Desktop Scanning to sRGB

Transcription

Desktop Scanning to sRGB
Presented at
IS&T and SPIE's 12th Annual Symposium on Electronic Imaging,
San Jose, CA, January 23-28, 2000
Conference 3963 on Color Imaging: Device-Independent Color,
Color Hardcopy, and Graphic Arts V
by
Jon Y. Hardeberg
Conexant Systems Inc.
Personal Imaging Division, Imaging Software Team, Redmond, WA
Conexant/PID/Jon Hardeberg/EI 2000
2
Outline
• Introduction
– Why is color a problem?
• Color Architecture
– ICC, L*a*b*, or sRGB?
– How, when, and where to convert
• Scanner Characterization
– Get the right color out of your scanner!
– Define the color conversion to sRGB
3
Introduction
• Scanners ‘speak’ color differently
Scanner A
File A
Scanner B
File B
≠
Original
Image
4
Introduction
• Printers ‘speak’ color differently
Printer A
≠
Digital file
Printer B
5
Introduction
• Imaging systems are open and complex
6
Introduction
• Devices do not speak the same color language
– Device-Dependent Color Spaces
• How to achieve color consistency throughout the
entire system?
– N input devices, M output devices
– N*M different colorspace relationships required
• Need for color management
– Each device dependent color space linked to a
Device-Independent Color Space
– Only N+M colorspace relationships
7
Introduction: Color Management
• Imaging devices characterized by profiles
• Image interchange through Device-Independent
Color Space
Profile
Profile
Profile
Device-Independent
Color Space
Profile
Profile
8
Outline
• Introduction
– ICC, L*a*b*, or sRGB?
9
Color Architecture
• From theory to practice
– Which device-independent color space to use?
– When to perform the color conversions?
• Three models/architectures
– ICC Color Management System architecture
– L*a*b* color space for image interchange
– sRGB color space for image interchange
10
Color Architecture: ICC Solution
Scanner RGB image
Scanner ICC profile defining
the color conversion from
Scanner RGB to XYZ
Color
conversion
engine
Printer ICC profile defining
XYZ to Printer CMY(K)
- Standard Color Management
System (CMS) architecture
- Image communicated in
Scanner’s RGB space
- Color conversion at print time
- Both scanner and printer profiles
must be known at print time
11
Color Architecture: ICC Solution
• Advantages:
– Emerging standard
– No rounding errors using intermediate colorspaces
• Disadvantages:
– Scanner profile must be embedded into the image
• Few image format allows embedded profiles
– CMS must be properly installed and used
• F.ex. Microsoft ICM 2.0, ColorSync 2.0
• Too complex for typical consumers
12
Color Architecture: L*a*b* Solution
L*a*b* image
Color
conversion
engine
Scanner profile defining
Scanner RGB to L*a*b*
Color
conversion
engine
Printer profile defining
L*a*b* to Printer CMY(K)
- Color Fax architecture
- Scan to L*a*b*
- Communicate L*a*b* image
- Print from L*a*b*
13
Color Architecture: L*a*b* Solution
• Advantages:
– Used for Color Fax
– L*a*b* is a “good” device-independent color space
– Large color gamut
• Disadvantages:
– Rounding/quantization errors
– Few PC imaging applications accept L*a*b* images
• Ambiguous encoding standards (Color Fax vs. TIFF)
– Color-space conversion before viewing
14
Color Architecture: sRGB Solution
sRGB image
Color
conversion
engine
Scanner profile defining
Scanner RGB to sRGB
Color
conversion
engine
Printer profile defining
sRGB to Printer CMY(K)
- Scan to sRGB
- Communicate sRGB image
- Print from sRGB
15
• Advantages:
– Simplicity
– All imaging applications accept images
– Images may be displayed without colorspace conversion
– Compatible with ICC Color Management System
– International Standard (IEC TC100)
• Disadvantages:
– Gamut mismatch for printing
– Rounding/quantization errors
16
Color Architecture: Conclusion
• Consider market segment
• Low cost imaging devices
