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CIS679: MPEG

CIS679: MPEG. MPEG. MPEG. MPEG: Motion Pictures Experts Group Standard for encoding videos/movies/motion pictures Evolving set of standards MPEG-1, MPEG-2, MPEG-4 Exploit motion prediction Apply the similar techniques as JPEG to individual image . MPEG Compression Steps.

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CIS679: MPEG

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  1. CIS679: MPEG • MPEG

  2. MPEG • MPEG: Motion Pictures Experts Group • Standard for encoding videos/movies/motion pictures • Evolving set of standards • MPEG-1, MPEG-2, MPEG-4 • Exploit motion prediction • Apply the similar techniques as JPEG to individual image

  3. MPEG Compression Steps • Data preparation • Similar to JPEG • Data processing • Motion prediction • DCT • Quantization • Entropy encoding

  4. Data Preparation • Subsample Chrominance signals • 4:1:1 in MPEG-1 • NTSC: Y=352*240, Cb=Cr=176*120 • PAL: Y=352*288, Cb=Cr=176*144 • Macroblocks • 16*16 samples (pixels) for the luminance component. • 8*8 samples (pixels) for the chrominance component.

  5. Data Processing • Apply motion prediction at the Macroblock level • In most cases, predictive encoding only makes sense for parts of images and not for the whole image. • Motion prediction used to figure out where this macroblock may be in the next frame • Forward prediction • Prediction based on previous frames • Backward prediction • Prediction based on future frames • Bidirectional prediction • Prediction based on both previous and future frames

  6. IPB Frames • I frames • Independently coded and no temporal prediction • Can start decoding once we have an I-frame • P Frames • Forward predicted frames • Based on previous I or P frames • B frames • Bi-directionally predicted frames, based on previous and following I and P frames • Group of Pictures (GOP): Set of frames between two I frames • IBBPBBPBBI or IPPPI, or II • MPEG decoders may have to buffer extra frames of data besides being displayed

  7. DCT • Apply DCT on the 8*8 data blocks • DC coefficients • AC coefficients

  8. Quantization • Apply Quantization on the frequency domain coefficients • Favor low frequency coefficients

  9. Entropy Encoding • Convert 2 dimensional symbols into 1 dimensional series • Use zig-zag or vertical scans • Apply run-length coding on the 1-D series • Apply Huffman coding

  10. Conclusion • Successive frames may have significantly same data • MPEG exploits motion prediction • Apply motion prediction at the Macroblock level • I, P, and B frames • The standard allows the use of I-frame only, I and P frames only or I-, P- and B-frames. • More about MPEG……

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