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Compression video overview

Compression video overview. 演講者：林崇元. Outline. Introduction Fundamentals of video compression Picture type Signal quality measure Video encoder and decoder Standard ’ s. Introduction. Why we need to compression Picture

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Compression video overview

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  1. Compression video overview 演講者：林崇元

  2. Outline • Introduction • Fundamentals of video compression • Picture type • Signal quality measure • Video encoder and decoder • Standard’s

  3. Introduction • Why we need to compression • Picture • A picture consists of three rectangular matrices representing luminance (Y) and two chrominance (Cb and Cr) values • The Y matrix has an even number of rows and columns • The Cb and Cr matrices are one-half the size of the Y matrix in each direction (horizontal and vertical).

  4. Introduction • Applications for image, video, and audio compression

  5. Introduction • Achieve high compression performance while keep good picture quality • Theorem • Spatial redundancy – DCT,DFT,subband,wavelet • Temporal redundancy – MC/ME • Statistical redundancy – VLC, Entropy coding • Perceptual redundancy – VQ

  6. Introduction • Tradeoffs in lossy compression

  7. Fundamentals of video compression • Use the technique of the JPEG • DCT based coding scheme • DCT transform (2D)

  8. Fundamentals of video compression • Use the technique of the JPEG • Discrete cosine transform

  9. Fundamentals of video compression • Use the technique of the JPEG • DCT based coding system Spatial-to-DCT domain transformation 8 x 8 DCT Image Discard unimportant DCT domain samples Quantization Lossless coding of DCT domain samples Entropy Coding

  10. Fundamentals of video compression • Quantization • Eyes are insensible to high-frequency components • The greater quantizer means greater loss • Lower frequency component has smaller quantizer, high frequency component has greater quantizer • The quantiation tables in the encoder and decoder are the same

  11. Fundamentals of video compression • Use the technique of the JPEG • The spatial domain is redundancy • For the DCT-based coding system on an image-by-image, one can achieve close to 14Mbits per second, which is too high for practical uses • For lower bit rate, we must introduce temporal redundancy

  12. Fundamentals of video compression • Temporal redundancy • The temporal correlation in an image sequence

  13. Fundamentals of video compression • Temporal redundancy • Instead of 3-D DCT, most video coders use a two-stage process to achieve good compression • Two-stage video coding process

  14. Fundamentals of video compression • Temporal redundancy • Motion estimation

  15. Fundamentals of video compression • Temporal redundancy • Full search algorithm

  16. Picture type • Video bit stream

  17. Picture type • Slice • One or more "contiguous'' macroblocks. The order of the macroblocks within a slice is from left-to-right and top-to-bottom. • Macroblock • A 16-pixel by 16-line section of luminance components and the corresponding 8-pixel by 8-line section of the two chrominance components. • Block • A block is an 8-pixel by 8-line set of values of a luminance or a chrominance component.

  18. Picture type • Intra picture • Coded using only information present in the picture itself • I-pictures provide potential random access points into the compressed video data. • I-pictures use only transform coding

  19. Picture type • Predicted picture • coded with respect to the nearest previous I- or P-picture. • P-pictures use motion compensation • Unlike I-pictures, P-pictures can propagate coding errors

  20. Picture type • Bidirectional picture • Coded use both a past and future picture as a reference • B-pictures provide the most compression and do not propagate errors

  21. Picture type • The choice of picture type • The MPEG algorithm allows the encoder to choose the frequency and location of I-pictures is based on the application's need for random accessibility and the location of scene cuts in the video sequence • The encoder also chooses the number of B-pictures between any pair of reference (I- or P-) pictures. This choice is based on factors such as the amount of memory in the encoder and the characteristics of the material being coded

  22. Picture type • Typical display order of picture types • Video stream composition • The MPEG encoder reorders pictures in the video stream to present the pictures to the decoder in the most efficient sequence

  23. Signal quality measure • SNR (signal-to-noise ratio) encoder input signal energy • SNR = 10log10 noise signal energy • PSNR (peak signal-to-noise ratio) • Instead of using the encoder input signal, one uses a hypothetical signal with a signal strength of 255

  24. Video encoder

  25. Video decoder

  26. MPEG-1 • Media storage • Optimal for frame size 352x240x30 • Bitrate : up to 1.5 Mbit/s • International standard in 1992

  27. MPEG-2 • Applications from storage to HDTV • Bitrate : standard definition TV:4-9 Mbit/s HDTV:15-25 Mbit/s • Interlaced/non-interlaced • Scalability • Capable of decoding MPEG-1 bitstream • International standard in 1994 • Single chip for video and audio

  28. MPEG-4 • Applications for multimedia communication • Bitrate : 10K-25 Mbit/s • Object – based coding • Natural and synthetic video • Scalability • Robust and error resilience • International standard in 1998 • Single chip for video and audio

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