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DATA COMPRESSION

DATA COMPRESSION. LOSSY COMPRESSION METHODS. 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). What it is…. Why use it?.

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DATA COMPRESSION

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  1. DATA COMPRESSION LOSSY COMPRESSION METHODS

  2. 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) What it is…

  3. Why use it? • Cost effective • Time Efficient

  4. How It’s 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

  5. 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 Σ Σ x P(x,y) m=0 n=0 • If there is no change to any one pixel out of the 64 in the chart, then 0’s are created for the spot.

  6. Quantization • After all that math, there’s more math… • Divide the number of bits needed for compression by a constant • Drop the fraction • # of bits and # of 0’s are now optimized for compression

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

  8. Means spatially compressing each frame while temporally compressing a set of frames Types: I-frames—intracoded frame= an independent frame from a set P-frames—predicted frame= related to the previous frame, without the possibility for changes being recorded B-frames—bidirectional frame= relative to preceding and future frames, never related to another B-frame MPEG’s

  9. Summation

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