Color FidelityColor Fidelityinin

MultimediaMultimedia

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 design filters for instruments & cameras image reproduction image correction

Multimedia Aspects

Rendering accurate color on various soft displays

Rendering hardcopy of softcopy Alternatives to hardcopy - journals Watermarking – calibration,

validation, breaking(?)

L

r

S

c = ST L r

Color Science Basics

Equation for eye c = STLr

Where S is the sensitivity of the eyeL is diagonal illuminant matrixr is vector of reflectances of object

Color Matching Functions defined by CIE

A is defined as a linear transformation of STristimulus values are defined by t = ATLr

Note: from any non-singular, linear transformation of A, the tristimulus values can be found

350 400 450 500 550 600 650 700 7500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (nm)

Cone Sensitivities

350 400 450 500 550 600 650 700 750-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Wavelength (nm)

CIE RGB Color M atching Functions

350 400 450 500 550 600 650 700 7500

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wavelength (nm)

CIE XYZ Color M atching Functions

CIEXYZCIELab

JND Thresholds

Color Space Uniformity

CIE Lab*Uniform Color Space

)]/()/([200*

)]/()/([500*

16)/(116*

nn

nn

n

ZZfYYfb

YYfXXfa

YYfL

where nnn ZYX ,, define the white point

008856.0,116/16787.7

008856.0, )(

3/1

xx

xxxf

Difference image may not relate toperceived difference

Difference image will relate toperceived difference

gamut

Record Device

Gamut Mapping

Display Device

I x, y,

ˆ I x,y,

Frecord

D Fdisplay 1

DICS

DICS

DDCS(1)

DDCS(2)

RGB L

a*

b*

Color Management

CIEDICS

One-to-one mapping onto CIE space

Device Independent Color Space

Device Dependent Color Space

Gamut of Display

CIE

gamut

DDCS

PrinterDyesub

An ExampleFdisplay

H .Fdisplay 1 Frecord .

Frecord 1

Desktop Scanner

Digital Lena

Printed Lena

Scanned Lena

Corrected Lena

H .

Color Camera/Scanner Modelc M HLrT

where M represents the scanner filter set

H represents optics and sensor functions

Goal:

Estimate tristimulus values from the recorded data

t Bc

Characterized for oneIlluminant.

Data gathered underanother illuminant.

1 2

Digital Camera

Digital Camera

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

c1, .. .,cp P-chan. Scanner

F1

FJ

L1

LJ

De-mosaic Problem

Color Image Communication

• Compression in luminance-chrominance space.

• RGB, CMYK, sRGB, CIEXYZ, CIELab.

Heat, Humidity, Paper Changes, Ink Changes

Digital DataDigital Color Printer

time=0 time=T

Output Control

RGB CIE

Output Device

Fdisplay-1

Output Device Characterization

PDDCS

DICSPDDCS

G1

G2

sRGB Approach

•Map printer DD values to DICS.

•Map DI values into sRGB gamut.

•Transform to sRGB values.

DICSsRGB Gamut

Printer Gamut

RGB Display SpaceCIE Color Space

Gamut

Output Device Gamut

RGB Display SpaceCIE Color Space

Gamut

Gamut Mapping

Gamut

Region B

Point A

Point CCIE 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 CIEXYZCIEXYZ

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?