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Principles of Video Compression

Learn about reducing video bit rates, exploiting correlations for quality, and motion estimation techniques in video compression. Dive into fundamental redundancy reduction principles and faster motion estimation methods used in video codecs.

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Principles of Video Compression

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  1. Principles of Video Compression Dr. S. M. N. Arosha Senanayake, Senior Member/IEEE Associate Professor in Artificial Intelligence Room No: M2.06 Email: arosha.senanayake@ubd.edu.bn Source: Chapter 3 of JPEG—Still Image Compression StandardJPEG2000 Standard for Image Compression: Concepts, Algorithms and VLSI Architecturesby Tinku Acharya and Ping-Sing Tsai 

  2. Topics today… • Introduction • Temporal Redundancy Reduction • Coding for Video Conferencing (H.261, H.263) SS-4306

  3. Introduction • Reduce video bit rates while maintaining an acceptable image quality • Exploit strong correlation both between successive picture frames and within the picture elements themselves • Insensitivity of the human visual system to loss of certain spatio-temporal visual information • Uses Interframe predictive coding • H.261, H.263, MPEG-1, 2 and 4 SS-4306

  4. Introduction [2] • Fundamental redundancy reduction principles: • Spatial redundancy reduction • Temporal redundancy reduction • Entropy coding SS-4306

  5. Temporal Redundancy Reduction • Use Interframe coding • Static parts of the image sequence, temporal differences will be close to zero, and hence are not coded • Parts that change between the frames, either due to illumination variation or to motion of the objects, result in significant image error, which needs to be coded SS-4306

  6. Temporal Redundancy Reduction [2] Interframe Motion compensated Interframe SS-4306

  7. Temporal Redundancy Reduction [3]MOTION ESTIMATION • Estimate the motion of moving objects by block matching algorithm (BMA) • Divide the frame into blocks of M × N pixels usually, square blocks of N2 pixels • For a maximum motion displacement of w pixels per frame • Match the current block of pixels against a corresponding block at the same coordinates but in the previous frame, within the square window of width N + 2w • Find the displacement on the basis of match criterion for best match SS-4306

  8. Temporal Redundancy Reduction [4]MOTION ESTIMATION The current and previous frames in a search window SS-4306

  9. Temporal Redundancy Reduction [4]MOTION ESTIMATION • Matching Functions • Mean Squared Error • Mean Absolute Error • To reduce processing cost, MAE is preferred to MSE and hence is used in all the video codecs SS-4306

  10. Temporal Redundancy Reduction [5]MOTION ESTIMATION • BMA in simple case requires (2w+1)2 computations • Costly • Motion estimations comprise almost 50–70 per cent of the overall encoder's complexity • Faster Motion estimation is required !!! SS-4306

  11. Temporal Redundancy Reduction [6]FASTER MOTION ESTIMATION • Reduce the number of search points by selectively checking only a small number of specific points • Assumption behind this being the distortion measure monotonically decreases towards the best matched point • Approaches • Two-dimensional logarithmic (TDL) • Three-step search (TSS) • Modified motion estimation algorithm (MMEA) SS-4306

  12. Temporal Redundancy Reduction [7]FASTER MOTION ESTIMATION Two dimensional Logarithmic Search SS-4306

  13. Temporal Redundancy Reduction [8]FASTER MOTION ESTIMATION • Try to find the maximum number of steps to reach the best estimation !!! SS-4306

  14. The assumption of monotonic variation of image intensity methods perform well for slow moving objects, such as those in video conferencing often converge to a local minimum of distortion subsample the image to smaller sizes, such that the motion speed is reduced by the sampling ratio Hierarchical block matching algorithm (HBMA) Temporal Redundancy Reduction [9]HIERARCHICAL MOTION ESTIMATION A three-level image pyramid SS-4306

  15. Temporal Redundancy Reduction [10]GENERIC INTERFRAME VIDEO CODEC Generic Interframe encoder used in standard video codecs, such as H.261, H.263, MPEG-1, MPEG-2 and MPEG-4 SS-4306

  16. Temporal Redundancy Reduction [11]GENERIC INTERFRAME VIDEO CODEC Generic Interframe decoder SS-4306

  17. Coding for Video Conferencing (H.261) • Allows bitrates between approximately 64 kbit/s and 1920 kbit/s • Interframe DCT-based coding technique • Interframe prediction is first carried out in the pixel domain • The prediction error is then transformed into the frequency domain, where the quantization for bandwidth reduction takes place • Motion compensation can be included in the prediction stage, although it is optional SS-4306

  18. Two types of frames I-Frame P-Frame I-Frame is usually sent a couple of seconds Motion vectors are always measured in the neighborhood of 15 pixels Coding for Video Conferencing (H.261) [2] Frame Sequence SS-4306

  19. Coding for Video Conferencing (H.261) [3] I-Frame Coding SS-4306

  20. Coding for Video Conferencing (H.261) [4] P-Frame Coding SS-4306

  21. Coding for Video Conferencing (H.261) [5] • Quantization • Step size is fixed, 31 even levels from 2 62 • scale between 1 to 31 • Exception : DC coeff in I-Frame , step size is 8 always used SS-4306

  22. Coding for Video Conferencing (H.261) [6] Encoder SS-4306

  23. Coding for Video Conferencing (H.261) [7] Decoder SS-4306

  24. Four Layers Picture Layer Group Of Blocks (GOB) Layer 11 x 3 Macroblocks, a GOB CIF contains 2x6 GOBS QCIF contains 3 GOBS Macroblock Layer Block Layer Video Bitstream Syntax Syntax of H.261 video bitstream SS-4306

  25. H.263 • An improved video coding standard for video conferencing & other audio visual services • Aimed at low bitrate communications of less than 64kbps • Predictive coding for inter-frames • Transform coding for intra-frames & difference macroblocks from inter-frame prediction • Supports notion of GOBs SS-4306

  26. H.263 Motion Compensation • Predicted MV of the current block • Finding MV when current block is on the border SS-4306

  27. Motion compensation involves half pixel precision H.263 Motion Compensation [2] SS-4306

  28. H.263 Optional Coding Modes • Unrestricted motion vector mode • Syntax based arithmetic coding (SAC) • Advanced prediction mode • PB-Frames SS-4306

  29. Reference:1. Chapter 3 of Principles of Video CompressionStandard Codecs: Image Compression to Advanced Video Coding by Mohammed Ghanbari 2. Chapter 10 of Ze-Nian Li & Mark S. Drew, "Fundamentals of Multimedia", Pearson Education, 2004 SS-4306

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