Color Management Concepts

Color Management Concepts — 1

ColorManagement Concepts

Michael Stokes
Microsoft Corporation
Draft v. 0.7, March 5, 2001
Contents

Foreword

Introduction

Color Science: Models and Algorithms

Models and Algorithms

Human Visual System

Viewing Environment Models

Measurement Methods

Defect and Artifact Recognition

Color Management

Color Data

Metadata Structures

Processing Sequences

Functional Structures

Color Imaging Workflows

Color Quality

Importance of Testing Methods

Considerations in Establishing Test Methods

Common Image Characteristics

Color Quality Metrics

A Process for Evaluating Image Quality

Image Quality Limitations of Computer Systems

Practical Suggestions

IEC Color Standards

Approach

Possible Architecture for Color Management in Open Systems

Results of International Standardization in IEC/TC 100

How Color Management Be Realized

Terminology......

About the Author

Endnotes

The information contained in this document represents the current view of Microsoft Corporation on the issues discussed as of the date of publication. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information presented. This document is for informational purposes only. MICROSOFT MAKE NO WARRANTIES, EXPRESS OR IMPLIED, IN THIS DOCUMENT.

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Microsoft thanks the International Electrotechnical Commission (IEC) for permission to use material from the IEC 61966 series of international standards. All extracts are copyright © IEC, Geneva, Switzerland. All rights reserved. Further information on the IEC, its publications and its role is available from IEC takes no responsibility for and will not assume liability for damages resulting from the reader’s misinterpretation of the referenced material referenced due to its placement and context in this publication. The material is reproduced or rewritten with their permission.

Color Management Concepts — 1

Foreword

The field of digital color reproduction is a congruence of several older industries merging together. These industries include broadcast television, motion pictures, slide reproduction, still photography, photofinishing, computer graphics, desktop publishing, paint formulation, presentation graphics, and graphics arts.

Color science has provided a scientific foundation for all of these industries with varying degrees of significance, but each industry has extended this foundation with empirical results specialized to its particular needs. Therefore, each of these industries encompasses a significant body of knowledge with respect to color reproduction issues. Much has been written about the traditional aspects of each field[1],[2],[3],[4],[5],[6],[7]

Researchers in color science4,[8],[9],[10] have continued to advance the scientific foundations over the last several decades, mostly independent of modern computer operating systems and networks. This has raised some issues in transitioning from the traditional methods to open computing environments, and the constraints they imposed.

The digital color reproduction emphasis began in 1980 with the Mavica (SONY) announcement. This led to a shift in many traditional imaging R&D budgets away from analog technologies to a strong investment in digital imaging. For most of these companies, this investment has yet to yield a net profit.

The advent of digital color processing applications in open systems, and in particular the World Wide Web, forced these industries to work within open computing environments and with each other. This has created a new technology field called digital color reproduction.

This new field inherited many of the older analog-based industries’ methods and standards. Furthermore, it combines the contributions of color science researchers with the constraints imposed by software operating systems, networks, applications, and devices that make up today’s digital computing environment.

The tensions among the traditional industries, together with the new digital technology, created an interesting and often conflict-filled new technical environment for digital color reproduction. Most current practitioners trace experiences directly to either one of the color or computer industries, and many claim priority in setting direction and standards in this new field. Many members of traditional industries are threatened by the idea of control of digital color processing by the operating system vendors or other traditional industries.

Amid these tensions, there has been a great reluctance to open up solutions for the betterment of the end users. With a few notable exceptions, this has resulted in the current amalgam of end-user solutions, none of which meet the need for transparent, predictable color reproduction, and most of which are incompatible with each other and with open, non-proprietary solutions.

The current chaos in this new field can be attributed to several mid-level companies who had control in one of the many contributing industries. In this new field, those companies are fighting to survive. To see the effects of the more general analog to digital conversion in the generic media industries, simply try and find a family owned photofinisher store or print shop in your neighborhood. In the 1960s, such individual shops were common in almost every retail center, but this is no longer true.

