Image compression

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Image compression
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Image Compression

• Why?
• Reducing transportation times
• Reducing file size
• A two way event - compression and decompression

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Compression categories

• Compression Image coding
• Still-image compression
• Compression of moving image

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INTERFRAME and INTRAFRAME PROCESSING
Intraframe Processing
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Group discussionDiscuss, which compression and
coding method you know!
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Image compression meters

• Compress ratio
• Original image size
• Compressed image size
• The larger the compression ratio, the smaller the
  result image

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Image compression

• Compression method is not same as the image
  file-interchange format.
• Example TIFF -file format supports several
  compression methods

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Why Can We Compress?

• Spatial redundancy
• Neighboring pixels are not independent but
  correlated
• Temporal redundancy

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Information vs Data
REDUNDANTDATA
INFORMATION
DATA INFORMATION REDUNDANT DATA
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Image compression fundamentals

• Same compression method is not to be used more
  than once.
• But you can use different methods at the same
  time, especially different lossless methods like
  LZW and PKZIP

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Image compression symmetry
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Color image compression

• RGB - apply the same compression scheme to the
  three color component images
• Convert the image from the RGB color space to a
  less redundant space, because RGB components
  carries a lot of same information.
• RGB --gt HSB, when Hue and Saturation components
  are well compressed

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Color imagecompression
HUE
BRIGHTNESS
SATURATION
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Lossless image compression

• Image can be decompressed back to original
• Used when images future purpose of use is not
  known, example space exploration imagery is often
  studied for years following its origination

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Run-Length Coding
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Run-length coding

• Codes the nearby pixels which has same brightness
  values in two values - Run-Length, RLE and
  brightness value
• Error sensitive
• Data explosion
• Data errors

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Huffman or Entropy Coding

• Converting the pixel brightness values in the
  original image to new variable-length codes,
  based on their frequency of occurrence in the
  image

Arrange values in descending frequency of
occurrence
Assign Huffman variable-length codes
Brightness Histogram
Huffman Code Image Data
Raw Image Data
Substitute Huffman codes
Append code list
0,10,0,1100 1111,11011
98,100,103, 87,86,95...
The flow of the Huffman coding operation.
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Huffman coding
character number Huffman code a
(97) 45 1 1 b (98) 23 1 0 01 c
(99) 2 0 0 000 d (100) 1 0 1 0010 CR
(13) 1 1 1 00111 LF (10) 1 0 00110
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Lossless or Lossy Compression

• Lossless compression
• There is no information loss, and the image can
  be reconstructed exactly the same as the original
• Applications Medical imagery, Archiving
• Lossy compression
• Information loss is tolerable
• Many-to-1 mapping in compression eg. quantization
• Applications commercial distribution (DVD,
  Blueray, WWW) and rate constrained environment
  where lossless methods can not provide enough
  compression ratio

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Predictive Coding

• Based on the assumption that pixels brightness
  can be predicted based on the brightness of the
  preceding pixel
• Codes only the brightness value of the pixel next
  to each other
• DPCM (Differential Pulse Code Modulation)

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DPCM (Differential Pulse Code Modulation)
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Block Coding

• Searching for repeated patterns (mostly in rows)
• Pixel patterns are put in Codebook
• Original images pixel pattern is replaced by
  codebook index in compressed image

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Block Coding

• LZW- compression (Lempel-Ziv-Welch)
• Compression ratio 21 - 31
• Starting with a 256 single-pixel long codebook -gt
  adding until it reaches its maximum length
• LZWHuffmann, where most common pixel patterns
  get shortest codes

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TRANSFORM CODING
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• Transform Coding
• A form of lossy block coding, but it does not use
  codebook
• Frequency domain
• Frequency transformation finds the essential data
  in the image and coding is accurate
• 88 pixel blocks
• Discrete Cosine Transform (DCT)

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File formats and compression methods

• Standards crucial, so pictures are transportable
  between different systems
• Compression standards
• CCITT group 3 and 4 (Fax-standard)
• Joint Bi-level Image Expert Group (JBIG)
• Joint Photographic Experts Group (JPEG)
• Motion picture
• CCITT Recommendation H.261 H.264
• Moving Picture Experts Group (MPEG)

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Why Do We Need International Standards?

• International standardization is conducted to
  achieve inter-operability .
• Only syntax and decoder are specified.
• Encoder is not standardized and its optimization
  is left to the manufacturer.
• Standards provide state-of-the-art technology
  that is developed by a group of experts in the
  field.
• Not only solve current problems, but also
  anticipate the future application requirements.

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Compression standards JPEG

• Joint Photographic Experts Group (JPEG)
• One of the most important image data compression
  standards
• Developed for highly detailed gray-scale and
  color images / photographs
• Most commonly used as a lossy image compression
  method, but lossless modes exist as well
• JPEG uses several cascaded compression modes
• Adjustable compression scheme à number of
  retained frequency components can be changed to
  achieve different compression ratios
• DCT gt Remove rare frequency components gt
  DPCM/RLE gt Huffman

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JPEG(Intraframe coding)

• First generation JPEG uses DCTRun length Huffman
  entropy coding.
• Second generation JPEG (JPEG2000) uses wavelet
  transform bit plane coding Arithmetic entropy
  coding.

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Why DCT Not DFT?

• DCT is similar to DFT, but can provide a better
  approximation with fewer coefficients

• The coefficients of DCT are real valued instead
  of complex valued in DFT.

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The 64 (8 X 8) DCT Basis Functions

• Each 8x8 block can be looked at as a weighted
  sum of these basis functions.
• The process of 2D DCT is also the process of
  finding those weights.

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Zig-zag Scan DCT Blocks

• Why? -- To group low frequency coefficients in
  top of vector.
• Maps 8 x 8 to a 1 x 64 vector.

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Original
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JPEG 271
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JPEG2000 271
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Motion compression standards

• Moving Picture Experts Group (MPEG)
• Intended for the mass distribution of motion
  video sequences
• Compression-asymmetric compression techniques
  require more processing time and computing power
  than the decompression ones
• In addition to coding techniques used with JPEG,
  MPEG utilizes interframe coding methods
• MPEG-1 use CD-ROM and Internet
• MPEG-2 use DVD and Digi-TV
• MPEG-4 most advanced technology (Blueray, www)