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Video Compression Fundamentals. Video Compression Standards. JPEG: ISO and ITU-T for compression of still image Moving JPEG (MJPEG) H.261: ITU-T SG XV for audiovisual service at p x 64Kbps MPEG-1, 2, 4, 7: ISO IEC/JTC1/SC29/WG11 for compression of combined video and audio.

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video compression standards
Video Compression Standards
  • JPEG: ISO and ITU-T
    • for compression of still image
  • Moving JPEG (MJPEG)
  • H.261: ITU-T SG XV
    • for audiovisual service at p x 64Kbps
  • MPEG-1, 2, 4, 7: ISO IEC/JTC1/SC29/WG11
    • for compression of combined video and audio
  • H.263: ITU-T SG XV
    • for videophone at a bit-rate below 64Kbps
  • JBIG: ISO
    • for compression of bilevel images
  • Non-standardized techniques
    • DVI: de facto standard from Intel for storage compression and real-time decompression
    • QuickTime: Macintosh
compressing digital video
Exploit spatial redundancy within frames (like JPEG: transforming, quantizing, variable length coding)

Exploit temporal redundancy between frames

Only the sun has changed position between these 2 frames

Compressing Digital Video

Current Frame

Previous Frame

simplest temporal coding dpcm

0 1 2 3

Simplest Temporal Coding - DPCM
  • Frame 0 (still image)
  • Difference frame 1 = Frame 1 – Frame 0
  • Difference frame 2 = Frame 2 – Frame 1
  • If no movement in the scene, all difference frames are 0. Can be greatly compressed!
  • If movement, can see it in the difference images
difference frames
Difference Frames
  • Differences between two frames can be caused by
    • Camera motion: the outlines of background or stationary objects can be seen in the Diff Image
    • Object motion: the outlines of moving objects can be seen in the Diff Image
    • Illumination changes (sun rising, headlights, etc.)
    • Scene Cuts: Lots of stuff in the Diff Image
    • Noise
coding mode i inter coding

Macro

Block

Previous

Current

Frame

Frame

Motion Vector

Coding Mode I (Inter-Coding)
  • Inter coding refers to coding with motion vectors
motion estimation and compensation
Macroblock: motion compensation unit

Motion Estimation

extracts the motion information from a video sequence

Motion information

one motion vector for forward-predicted macroblock

two motion vectors for bidirectionally predicted macroblocks

Motion Compensation

reconstructs an image using blocks from the previous image along with motion information, I.e., motion vectors

Motion Estimation and Compensation
coding mode ii intra coding
INTRA coding refers to coding without motion vectors

The MB is coded all by itself, in a manner similar to JPEG

Macro

Block

Previous

Current

Frame

Frame

Coding Mode II (Intra-Coding)
i picture coding
I-Picture Coding
  • Two possible coding modes for macroblocks in I-frames
    • Intra- code the 4 blocks with the current quantization parameters
    • Intra with modified quantization: scale the quantization matrix before coding this MB
  • All macroblocks in intra pictures are coded
  • Quantized DC coefficients are losslessly DPCM coded, then Huffman as in JPEG
i picture coding1

8

8

8

8

I-Picture Coding
  • Use of macroblocks modifies block-scan order:
  • Quantized coefficients are zig-zag scanned and run-length/Huffman coded as in JPEG
  • Very similar to JPEG except (1) scaling Q matrix separately for each MB, and (2) order of blocks
p picture coding many coding modes
P-Picture Coding: many coding modes
  • Motion compensated coding: Motion Vector only
  • Motion compensated coding: MV plus difference macroblock
  • Motion compensation: MV & difference MB with modified quant. scaling
  • MV = (0,0), just send difference block
  • MV=(0,0), just send diff block, with modified quantization scaling
  • Intra: the MB is coded with DCTs (no difference is computed)
  • Intra with modified quantization scaling

Motion Vector

DCT

how to choose a coding mode
How to choose a coding mode?
  • MPEG does not specify how to choose mode
    • Full search = try everything…the different possibilities will lead to different rate/distortion outcomes for that macroblock

distortion

• MV, no difference

• MV, plus difference

• Intra

rate

how to choose a coding mode1
How to choose a coding mode?
  • Tree search: use a decision tree
  • For example:
    • First find the best-match block in the search window. If it’s a very good match, then use motion compensation. Otherwise, don’t.
    • If you decided to use motion compensation, then need to decide whether or not to send the difference block as well. Make decision based on how good a match it is.
    • If you decided to send the difference block, then have to decide whether or not to scale the quantization parameter… check the current rate usage…
motion estimation for different frames
Motion estimation for different frames

X

Z

Available from earlier frame (X)

Available from later frame (Z)

Y

b picture coding
B-Picture Coding
  • B pictures have even more possible modes:
    • Forward prediction MV, no difference block
    • Forward prediction MV, plus difference block
    • Backward prediction MV, no difference block
    • Backward prediction MV, plus difference block
    • Interpolative prediction MV, no difference block
    • Interpolative prediction MV, plus difference block
    • Intra coding
    • Some of above with modified Quant parameter