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2.3 Video Compression

2.3 Video Compression. 2.3.1 MPEG MPEG stands for Moving Picture Experts Group (a committee of ISO). Main goal of MPEG-1 : compress a video signal (with audio) to a data stream of 1.5 Mbit/s, the data rate of a T1 link in the U.S. and the rate that can be streamed from a CD-ROM.

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2.3 Video Compression

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  1. 2.3 Video Compression • 2.3.1 MPEG • MPEG stands for Moving Picture Experts Group (a committee of ISO). • Main goal of MPEG-1: compress a video signal (with audio) to a data stream of 1.5 Mbit/s, the data rate of a T1 link in the U.S. and the rate that can be streamed from a CD-ROM.

  2. Goals of the MPEG-1 Compression Algorithm • Random access within 0.5 s while maintaining a good image quality for the video • Fast forward / fast rewind • Possibility to play the video backwards • Allow easy and precise editing.

  3. MPEG Frame Types • MPEG distinguishes four types of frames: • I-Frame (Intra Frame) • Intra-coded full image, very similar to the JPEG image, encoded with DCT, quantization, run-length coding and Huffman coding • P-Frame (Predicted Frame) • Uses delta encoding. The P frame refers to preceding I- and P-frames. DPCM encoded macro blocks, motion vectors possible. • B-Frame (Interpolated Frame) • "bidirectionally predictive coded pictures„. The B frame refers to preceding and succeeding frames, interpolated the data and encodes the differences. • D-Frame • "DC coded picture", only the DC coefficient of each block is coded (upper left-hand corner of the matrix), e.g., for previews.

  4. „Group of Pictures“ in MPEG • The sequence of I, P and B frames is not standardized but can be chosen according to the require-ments of the application. This allows the user to chose his/her own compromise between video quality, compression rate, ease of editing, etc.

  5. MPEG Encoder

  6. entropy inverse decoder quantizer IDCT previous picture store 1/2 x u M future picture store 0 motion compensation MPEG Decoder

  7. Temporal Redundancy and Motion Vectors • "Motion Compensated Interpolation" On the encoder side the search range can be chosen as a parameter: the larger the search range, the higher the potential for compression, but the longer the run time of the algorithm.

  8. MPEG-2 • MPEG-2 extends MPEG-1 for higher bandwidths and better image qualities, up to HDTV. It was developed jointly by ISO and ITU-T (where the standard is called H.262). • MPEG-2 defines scalable data streams which allow receivers with different bandwidth and processing power to receive and decode only parts of the data stream.

  9. Scalability in MPEG-2 (1) • SNR scalability:Each frame is encoded in several layers. A receiver who only decodes the base layer will get a low image quality. A receiver decoding additional (higher) layers gets a better image quality. An example is color sub sampling: the base layer contains only one quarter of the values for the U and V components, compared to the Y components. The enhancement layer contains the U and V components in full resolution, for better color quality. • Spatial scalability:The frames are encoded with different pixel resolutions (e.g., for a standard TV set and for an HDTV TV set). Both encodings are transmitted in the same data stream.

  10. Scalability in MPEG-2 (2) • Temporal scalability: The base layer contains only very few frames per second, the enhancement layers additional frames per second. Receivers decoding the higher layers will thus get a higher frame rate (i.e., a higher temporal resolution). • Data partitioning: The data stream is decomposed into several streams with different amounts of redundancy for error correction. The most important parts of the stream are encoded in the base layer, e.g., the low-frequency coefficients of the DCT and the motion vectors. This layer can then be enriched with an error correcting code for better error resilience than the enhancement layers where errors are not as harmful.

  11. MPEG-2 Video Profiles

  12. MPEG-4/ASP (1) • Originally, ISO and ITU-T had planned a standard MPEG-3 for HDTV at very high bit rates. This work was later integrated into MPEG-2. This explains why there is no MPEG-3 standard. • MPEG-4 is also known as MPEG-4 part 2 or MPEG-4/ASP (Advanced Simple Profile). It was originally planned for video at very low bit rates (e.g., for wireless PDAs). Later the ISO committee decided to concentrate on an entirely new technology, namely encoding in the form of sets of objects overlaid to form an image. The encoding technique can be chosen separately for each object. This object-oriented encoding also opens up much richer possibilities for processing on the receiver side.

  13. MPEG-4/ASP (2) • History of MPEG-4 • Competitive tests for video functionalities in November 1995 • Development of the standards from 1996 till 1998 • Stable final committee draft for the video part in March 1998 • The standard is fixed at the end of 1999. • Features of MPEG-4 • A scene is constructed of multiple independent objects. • Objects are merged into a scene on the decoder side. • A combination of different object types and different coding methods is possible. • An improved coding efficiency with bit rates between 5 kbit/s and 50 Mbit/s is provided. • The error-resilient video coding is optimized for mobile and packet networks.

