－基于内容的交互（Content-basedInteractivity）FurtherdatareductionisachievedbynotfurtherprocessingblockswhereallDCTcoefficientsarezero.Thisiscodedbyinsertingasix-bitvalueintotheencodeddatastream.Otherwise,theDC-andAC-coefficientsarethenencodedusingthesametechnique.Next,run-lengthencodingisappliedandavariablelengthcodingisdeterminedaccordingtoanalgorithmsimilartoHuffman.motionvectorsofadjacentmacroblocksareDPCMcoded.Theresultisagaintransformedintovariable-lengthcodedwordsusingatable.BframesBframes(bidirectionallypredictivecodedpictures)requireinformationfrompreviousandfollowingIand/orPframes.BframesyieldthehighestcompressionratioattainableinMPEG.ABframeisdefinedasthedifferencefromapredictionbasedonapreviousandafollowingIorPframe.Itcannoteverserveasareferenceforpredictioncodingofotherpictures.AmacroblockcanbederivedfrommacroblocksofpreviousandfollowingPand/orIframes.apredictioncaninterpolatetwosimilarmacroblocks.twomotionvectorsareencodedonedifferenceblockisdeterminedbetweenthemacroblocktobeencodedandtheinterpolatedmacroblock.SubsequentquantizationandentropyencodingareperformedasforP-frame-specificmacroblocks.Bframesneednotbestoredinthedecoder.DframesDframes(DCcodedpictures)areintraframe-codedandcanbeusedforefficientfastforward.DuringtheDCT,onlytheDC-coefficientsarecoded;the