10_CM0340_JPEG - 426 JJ II J I Back Close Compression...

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Unformatted text preview: 426 JJ II J I Back Close Compression: Images (JPEG) What is JPEG? • JPEG: Joint Photographic Expert Group — an international standard since 1992. • Works with colour and greyscale images • Up to 24 bit colour images ( Unlike GIF ) • Target photographic quality images ( Unlike GIF ) • Suitable for many applications e.g., satellite, medical, general photography... 427 JJ II J I Back Close Basic JPEG Compression Pipeline JPEG compression involves the following: • Encoding • Decoding – Reverse the order for encoding 428 JJ II J I Back Close Major Coding Algorithms in JPEG The Major Steps in JPEG Coding involve: • Colour Space Transform and subsampling (YIQ) • DCT (Discrete Cosine Transformation) • Quantisation • Zigzag Scan • DPCM on DC component • RLE on AC Components • Entropy Coding — Huffman or Arithmetic We have met most of the algorithms already : • JPEG exploits them in the compression pipeline to achieve maximal overall compression. 429 JJ II J I Back Close Quantisation Why do we need to quantise: • To throw out bits from DCT. • Example : (101101) 2 = 45 (6 bits). Truncate to 4 bits: (1011) 2 = 11 . Truncate to 3 bits: (101) 2 = 5 . • Quantisation error is the main source of Lossy Compression. • DCT itself is not Lossy • How we throw away bits in Quantisation Step is Lossy 430 JJ II J I Back Close Quantisation Methods Uniform quantisation • Divide by constant N and round result ( N = 4 or 8 in examples on previous page). • Non powers-of-two gives fine control (e.g., N = 6 loses 2.5 bits) 431 JJ II J I Back Close Quantisation Tables • In JPEG, each F[u,v] is divided by a constant q(u,v). • Table of q(u,v) is called quantisation table ....
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This note was uploaded on 01/26/2012 for the course CM 0340 taught by Professor Davidmarshall during the Fall '09 term at Cardiff University.

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10_CM0340_JPEG - 426 JJ II J I Back Close Compression...

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