Lecture 10Introduction to MPEG Video Compression - MPEG-1/2 Video coding - MPEG-4 video codingLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong1MPEG-1/2 Video codingLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong2References: MPEG 1/2References: MPEG-1/2hfbk Chapter 11 of textbook Chapter 16 of Sayood, K., Introduction to Data py Compression, 2ndEdition, Morgan Kauffman, 2000. Le Gall, D., “MPEG: A video Compression Standard for Multimedia Applications”Standard for Multimedia Applications , Communications of the ACM, Vol. 34, No. 4, April 1991April, 1991.Lecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong3Overview of MPEG 1Overview of MPEG-1 MPEG introduced earlier for audio coding MPEG-1 video This is part 2 of MPEG-1 Recall part 3 was for audio (MP3)p() Achieves bit rates around 1 to 1.5 Mbps Near VHS quality at 1 2 MbpsNear VHS quality at 1.2 Mbps Initial applications were for digital storage and retrieval of videoretrieval of videoLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong4Overview of MPEG 2Overview of MPEG-2 Intended as a generic, application independent standard E.g. Used in DVD and HDTV Bit rates could be from 4 Mbps up to 80 Mbps Bit rates could be from 4 Mbps up to 80 Mbps MPEG-2 defines a variety of different options that can be chosen depending on the applicationppLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong5MPEG Video CompressionMPEG Video Compression Basic building blocks are: Motion estimation and compensationp Transformation of frames using the DCT Quantisation of the DCT coefficients Quantisation of the DCT coefficientsLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong6MPEG 1 FramesMPEG-1 Framesh fdfi Each frame made up of 3 matrices 1 for luminance (black and white) Formed from red, blue and green colour components 2 for chrominance (colour differences) Formed from luminance and colour components MPEG-1 macroblock A 1616 luminance block Two 88 chrominance blocksTwo 88 chrominance blocks Lecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong7MPEG 1 Frame TypesMPEG-1 Frame Typesl ildd MPEG-1 exploits temporal redundancy Recall that differential coding has random g access and transmission error problems Hence MPEG-1 frame sequence is broken upHence, MPEG 1 frame sequence is broken up into sections Combine intraframe and interframe coding within Combine intraframe and interframe coding within each section Lecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong8MPEG 1 Frame Types (2)MPEG-1 Frame Types (2) Each frame is categorised into three types: I Frames: Intraframe coded frames Thfd dithtfttf tf These frames are coded without reference to any past or future frames P Frames: Predicted frames Coded using reference to past I or P frames e.g. using differential codingCoded using reference to past I or P frames e.g. using differential coding B Frames: Bidirectionally predicted frames Coded using reference to past and future I or P frames The I and P frames act as anchors for the predicted P and B framesLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong9MPEG 1 Group Of Pictures (GOP)MPEG-1 Group Of Pictures (GOP) A GOP is a group of I, P and B frames The GOP must have at least one I frame, to actThe GOP must have at least one I frame, to act as a reference point for the P and B frames A GOP llhdilh A GOP allows the user to display short sections of the video that contain at least one I frameLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong10GOP ExampleGOP Example?P frames predicted using the most recent I frameI frame?B frames predicted using a past and future I frame and P frameLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong11future I frame and P frameDisplay order versus transmit orderDisplay order versus transmit order The display order is the order in which each the frames are viewed by the usery The transmit order is the order in which the quantised frames are transmittedquantised frames are transmitted Generally different to the display orderLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong12Display order versus transmit order (2)Type I1B1B2P1B3B4I2No.1234567Display orderType IPBBIBBType I1P1B1B2I2B3B4No.1423756Transmit orderLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong13Transmit orderWhy the order difference?Why the order difference? You dont need to wait for the future anchor frame before you can decode the current B y frame In the above example you need P so that you In the above example, you need P1so that you can decode B1and B2 Sending P1before B1and B2allows you to display frames fasterdisplay frames fasterLecture 10Copyright 2008-2011, Christian Ritz, University of Wollongong14Mot