Color Fidelity in Multimedia H. J. Trussell Dept. of Electrical and Computer Engineering North Carolina State University Raleigh, NC

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

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

Color Science Basics Equation for eye c = S T Lr 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 = A T Lr Note: from any non-singular, linear transformation of A, the tristimulus values can be found

Color Space Uniformity

CIE Lab* Uniform Color Space where define the white point

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

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

Device Independent Color Space

Device Dependent Color Space

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

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

Characterized for one Illuminant. Data gathered under another illuminant.

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

Input Device Design P-chan. Scanner

De-mosaic Problem

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

Output Control

RGB CIE Output Device Output Device Characterization

PDDCS DICS PDDC S G1 G2

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

Output Device Gamut

Gamut Mapping

Gamut Region B Point A Point C CIE Space

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. Appearance Concerns

1 2 Monitor CIEXYZ

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