motion estimation and compensation for very low bitrate video coding

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18 Chapter 1. Digital Video Coding at Very-Low BitRate other cases one step is not used. However, the di erent steps have to be coherent if one wants the scheme to be e cient: the type of analysis made often guides the rest of the algorithm. Up to now, two di erent philosophies have been used in (VLBR) video coding. The rst class is the block-based one [62, 96], where pictures are divided into subblocks according to an a priori grid: every subblock may be independently coded according to its own characteristics. The other class is the segmentation-based or object-based one. Such schemes [85, 84, 34] aim to better preserve essential characteristics: high compression ratios are obtained by removing insigni cant objects and by encoding textures more coarsely, while contours are considered as essential features for image description. After a detection of the di erent pictured regions (objects), every region is separately coded: its shape (contour) is rst described and is followed by a texture (or motion) information. 1.2.2 Video Compression Original images Intra-images encoder Motion compensation Motion estimation Frame memory Intra-images decoder Motion vectors Multiplexer to channel coder Figure 1.2: Typical video coder: intra-images, motion estimation & compensation and residues are used Independently of these di erent approaches of the image contents, one can point out that both categories of (VLBR) coding schemes make an
1.2 Video Coding 19 exhaustive use of the two following types of pictures: Intra-images: When coding an image in intra mode, only the spatial correlation inherent to the picture itself is exploited. In fact, an intra-image is an isolated picture and is compressed as such (cf. the JPEG [135] algorithm for still pictures compression). Inter-images: These images are speci c to video sequences in comparison with still pictures. They allow the coder to exploit the spatio-temporal correlation between the present image and the previous one(s) within a sequence. The classical tool used to achieve compression on such images is motion estimation & compensation which allows one to obtain a prediction of the present image from the previous one(s). { Residues: As the estimation & compensation of an interimage is not perfect, the di erence between the estimate and the original image is computed: the so-called Displaced Frame Di erence (DFD) has to be sent if it is relevant enough. It can only be transmitted on its own basis, i.e. as an intra-image. The rst image of a sequence must of course be encoded as an intraimage. For the subsequent images, a scene-cut detector compares the new image with the previous one(s) to check if some temporal correlation still exists. In the a rmative, the new image will be inter-coded. Otherwise, the coder considers that a scene-cut has occurred: the new image di ers so much from the previous one(s) that it is better to encode it as an intra-image. The user may also enforce intra-image coding to take place every x seconds to o er speci c functions like fast retrieval, temporal scalability or resynchronization in case of noisy channel. According to these considerations, one can a rm that the scheme presented on gure 1.2 (from [13]) is typical of most (VLBR) video coders. The rst image is intra-coded. Then the second image is motion estimated on the basis of the rst one (original or reconstructed). The motion vectors of course need to be sent to the decoder in order to allow it to behave the same way. The motion vector eld is applied to the rst image thereby obtaining a prediction of image two, which is used for the DFD computation. This DFD is the residual image that also needs to be coded. The process goes on until some scene cut arises or until a de ned threshold enforces to start again with an intra-image.
Page 1: LABORATOIRE DE TELECOMMUNICATIONS E
Page 5: Avant-propos Mener a bien une these
Page 8 and 9: Resume L'estimation du mouvement re
Page 10 and 11: x 2.1.1 Apparent versus Real Motion
Page 12 and 13: xii A VLBR-like video sequences 167
Page 14 and 15: 2 Introduction Ancient Greece initi
Page 16 and 17: 4 Introduction Meanwhile, the desir
Page 18 and 19: 6 Introduction However, one can que
Page 20 and 21: 8 Introduction compensation in a vi
Page 22 and 23: 10 Introduction Once the proposed s
Page 25 and 26: Chapter 1 Digital Video Coding at V
Page 27 and 28: 1.2 Video Coding 15 These di erence
Page 29: 1.2 Video Coding 17 { The redundanc
Page 33 and 34: 1.4 Some (Very-) Low BitRate Codecs
Page 49: 1.5 Discussion 37 1.5 Discussion Th
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68 Chapter 2. Motion in the Framewo
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78 Chapter 3. Multiscale Block Matc
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Chapter 4 Image Pre-Processing for
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4.1 Intuitive Rationale 97 (a) (b)
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4.1 Intuitive Rationale 99 (a) (b)
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4.2 Rate Distortion Conditions 101
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4.3 Experimental Results 107 Pre-pr
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4.3 Experimental Results 109 Mean P
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4.3 Experimental Results 111 From t
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4.3 Experimental Results 113 Origin
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4.4 Conclusion 119 4.4 Conclusion T
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122 Chapter 5. Mesh-Based Motion Co
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162 Conclusion so as to adapt the s
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164 Conclusion gain is not as obvio
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Appendix A VLBR-like video sequence
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VLBR-like video sequences 169 Coast
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Appendix B Rate Distortion theory A
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B.1 Information Theory 173 B.1 Info
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B.2 Discrete Memoryless Sources and
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B.3 Rate Distortion Function 177 B.
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B.4 Extension to Moving Pictures 17
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B.4 Extension to Moving Pictures 18
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184 Chapter C. Markov Random Fields
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186 Chapter C. Markov Random Fields
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Appendix D Complements to Chapter 5
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D.2 Pseudo-code of the implementati
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Bibliography [1] COST 211ter Simula
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Bibliography 205 [21] E. Decenciere
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Bibliography 207 [41] R.M. Gray. Ve
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Bibliography 209 [65] B. Macq, M.P.
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Bibliography 211 [86] H.G. Musmann,
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Bibliography 213 [110] M.P. Queluz
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Bibliography 215 [132] F. Vermaut,
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