• Reduce complexity
• Use sRGB as default colorspace
17
“The goal of sRGB is to
develop an “80%” solution
that puts a single “stake in
the ground” on a single
recommendation that solves
most of the color communication problems for office,
home and web users.”
– HP’s Michael Stokes et. al,
http://www.srgb.com
18
How to convert to sRGB
• Two computational methods:
– 3rd order polynomial functions
– Interpolation in 3D LUTs
Scanner Color Quality, RGB to L*a*b*
Scanner Color Quality, RGB to sRGB
Printer Color Quality
PC speed
DSP speed
Memory economy
Flexibility
3D LUT
Polynomial
+
+
+
+
++
++
+
+
+
-
19
Where to convert to sRGB
Storage,
Distribution,
etc.
PC
Device
Imaging
Application
Perform colorspace
conversion in the
application?
Perform the
conversion in the
application?
TWAIN
Filter
Intercept the
TWAIN data flow to
convert to sRGB?
Let the system’s CMS
perform conversion from
sRGB to Printer CMY(K)?
Perform conversion
to sRGB in the
“scanner driver”?
Perform conversion from
sRGB to Printer’s CMY(K)
in the printer driver?
TWAIN
Data
Source
Embedded
Image
Processing
Scan
Sensor
Convert from Scanner
RGB to sRGB in the
device?
Develop new sensors
that produce sRGB
directly?
Perform
conversion in
the device?
Imaging
Application
ICM 2.0
Printer
Driver
Embedded
Image
Processing
Printing
Engine
20
TWAIN Filter: Principle
•
•
•
Plug-in concept
– Seamless integration into any
TWAIN-capable application
Transmit most of the TWAIN
commands to the original Data TWAIN
commands
Source
– Appear to the application as a
TWAIN Data Source
Intercept image data
– Perform colorspace conversion to
sRGB
– Perform other image processing
tasks, OCR etc.
Device specific
Imaging
Application
TWAIN Data
Source
Manager
TWAIN
Filter
TWAIN
Data
Source
Image data
(sRGB color space)
Color
Conversion
and Image
Processing
Library
Image data
(Scanner RGB)
commands
Scan
Sensor
21
TWAIN Filter: User interface
22
Outline
• Introduction
– sRGB color space for image interchange
23
Scanner Characterization: Goal
• Desktop Scanning to sRGB!
Scanner RGB
values (device
dependent)
?
Sought sRGB
values (device
independent)
(RsRGB , GsRGB , BsRGB ) = f (Rraw , Graw , Braw )
24
Scanner Characterization: Methods
No characterization
Gamma correction
255x1 LUT
Scanner
RGB
sRGB
Simple method
Linear regression
matrix
(3x3 coefficients)
Scanner
RGB
Estimated
linear
sRGB
Gamma correction
255x1 LUT
sRGB
Complex method
Scanner
RGB
CUBIC
ROOT
3rd order regression
matrix
(3x20 coefficients)
Estimated
L*a*b*
Known
formulae
sRGB
25
Scanner Characterization: Setup
Colorimetric
data
Color Target
Colorimetric
characterization
by
regression
Parameters of
the polynomial
3-D polynomial
transformations
CMS
Scanner Profile
(3D lookup-table)
26
Scanner Characterization: Experiment
• Applied to 4 scanners in this study
– Canon MultiPass C5500 (MFP, CCD)
– Compaq A900 (MFP, CCD)
– Hewlett-Packard OfficeJet R40xi (MFP, CCD)
– Microtek SlimScan C3 (flatbed, CIS)
27
Scanner Characterization: Results
• Reporting mean and max ∆Eab differences
No
Simple
Complex
Canon
Mean
Max
8.9
34.9
5.1
19.9
1.4
7.7
Compaq
Mean
Max
10
36.3
4.8
21.4
1.4
9.2
HP
Mean
19
12.2
2.8
Max
51.7
31.6
14.6
Microtek
Mean
Max
11.4
36.9
7.5
26.8
2.2
10.7
28
• Mean ∆Eab
20
18
16
14
12
10
8
6
Microtek
4
HP
2
Compaq
0
No
Canon
Simple
Complex
29
• Max ∆Eab
60
50
40
30
20
Microtek
10
HP
Compaq
0
No
Canon
Simple
Complex
30
31
Summary: Desktop Scanning to sRGB
• Use sRGB color space for image interchange
– Convert to sRGB at scan-time
– TWAIN Filter technology
– Unambiguous color
• Proposed scanner characterization method
– Accurate and consistent colors
32
Questions?
• More info:
– jon.hardeberg@conexant.com
– http://www.conexant.com/
– http://www.deviceguys.com/jonh/
33

Desktop Scanning to sRGB

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