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  1. Chapter 10Video Multimedia Systems

  2. Key Points • The display of moving pictures depends on persistence of vision. • Uncompressed video requires 26MBytes per sec (NTSC) or 31MBytes per sec (PAL). • Digitization may be performed in the camera (e.g. DV) or using a capture card attached to a computer. • NTSC, PAL and SECAM are analogue video standards. All three use interlaced fields.

  3. Key Points • CCIR 601 is a standard for digital video. It uses Y'CBCR colour with 4:2:2 chrominance sub-sampling. The data rate is 166Mbits per sec. • Video compression can make use of spatial (intra-frame) and temporal (inter-frame) compression. Spatial compression is still-image compression applied to individual frames. Temporal compression is based on frame differences and key frames. • Motion JPEG applies JPEG compression to each frame. It is usually performed in hardware. • Cinepak, Intel Indeo and Sorenson are popular software codecs used in multimedia. They are based on vector quantization.

  4. Key Points • MPEG video is an elaborate codec that combines DCT-based compression of key frames (I-pictures) with forward and backward prediction of intermediate frames (P-pictures and B-pictures) using motion compensation. • QuickTime is a component-based multimedia architecture providing cross-platform support for video, and incorporating many codecs. It has its own file format that is widely used for distributing video in multimedia. • Digital video editing is non-linear (like film editing).

  5. Key Points • Most digital post-production tasks are applications of image manipulation operations to the individual frames of a clip. • For delivery using current technology, it may be necessary to sacrifice frame size, frame rate, colour depth, and image quality. • Streamed video is played as soon as it arrives without being stored on disk, so it allows for live transmission and `video on demand'.

  6. Moving Pictures • All current moving pictures depend on the following phenomena • Persistence of vision • A lag in the eye's response results 'after-images' • Fusion frequency • If a sequence of still images is presented above this frequency, we will experience a continuous visual sensation • Depend on brightness of image relative to viewing environment • Below this frequency will perceived flickering effect

  7. Generate Moving Pictures • Video • Use video camera to capture a sequence of frames • Animation • Generate each frame individually either by computer or by other means

  8. Digital Video • A video sequence consists of a number of frames • Each frame is a single image produced by digitizing time-varying signal generated by video camera

  9. Digital Video • Think about the size of the uncompressed digital video • NTSC video format • Bitmapped images for video frame • 640  480 pixels with 24-bit color = 0.9 MB/frame • 30 frames per second • 900 kb/frame  30 frames/sec = 26 MB/sec • 60 seconds per minute • 26 MB/sec  60 secs/minute = 1,600 MB/minute • Strains on current processing, storage and data transmission !

  10. Create Digital Video • Get analog/digital video signal from • video camera • video tape recorder (VTR) • broadcast signal • Digitize analog video & compress it

  11. Digitizing Analog Video • In computer • Video capture card • Convert analog to digital & compress • Can also decompress & convert digital to analog • Compress through • Video capture card (hardware codec) • Software (software codec)

  12. Digitizing Analog Video • In camera • Digitize and compress using circuitry inside camera • Transfer digitized signal from camera to computer through • IEEE 1394 interface (FireWire): 400 Mb/sec • USB: 12Mb/sec(version 1.1) ~ 480 Mb/sec(version 2.0)

  13. Digitize in Computer v.s. Camera • Digitize in camera • Advantage • Digital signals are resistant to corruption when transmitted down cables and stored on tape • Disadvantage • User has no control between picture quality and data rate (file size)

  14. Analog Video Standard

  15. Display Video on TV Cross-section of CRT Delta-delta shadow-mask CRT (Scan From “Computer Graphics: Principles and Practice”)

  16. Field and Interlace • Transmitting many entire pictures in a second requires a lot of bandwidth • Field • Divide each frame into two fields • One consisting of the odd-numbered lines of each frame, the other of the even lines • Interlace • Each frame is built up by interlacing the fields • PAL • 50 fields/sec => 25 frames/sec • NTSC • 59.94 fields/sec => 29.97 frames/sec

  17. Display Video on Computer • Progressive scanning • Write all lines of each frame to frame buffer • Refresh whole screen from frame buffer at high rate

  18. Display Video TV … … frame i frame i+1 frame i+2 frame i+3 … Computer Monitor

  19. Field & Interlace Artifacts A video clip of flash light on the water surface Combine previous two analog video for progressive display Odd lines of frame i Even lines of frame i+1

  20. Field & Interlace Artifacts

  21. Prevent Interlace Artifacts • Average two field to construct a single frame • Discard half fields and interpolate remained fields to construct a full frame • Convert each field into a single frame (reduce frame rate but much better !)

  22. Types of Analog Video • Component video • Three components: Y (luminance), U and V (color) • Often use in production and post-production • Composite video • Combine three components into a signal • Color component (U and V) is allocated half bandwidth as the luminance (Y) • Often use in transmission • S-video • Separates the luminance from the two color (total two signals)

  23. Parameter Value YUV encoding 4:2:2 Sampling frequency for Y (MHz) 13.5 Sampling frequency for U and V (MHz) 6.75 No of samples per line 720 No of levels for Y component 220 No of levels for U,V components 225 Digital Video Standards • CCIR 601 (Rec. ITU-R BT.601) • specifies the image format, and coding for digital television signals

  24. NTSC System 1 2 3 … 1 2 3 … 480 480 1 2 3 … 640 1 2 3 … 720 CCIR 601 standard Pixels are not square Analog to digital Pixels are square Perplexing

  25. 4 : 2 : 2 sampling (co-site) 4 : 2 : 0 sampling (not co-site) 4 : 1 : 1 sampling (co-site) Y samples CB and CR samples CCIR 601 Sampling

