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DCT

DCT. Somewhere in Cinque Terre, May 2005. cs195g: Computational Photography James Hays, Brown, Spring 2010. Slides from Alexei Efros. Campbell-Robson contrast sensitivity curve. Depends on Color. R. G. B. Lossy Image Compression (JPEG). Block-based Discrete Cosine Transform (DCT).

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DCT

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  1. DCT Somewhere in Cinque Terre, May 2005 cs195g: Computational Photography James Hays, Brown, Spring 2010 Slides from Alexei Efros

  2. Campbell-Robson contrast sensitivity curve

  3. Depends on Color R G B

  4. Lossy Image Compression (JPEG) Block-based Discrete Cosine Transform (DCT)

  5. Using DCT in JPEG • The first coefficient B(0,0) is the DC component, the average intensity • The top-left coeffs represent low frequencies, the bottom right – high frequencies

  6. Image compression using DCT • DCT enables image compression by concentrating most image information in the low frequencies • Loose unimportant image info (high frequencies) by cutting B(u,v) at bottom right • The decoder computes the inverse DCT – IDCT • Quantization Table • 3 5 7 9 11 13 15 17 • 5 7 9 11 13 15 17 19 • 7 9 11 13 15 17 19 21 • 9 11 13 15 17 19 21 23 • 11 13 15 17 19 21 23 25 • 13 15 17 19 21 23 25 27 • 15 17 19 21 23 25 27 29 • 17 19 21 23 25 27 29 31

  7. Block size in JPEG • Block size • small block • faster • correlation exists between neighboring pixels • large block • better compression in smooth regions • It’s 8x8 in standard JPEG

  8. JPEG compression comparison 89k 12k

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