Desktop Scanning to sRGB Presented at IS&T and SPIE's 12th - - PDF document

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

Desktop Scanning to sRGB

Jon Y. Hardeberg

Conexant Systems Inc. Personal Imaging Division, Imaging Software Team, Redmond, WA

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

Introduction

• Scanners ‘speak’ color differently

File B File A Scanner A Scanner B Original Image

≠

Introduction

• Printers ‘speak’ color differently

Printer A Printer B Digital file

≠

Introduction

• Imaging systems are open and complex

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

Introduction: Color Management

Profile

Device-Independent Color Space

Profile Profile Profile Profile

• Imaging devices characterized by profiles
• Image interchange through Device-Independent

Color Space

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

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

Color Architecture: ICC Solution

Scanner ICC profile defining the color conversion from Scanner RGB to XYZ Printer ICC profile defining the color conversion from XYZ to Printer CMY(K) Scanner RGB image Color conversion engine

• 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

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

Color Architecture: L*a*b* Solution

Scanner profile defining the color conversion from Scanner RGB to L*a*b* Printer profile defining the color conversion from L*a*b* to Printer CMY(K) L*a*b* image Color conversion engine Color conversion engine

• Color Fax architecture
• Scan to L*a*b*
• Communicate L*a*b* image
• Print from L*a*b*

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

Color Architecture: sRGB Solution

Scanner profile defining the color conversion from Scanner RGB to sRGB Printer profile defining the color conversion from sRGB to Printer CMY(K) sRGB image Color conversion engine Color conversion engine

• Scan to sRGB
• Communicate sRGB image
• Print from sRGB

Color Architecture: sRGB Solution

• 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

Color Architecture: Conclusion

• Consider market segment
• Low cost imaging devices
• Reduce complexity
• Use sRGB as default colorspace

Color Architecture: sRGB Solution

“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 communi- cation problems for office, home and web users.”

– HP’s Michael Stokes et. al, http://www.srgb.com

How to convert to sRGB

• Two computational methods:

– 3rd order polynomial functions – Interpolation in 3D LUTs

3D LUT Polynomial Scanner Color Quality, RGB to L*a*b*

+ ++

Scanner Color Quality, RGB to sRGB

+ +

Printer Color Quality

+

• PC speed

+

• DSP speed
• +

Memory economy

• +

Flexibility

++

• Conexant/PID/Jon Hardeberg/EI 2000

Where to convert to sRGB

Scan Sensor TWAIN Data Source Imaging Application TWAIN Filter Embedded Image Processing

PC Device

Develop new sensors that produce sRGB directly? Convert from Scanner RGB to sRGB in the device? Perform conversion to sRGB in the “scanner driver”? Intercept the TWAIN data flow to convert to sRGB? Perform conversion from sRGB to Printer’s CMY(K) in the printer driver? Let the system’s CMS perform conversion from sRGB to Printer CMY(K)? Perform conversion in the device? Imaging Application Printing Engine ICM 2.0 Printer Driver Embedded Image Processing Perform colorspace conversion in the application? Storage, Distribution, etc. Perform the conversion in the application?

TWAIN Filter: Principle

Scan Sensor TWAIN Data Source Imaging Application TWAIN Data Source Manager TWAIN commands Image data (Scanner RGB)

• Plug-in concept

– Seamless integration into any TWAIN-capable application

Color Conversion and Image Processing Library Image data (sRGB color space)

TWAIN Filter

• Transmit most of the TWAIN

commands to the original Data 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 commands

TWAIN Filter: User interface

Outline

• Introduction

– Why is color a problem?

• Color Architecture

– sRGB color space for image interchange – How, when, and where to convert

• Scanner Characterization

– Get the right color out of your scanner! – Define the color conversion to sRGB

Scanner Characterization: Goal

Scanner RGB values (device dependent) Sought sRGB values (device independent)

?

( ) ( )

raw raw raw sRGB sRGB sRGB

B G R f B G R , , , , =

• Desktop Scanning to sRGB!

Scanner Characterization: Methods

No characterization

Scanner RGB Gamma correction 255x1 LUT sRGB Scanner RGB Linear regression matrix (3x3 coefficients)

Simple method

Gamma correction 255x1 LUT sRGB Estimated linear sRGB CUBIC ROOT 3rd order regression matrix (3x20 coefficients) sRGB Estimated L*a*b*

Complex method

Scanner RGB Known formulae

Scanner Characterization: Setup

Colorimetric data Colorimetric characterization by regression 3-D polynomial transformations

CMS

Color Target

Scanner Profile (3D lookup-table)

• 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)

Scanner Characterization: Experiment

Mean Max Mean Max Mean Max Mean Max No 8.9 34.9 10 36.3 19 51.7 11.4 36.9 Simple 5.1 19.9 4.8 21.4 12.2 31.6 7.5 26.8 Complex 1.4 7.7 1.4 9.2 2.8 14.6 2.2 10.7 Canon Compaq HP Microtek

• Reporting mean and max ∆

∆Eab differences

Scanner Characterization: Results

No Simple Complex Canon Compaq HP Microtek 2 4 6 8 10 12 14 16 18 20

• Mean ∆

∆Eab

Scanner Characterization: Results

No Simple Complex Canon Compaq HP Microtek 10 20 30 40 50 60

• Max ∆

∆Eab

Scanner Characterization: Results

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

Questions?

• More info:

– jon.hardeberg@conexant.com – http://www.conexant.com/ – http://www.deviceguys.com/jonh/