DATA COMPRESSION

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DATA COMPRESSION

• LOSSY COMPRESSION METHODS

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What it is

• A compression of information that is acceptable
  in pictures or videos, but not texts or programs.
• Methods of Lossy Data Compression are
• Joint Photographic Experts Group (JPEG)
• Motion Picture Experts Group (MPEG)

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Why use it?

• Cost effective
• Time Efficient

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How Its Done!

• Change the picture into a linear set of numbers
  that reveals the redundancies.
• Redundancies can be removed.
• The set of bits is simplified.
• Each block of 64 pixels goes through the discrete
  cosine transform (DCT)

P(x,y)
T(x,y)
Value of each pixel before DCT
Value after DCT
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Operation

• If a block has two different sections, each
  uniform in its respective section, you can
  decrease the amount of operations performed to
  display the image by 4800 times. How?
• By doing the DCT operation
• 7 7
• 0.25 x c(x) x c(y) x S S x P(x,y)
• m0 n0
• If there is no change to any one pixel out of the
  64 in the chart, then 0s are created for the
  spot.

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Quantization

• After all that math, theres more math
• Divide the number of bits needed for compression
  by a constant
• Drop the fraction
• of bits and of 0s are now optimized for
  compression

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Compression

• After quantization, redundant 0s are removed
• Done by zigzag linearization rather than row by
  row, to conceal fine changes.

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MPEGs

• Means spatially compressing each frame while
  temporally compressing a set of frames

• Types
• I-framesintracoded frame an independent frame
  from a set
• P-framespredicted frame related to the previous
  frame, without the possibility for changes being
  recorded
• B-framesbidirectional frame relative to
  preceding and future frames, never related to
  another B-frame

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Summation
All received data need not be an exact replica of the sent data MPEG is used to compress video bits
JPEG is a method to compress pics and graphics Spatial compression is similar to JPEG, compressing extraneous bits
DCT, quantization, and compression are tools for compression Temporal compression removes redundant frames