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10_CM0340_JPEG - Compression Images(JPEG What is JPEG JPEG...

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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...
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427 JJ II J I Back Close Basic JPEG Compression Pipeline JPEG compression involves the following: Encoding Decoding – Reverse the order for encoding
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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.
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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
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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)
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431 JJ II J I Back Close Quantisation Tables In JPEG, each F[u,v] is divided by a constant q(u,v).
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