While there has been incredible consolidation in these industries, a major portion of the consumer work has been quickly migrating to the actual consumers and their personal computers. It is common for home users to take digital pictures with their digital camera and to create their own greeting cards or invitations. These are just two examples of the more general analog to digital media transition that has stripped the traditional analog industries of their most profitable product mix.

Given this business climate, it is not surprising that these traditional analog companies are often the most vocal opponents to open standards for digital color reproduction. This is apparent in attacks made on standards without constructive proposals and attempts to control and divert more general open standards to primarily serve a single analog industry’s needs by making the standard essentially emulate the traditional analog workflow.

Some of this resistance is due to a business strategy based on proprietary solutions instead of open technology. Many traditional industries have thrived on using trade secrets and patents to protect niche markets or to control positions. While intellectual property remains a core component of technological innovation, it is important to have a productive balance with standards to help allow technologies to thrive.

The U.S. Patent Office’s lack of expertise in granting software patents related to color reproduction significantly hampered progress. Today, many companies seem to be as concerned about protecting or violating intellectual property rights as they are about creating technical breakthroughs or more importantly, providing any consumer benefits.

Interestingly, chaos and in-fighting are not new to the color reproduction industry. In fact, Adrian Cornwell-Clyne, in his classic monograph Color Cinematography, summarized a similar set of conditions in 1951 with the following statement,

The public history of “processes” of color cinematography is on the whole discouraging and disconcerting, but the reader may be assured that the private history is hardly credible, and will, if ever it be made known, constitute a singular commentary upon the least rational aspects of our society and its culture.

It can only be hoped that the digital color reproduction field will mature and become as technical and commercially successful as the color motion picture industry has become. The difficult part for the digital color reproduction field has been combining all these divergent fields together with the capabilities of current computer systems and networks in an open architecture. There is some evidence of this maturity occurring with the development of IEC multimedia color measurement and management standards that provide basic standards in a heterogeneous device environment.

I am fortunate to have worked in many of these divergent fields and been in the midst of several efforts to resolve the open architectural issues. These experiences have allowed me to publish many algorithms that in the past have been rarely published or explained, though well known to those skilled in this field.

Open standards for digital color reproduction are inevitable. Both the end users and the major color vendors want results that only standards can provide. I hope this paper will be one more step on the path of making open standards for digital color reproduction a reality.

Michael Stokes
Color Architect
Microsoft Corporation

Introduction

This paper is a practical discussion of digital color reproduction issues, based on the author’s experience in this field.

What’s in This Paper?

This paper combines conceptual explanations and practical guidance with an overview of devices, communication protocols, user intentions, viewing conditions, and color management systems. It also provides extensive references to prior art and current research.

For areas covered adequately elsewhere, this paper includes overviews with references to other sources of information. The goal is to provide clear explanations of the major issues involved in implementing a digital color reproduction system.

Each component in a digital color reproduction system is described in this paper, with explanations of the underlying concepts. This paper also describes how to characterize color capabilities with respect to how the human visual system perceives color. The common protocols to communicate color between devices and across systems are also described.

This paper also addresses the critical component of preserving the users’ intentions throughout the reproduction process, together with a discussion of issues and methods for compensating for each device’s viewing conditions.

Who Is This Paper For?

This paper will help you solve color reproduction problems, whether you are building a color printer, writing a device driver for a digital camera, attempting to calibrate a color monitor, designing an application’s interface with a color management system, or simply trying to understand and improve your system’s color performance.

Many books have been written about various aspects of digital color reproduction, but most have only provided overviews of particular concepts or implementation details for particular software products. None have provided all the relevant information to support creating a complete digital color reproduction system that works in an open environment such as the World Wide Web.

Device Engineers. A color engineer for a device manufacturer is often responsible for providing device profiles for the product along with suggestions for improving product design. This paper provides real-world examples of how to build color-managed devices.

Color Researchers. A color scientist or academic researcher typically understands the relevant science and theories involved in color reproduction but is often astounded at the practical implementations and limitations in real-world products, software, and systems.

This paper provides insight into how conceptual theory is often implemented in practical products and why such decisions are made. This information could lead to the development of more relevant theories and research for real-world products.

