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Color Fidelity in Multimedia H. J. Trussell Dept. of Electrical and Computer Engineering North...
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Transcript of Color Fidelity in Multimedia H. J. Trussell Dept. of Electrical and Computer Engineering North...
Color FidelityColor Fidelityinin
MultimediaMultimedia
H. J. Trussell
Dept. of Electrical and Computer Engineering
North Carolina State University
Raleigh, NC 27695-7911
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?