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TTM4142 Networked Multimedia Systems

Video BasicsImage and Video

Lossless Compression

Leif Arne Rønningen

Autumn 2008

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Graphics and Image Data Representations

• Large number of (file) formats, PDF, JPEG, GIF, PNG, BMP

• 8 bit Gray-Level Images

• 24 bit Color Images, RGB

• Color Lookup Table - LUT

• Dithering

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Color in Image and Video

Spectral power distribution of daylight

Wavelength (nm)

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Human Vision

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CIE tristimulus values

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Color Models in VideoYUV – PAL and CCIR 601 digital videoYCrCb – JPEG, MPEG, ITU-R 601-4

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Fundamental Video Concepts

• Component Video, RGB, YUV, YCrCb

• Composite Video, chrominance and luminance signals on single carrier

• S-Video, separated video, one wire for luminance, one wire for composite chrominance

• Analog Video, PAL, interlaced scan, blanking intervals h, v

• Digital Video, YCrCb - chroma subsampling

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Digital Video

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HDTV

H.264, (MPEG-4 part 10)

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Lossless Compression AlgorithmsInformation theoryAlphabet S =

Entropy

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Run-length Coding

The string AABACCCCCCCCCCDEAB can be coded as

!02ABA!10CDEAB

Which is 14 instead of 18 characters

! Must be reserved

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LZW Compression

• With this algorithm, the encoder and decoder build up the same dictionary of recognized strings from a stream dynamically. Each string is given a codeword. The encoder sends short codewords instead of long strings.

• See Li and Drew for details

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Arithmetic Coding, Sequence to be coded: CAEE$

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Arithmetic Coding

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Arithmetic Coding, Decoding

• The coded sequence CAEE$ ends up with the interval [0.33184, 0.3322)

• Find the shortest binary number that is within this interval, 0.01010101 <=> 0.33203125 (procedure 7.2 in book)

• Note that only 8 bits is needed!

• For decoding, go the opposite way of that in the previous slide

• 0.33203125 is in Low-High interval for C

• (0.33203125-0.3)/0.2 = 0.16015624, an A

• and so on

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Comparing some lossless compression schemes