Image Compression

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DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF
JOENSUU JOENSUU, FINLAND

• Image Compression
• Lecture 20
• MPEG
• Alexander Kolesnikov

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Video images compression ratios
NTSC TV HDTV Film quality Channel
Bit rate 168 Mb/s 933 Mb/s
2300 Mb/s PC LAN 30 kb/s 5,6001
31,0001 76,0001 Modems 56 kb/s
3,0001 17,0001 41,0001 ISDN
64 - 144 kb/s 1,1661 6,4001
16,0001 T-1, DSL 1.5 Mb/s
1121 6221
1,5001 Ethernet 10 Mb/s
171 931
2301 T-3 42 Mb/s
41 221
541 Fiber optic 200 Mb/s 11
51 111
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Mideo Images and Still Images

• Video images are three-dimensional
  generalization
• of still images, where the third dimension is
  time
• Each frame of a video sequence can be compressed
  by
• any image compression algorithm
• Motion JPEG (M-JPEG) the images are separately
• coded by JPEG

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Correlation

• Lets take an advantage of the temporal
  correlations
• i.e. the fact that subsequent images resemble
  each
• other very much
• Still Images spatial correlation
• Video Images spatial and temporal correlation

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MPEG-1

• The MPEG algorithm relies on two basic techniques
• Block based motion compensation
• DCT based compression

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MPEGs

• MPEG-1 (1992) VideoCD
• MPEG-2 (1994) DVD, digital TV, SVCD
• about 501 compression, typically 3-10 Mbps
• MPEG-3 was abandoned
• MPEG-4 (1999) DivX (starting from Version 5)
• designed specially for low-bandwidth
• MPEG-7 (gt1998)
• searching and indexing of a/v data, using
  Description
• Tools

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MPEG-1 Blocks

• The pictures are divided into 16x16 macroblocks,
  
• each consisting of four 8x8 elementary blocks.
  
• The choice of the prediction method is chosen
  for each
• macroblock separately.

• The intra-coded blocks are quantized differently
  from
• the predicted blocks
• Intra-coded blocks contain information in all
  frequencies
• and are quantized differently from the
  predicted blocks
• The predicted blocks, contain mostly high
  frequencies
• and can be quantized with more coarse
  quantization
• tables.

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MPEG-1 Inter-block Prediction
I
I
P
P

• Bidirectional prediction
• Forward prediction
• Backward prediction
• Intra coding.

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MPEG-1 Predictions schemes
I Intra pictures are coded as still images by
DCT. P Predicted pictures are coded with
reference to a past picture. The difference
between the prediction and the original
picture is then compressed by DCT. B
Bidirectional pictures, the prediction can be
made both to a past and a future frame.
Bidirectional pictures are never used as
reference.
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Motion estimation and compensation

• The prediction block in the reference frame is
  not
• necessarily in the same coordinates than the
  block in
• the current frame.
• Because of motion in the image sequence, the
  most
• suitable predictor for the current block may
  exist
• anywhere in the reference frame.
• The motion estimation specifies where the best
  prediction
• (best match) is found.
• Motion compensation consists of calculating the
  difference
• between the reference and the current block.

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Motion estimation 1

• Exhaustive search block matching

Slow!
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Motion estimation 2

• Hierarchical block matching

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Compression ratios
NTSC TV HDTV Film quality Channel
Bit rate 168 Mb/s 933 Mb/s
2300 Mb/s PC LAN 30 kb/s 5,6001
31,0001 76,0001 Modems 56 kb/s
3,0001 17,0001 41,0001 ISDN
64 - 144 kb/s 1,1661 6,4001
16,0001 T-1, DSL 1.5 Mb/s
1121 6221
1,5001 Ethernet 10 Mb/s
171 931
2301 T-3 42 Mb/s
41 221
541 Fiber optic 200 Mb/s 11
51 111
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Object-based coding
VOP- Video object plane VOL- Video object layer
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Video content analysis

• Bring video sequence into chunks, each with
• consistent content shots
• Group similar shots into scenes
• Describe connections between scenes
• Associate shots/scenes with semantics for
  future query