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Color Fidelity in Multimedia

Color Fidelity in Multimedia. H. J. Trussell Dept. of Electrical and Computer Engineering North Carolina State University Raleigh, NC 27695-7911 hjt@eos.ncsu.edu. Basic Color Problems. describe color measure color coordinates color matching, profiling, calibration

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Color Fidelity in Multimedia

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  1. Color FidelityinMultimedia H. J. Trussell Dept. of Electrical and Computer Engineering North Carolina State University Raleigh, NC 27695-7911 hjt@eos.ncsu.edu

  2. Basic Color Problems • describe color • measure color coordinates • color matching, profiling, calibration • design filters for instruments & cameras • image reproduction • image correction

  3. Multimedia Aspects • Rendering accurate color on various soft displays • Rendering hardcopy of softcopy • Alternatives to hardcopy - journals • Watermarking – calibration, validation, breaking(?)

  4. Color Science Basics Equation for eye c = STLr Where S is the sensitivity of the eye L is diagonal illuminant matrix r is vector of reflectances of object Color Matching Functions defined by CIE A is defined as a linear transformation of S Tristimulus values are defined by t = ATLr Note: from any non-singular, linear transformation of A, the tristimulus values can be found

  5. Color Space Uniformity

  6. CIE Lab*Uniform Color Space definethe white point where

  7. Difference image may not relate to perceived difference Difference image will relate to perceived difference

  8. Color Management L a* b* RGB Record Device D Display Device Gamut Mapping

  9. Device Independent Color Space

  10. Device Dependent Color Space

  11. An Example Dyesub Printer Digital Lena Desktop Scanner Corrected Lena Printed Lena Scanned Lena

  12. Color Camera/Scanner Model where M represents the scanner filter set H represents optics and sensor functions Goal: Estimate tristimulus values from the recorded data

  13. Characterized for one Illuminant. Data gathered under another illuminant.

  14. To determine the appearance of an image under many different lighting conditions You must record more than 3 channels! Problems: Time to record Space to store

  15. Input Device Design P-chan. Scanner

  16. De-mosaic Problem

  17. Color Image Communication • Compression in luminance-chrominance space. • RGB, CMYK, sRGB, CIEXYZ, CIELab.

  18. Output Control

  19. Output Device Characterization Output Device RGB CIE

  20. PDDCS PDDCS DICS G1 G2

  21. sRGB Approach • Map printer DD values • to DICS. • Map DI values into • sRGB gamut. • Transform to sRGB • values.

  22. Output Device Gamut

  23. Gamut Mapping

  24. Point C Region B CIE Space Gamut Point A

  25. Appearance Concerns If viewing conditions the same, CIE works well to indicate color sample matching. Cost functions must consider color space uniformity. (all CIE spaces are not the same) Pixel to pixel differences in CIELab for pictorial images may not relate to appearance. Need usable color appearance models.

  26. Monitor CIEXYZ CIEXYZ CIEXYZ 1 2

  27. Summary • Color is complicated to get right • There are some really neat math problems in color • Multimedia depends on color for its glitz • Who is willing to pay for accurate color?

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