Application Programmers. An application programmer is frequently responsible for providing color support for a product with little information on how to do. This paper provides this information.

Graphics and Media Professionals. The graphicsarts prepress separator, television video colorist, and motion picture film engineer frequently find themselves interfacing with a digital world that seems naïve about their field of expertise. This paper provides explanations to establish common ground between the legacies of analog color reproduction systems and the reality of digital color systems today. This includes explanations on when and how to use device profiles and when to edit them, along with comparisons of current software utilities.

Web Designers. Web page authors simply want to use color graphics and images with assurance that their clients will see the same thing they see, without any unpleasant surprises. This paper provides practical help in improving the current situation and insight into what the future will bring.

The World Wide Web Consortium has standardized on using IEC 61966-2-1 (sRGB) as the default color space for HTML, CSS, SMIL and other web standards that are fundamental to web designers. This paper provides insight into sRGB and provides methods web designers can use to verify the color content they design will be delivered with appropriate fidelity to end users. Without this standard in place, it is easy for web designers to capture an image of a piece of clothing and design the product web page on their computer system and display and then have end users perceive a variety of different colors in the final browsed page. This leads to customer returns and complaints about mismatching color where if proper standards were in place could have been avoided. This is potentially unpleasant when presenting the final web design to a client and their system is different from the designers and no standard color management was used to account for this mismatch.

End Users. End users of applications just want color to work without surprises, preferably without intervention. In spite of all the research and resources devoted to digital color reproduction, and the many press releases about new products and standards, most users are confused and almost never achieve the color results they want.

This paper explains why it is difficult to achieve desired results today, what is being done about it, and what users can do to improve their results.

Introduction to Digital Color Reproduction

Digital color reproduction is derived from several traditional industries with conflicting needs, priorities, and solutions. The field of digital color reproduction can be divided into three broad categories:

Color Science: Models and algorithms. The various models and algorithms for color reproduction represent mostly independent pieces of digital color reproduction systems. This includes what is known about modeling the human visual system.
Color Management. Color management takes the models and algorithms of color science and provides the practical engineering necessary to transform these into real world products. Color management consists of device model processing sequences, data and metadata structures, functional structures and workflow designs. Device model processing sequences are the processing sequences take these algorithms and connect them together in appropriate sequences to address particular devices and situations.

The data and metadata structures provide a means for communicating the color information as well as the parameters of each individual model or algorithm in the processing sequences, within the limitations of the overall software environment. The functional structures provide software support within the overall software environment to allow the data and software to communicate and function. The workflow designs provide practicable limitations for both the functional software and color reproduction results.

Color Quality. Finally, color quality provides the metrics and support to judge whether the models, algorithms and color management has produced an acceptable color reproduction.

Issues and limitations in each of these categories have a profound impact on the reasonable solutions in the other categories. It is difficult to attempt to implement solutions in digital color reproduction without a clear understanding of all the issues involved.

To properly understand individual issues in detail, one must first have a broad understanding of all the issues and how they are interrelated.

Because digital color reproduction is a relatively young field, many issues have not been resolved.

Color Science: Models and Algorithms

Models and Algorithms

The various models and algorithms for color reproduction represent mostly independent pieces of digital color reproduction systems and represent the basic aspects of color science.

The models involved in digital color reproduction include:

The human visual system
Color appearance models, to compensate for differing viewing conditions
Gamut mapping methods, to compensate for limited device gamuts
Device mapping and measurement methods, to correlate device space color states with human visual system colors
A set of user intent algorithms—color enhancement and media intents
Channel-generation algorithms—black generation
Continuous-to-discrete algorithms—half-toning, error diffusion, andsoon
Mechanism compensation issues—banding compensation
Ink/media compensation issues—ink limiting

Each model or algorithm should have a clear input and output color space definition, including viewing conditions. This is what these transformations are about—transforming colors from one color space or viewing condition into another. In some instances, the actual dimensions stay the same for input and output, but the meaning of the color values is quite different, as in the case of tone correction algorithms.