  26. Compression & Data Stream Standards • Sampling produces a digital representation of a video signal • This must be compressed and then formed into a data stream for transmission • Further standards are needed to specify the compression algorithm and the format of the data stream

  27. Compression & Data Stream Standards • DV standard • For semi-professional & news-gathering • MPEG-2 standard • For family use • Organized into different profiles and levels • The most combination is Main Profile at Main Level (MP@ML) • Used for digital television broadcasts & DVD video

  28. Introduction to Video Compression • Adapted to consumers’ hardware, video data needs to be compressed twice • First during capture • Then again when it is prepared for distribution

  29. Video Compression • Digital video compression algorithms operate on a sequence of bit-mapped images • Spatial compression (intra-frame) • Compress each individual image in isolation • Temporal compression (inter-frame) • Store the differences between sub-sequences of frames

  30. Spatial Compression • Compress method is similar to image compression • Lossless • No information loss • Compression ratios is lower • Lossy • Some information loss • Compression ratios is higher • Why recompressing video is unavoidable • The compressor used for capture are not suitable for multimedia delivery • For post-production

  31. Temporal Compression • Key frames • Certain frames in a sequence are designated as key frames • Difference frame • Each of the frames between the key frames is replaced by a difference frame • Records only the differences between the frames

  32. Time Required for Compression & Decompression • Symmetrical • Compression & decompression of a piece of video take the same time • Asymmetrical • Compression & decompression of a piece of video not take the same time • Generally Compression takes longer time

  33. Motion JPEG (MJPEG) • A popular approach to compressing video during capture • Applying JPEG compression to each frame (No temporal compression) • Therefore it is called “Motion JPEG”

  34. DV • Compression based on DCT transform • Perform temporal compression (motion compensation) between two fields of each frame • Quality is varied dynamically to maintain constant data rate

  35. MJPEG v.s DV

  36. Software Codecs for Multimedia • Popular software codecs • MPEG-1 • Cinepak • Intel Indeo • Sorenson

  37. Source Output Reconstruction encoder decoder Table lookup Find closest code-vector Group into vectors Unblock index code book index code book Vector Quantization

  38. MPEG • Stand for Motion Picture Experts Group (Joint of the ISO and the IEC) • Works on standards for the coding of moving pictures and associated audio

  39. MPEG Family • MPEG – 1 • Coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mb/s • MPEG – 2 • Generic coding of moving pictures and associated audio • For broadcasting & studio work • MPEG – 3 • no longer exists (has been merged into MPEG-2) • MPEG – 4 • Very low bit rate audio-visual (integrated multimedia) coding

  40. MPEG Family • MPEG – 7 • Multimedia content description interface • MPEG – 21 • Vision statement • To enable transparent & augmented use of multimedia resources across a wide range of networks and devices • Objectives • To understand how the elements fit together • To identify new standards which are required if gaps in the infrastructure exist • To accomplish the integration of different standards

  41. MPEG–1 Standard • Defines a data stream syntax and a decompressor, allowing manufacturers to develop different compressors • MPEG-1 compression • Temporal compression based on motion compensation • Spatial compression based on quantization & coding of frequency coefficients produced by a DCT of the data

  42. MPEG –1 Objective • Medium quality video (VHS-like) • Bit rate < 1.5 Mb/s • 1.15 Mb/s for video • 350 kb/s for audio & additional data • Asymmetrical application • Store video & audio on CD-ROM • Picture format : SIF (Source Input Format) • 4:2:0 sub-sampled • Frame size @ frequency rate • 352  288 @ 25 HZ • 352  240 @ 30 HZ

  43. An object moving between frames Area of potential change

  44. Motion Compensation • Divide each frame into macroblocks of 16  16 pixels • Predict where the corresponding macroblock in next frame • Try all possible displacements within a limited range • Choose the best match • Construct difference frame by subtracting each macroblock from its predicted counterpart • Keep the motion vectors describing the predicted displacement of macroblocks between frames

  45. Picture Type • I (intra) pictures • Code without reference to other pictures • Low compression rate • P (predicted) pictures • Code using motion compensated prediction from a past I or P picture • Higher compression rate than I picture • B (bidirectional-predicted) pictures • Code bidirectional interpolation between the I or P picture which preceded & followed them • Highest compression rate All are compressed using the MPEG version of JPEG compression

  46. P I I P B B B B B B B B 01 11 02 03 04 05 06 12 13 14 15 16 21 Group of Pictures (GOP) An MPEG sequence in display order P I I I P B B B B B B B B 01 06 04 02 03 11 05 14 12 13 21 15 16 An MPEG sequence in bitstream order (decode order) I

  47. MPEG-1 視頻壓縮技術 • 運動補償 (Motion Compensation) • 頻率變換 (Frequency Transform) • 可變長度編碼 (Variable Length Coding) • 彩色信號 subsampling • 量化 (Quantization) • 預測編碼 • 圖像插值

  48. QuickTime • Apple, 1991 • Time base, non-linear editing • Component-based architecture • Compressor components • Cinepak, Intel Indeo codec • Sequence grabber components • Movie control component • Transcoder • Translate data between different formats • Video digitizer component • Support MPEG-1, DV, OMF, AVI, OpenDML

  49. Digital Video Editing & Post-production • Editing • Compositing • Reverse shot • Conversation between two people

  50. Film & Video Editing • Traditional • In point and out point • Timecode • SMPTE timecode • Hours, minutes, seconds, frames • VHS • Two copying operations is to produce serious loss of quality • Constructed linearly