  14. MPEG-4/ASP (3) • Separate encoding of background and foreground. The background is static.

  15. MPEG-4/ASP (4) • Decoding of an MPEG-4 system stream

  16. MPEG-4/ASP (5) • Object hierarchy for our decoding example

  17. MPEG-4/ASP (6) • Scalability by „layered encoding“ in MPEG-4

  18. MPEG-4/ASP (7) • The MPEG-4/ASP standard describes how to encode the objects. The automatic segmentation of the objects (i.e., how to identify single objects) is not specified. • Object segmentation techniques: • Blue screening • Automatic segmentation based on color or motion • Semi-automatic segmentation: A user selects an object, the tracking is then done automatically.

  19. MPEG-4/ASP (8) • Features of MPEG-4/ASP • Similarity to previous standards like MPEG-1 or MPEG-2: • based on a DCT • quantization is similar to MPEG-2 • supports B-frames. • Improvements: • additional coding of objects which can overlay an image • global motion compensation • ¼ pixel precision for motion compensation • better image quality for low-bandwidth encoding. • Critical: • computationally intensive • compression rates are not significantly better (in comparison with MPEG-2).

  20. High Level Main Profile Studio Profile Main Level CPB Low Level Simple Core MPEG-4/ASP (9): Comparison of Resolutions and Bitrates 2160P (Quad HDTV) (3840x2160) 1080P (Full HD)(1920x1080) 720P (HD Ready)(1280x720) Standard TV (720x576) CIF (352x288) QCIF (176x144) resolution bit rate 64K 384K 1.5M 2M 4M 15M 38M 60M 90M 1800M MPEG-1 Constrained Parameter Bitstream (CPB) MPEG-2 MPEG-4 Part 2

  21. 2.3.2 ITU Recommendation H.261 • Also known as „p*64 kbit/s“ • A video coding technique for video data at px64 kbit/s. • Originally developed for ISDN • Parameter p in [1,30] • p small implies low image quality at low data rates. An example is video telephony with p=1 or p=2. • p larger implies better video quality at higher data rates. Typical is p=6 for com-pany video conferencing over six parallel ISDN B-channels. • Intraframe-Coding: based on the DCT. Very similar to JPEG but there is only one quantization factor for all values of the block (no quantization table). • Interframe-Coding: very similar to the P frames in MPEG-1. • There are no B frames in H.261.

  22. Important Parameters of H.261 CIF = Common Intermediate Format Hierarchy of the elements of the data stream: Structure ElementDescription picture a full frame group of blocks 33 macro blocks macro block 16 x 16 Y, 8 x 8 Cb, Cr block 8 x 8 pixels (unit for the DCT)

  23. The H.261 Encoder

  24. Status of H.261 • Very widely used in practice, many products available in the market from many manu-facturers. Has replaced earlier proprietary standards for video telephony. Currently getting replaced by newer versions, such as H.263 and H.264. • Pure software implementations are available as well as stand-alone hardware solu-tions (“black boxes“) and combined solutions, mainly for the PC.

  25. H.263 • H.263 is the successor standard of H.261 at ITU-T, incorporating much of the experience gained with MPEG-1. • Some differences between H.263 and H.261 are: • There are five image sizes instead of two. • There is a bi-directional interpolation where exactly one B frame follows each P frame. • There are negotiable options that allow to tailor the algorithm for specific applications. For example, arithmetic coding can be chosen instead of run-length/Huffman coding in the entropy encoding step.

  26. H.264, MPEG-4/AVC (1) • H.264 is also known as MPEG-4 Part 10, MPEG-4/AVC (Advanced Video Coding), or ISO/IEC 14496-10. • Developed by the Joint Video Team (JVT): joint work by experts of the ITU-T Video Coding Experts Group (VCEG) and the ISO/IEC Moving Picture Experts Group (MPEG) • First version of the standard completed in May 2003. Goals • Good video quality at significantly lower bit rates than previous standards (MPEG-2, H.263, MPEG-4 Part 2/ASP) • The coding complexity should not be much higher in comparison with previous standards. • High flexibility: support of very low/high bit rates, very low/high video resolutions (HDTV), DVD storage, ITU-T multimedia telephony.

  27. H.264, MPEG-4/AVC (2) • Comparison of H.264 with older standards

  28. H.264, MPEG-4/AVC (3) • Comparison of H.264 with older standards (cont.) • The variable block size and the deblocking filter improve the visual quality most.

  29. H.264, MPEG-4/AVC (4) The acceptance of H.264 is very high due to the high coding efficiency. Many app-lications have been developed based on H.264: • HDTV: used for HD DVD, Blu-ray Disc and high-resolution TV (DVB-S2) • Portable video: DVB-H and DMB (Digital Multimedia Broadcasting) • Codecs are available for PCs (e.g., QuickTime V.7), video conferencing systems and camcorders.

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