DOC

sRGB Color Management Case Studies

By Heather Garcia,2014-06-17 20:48
17 views 0
sRGB Color Management Case Studies

Windows Platform Design Notes

    Designing Hardware for the Microsoft Windows Family of Operating Systems

    sRGB Color Management Case Studies Michael Stokes

    Color Architect, Microsoft

    Draft Version 0.9 March 22, 2001

    Abstract This paper presents background information and case studies concerning the use of sRGB color management in workflows for digital photography and archiving. This paper also discusses alternative color space proposals and describes how sRGB provides superior solutions for a variety of applications, including professional digital photography and image archiving. To further illustrate this success, case studies are provided to illustrate the efficacy of sRGB as a robust, solid, simple, and easy-to-use standard color space.

    Finally, this paper includes answers to frequently asked questions, comparing the color management capability of Microsoft? Windows? operating systems with capabilities

    supported in Apple Macintosh systems.

    Contents Introduction ............................................................................................................................................ 2

    Some Background Concepts ............................................................................................................................2

    History of sRGB (IEC 69166-2-1) .....................................................................................................................5 Misconceptions about sRGB ............................................................................................................................6

    Case Studies for sRGB Color Management ........................................................................................... 7

    National Geographic Society ............................................................................................................................7 U.S. Library of Congress...................................................................................................................................8

    Phographic Film Study ......................................................................................................................................8

    Windows vs. Apple Color Management .................................................................................................. 9 References ........................................................................................................................................... 10 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 MAKES NO WARRANTIES, EXPRESS OR IMPLIED, IN THIS DOCUMENT. Microsoft Corporation may have patents or pending patent applications, trademarks, copyrights, or other intellectual property rights covering subject matter in this document. The furnishing of this document does not give you any license to the patents, trademarks, copyrights, or other intellectual property rights except as expressly provided in any written license agreement from Microsoft Corporation.

    Microsoft, Win32, Windows, and Windows NT are trademarks or registered trademarks of Microsoft Corporation in the United States and/or other countries. Other product and company names mentioned herein may be the trademarks of their respective owners. ? 2001 Microsoft Corporation. All rights reserved.

     sRGB Color Management Case Studies 2

    Introduction

    Microsoft and Hewlett-Packard initiated a collaborative effort several years ago to develop and standardize a common RGB color space. This effort involved experts from around the world and across several industries, and it resulted in the sRGB color space being unanimously approved and published as the formal international standard IEC 61966-2-1.This paper provides some background and describes how the sRGB color space addresses many problems in the industry today.

    In the past few years, misplaced criticisms have resulted in a narrow market desire for alternative RGB color space solutions. From among the over 20 alternatives proposed in recent yearsmany to address the supposed limitations of sRGBthis paper examines

    two of these spaces, ROMM RGB and Adobe 1998 RGB, and compares them with sRGB.

    Finally, some case studies are presented that show how sRGB does indeed meet the needs of professional photographers, archivists, and printers.

    For a comprehensive background review of color science and color management issues, see "Color Management Concepts" at http://www.microsoft.com/hwdev/color/.

    Some Background Concepts

    The two fundamental problems in managing digital reproduction of color are: ; The human visual system does not “see” color in the same manner devices produce

    or capture color.

    ; Different device technologies such as digital cameras, CRTs, LCDs, ink-jet

    printers, laser printers, and presses do not “understand” or process color in a

    common manner.

    Color science is the scientific discipline of understanding how the human visual system perceives color and light as well as understanding how devices capture and produce color. Some estimate that 40 percent of the human brain is dedicated to visual processing, so it is not surprising that color science is only beginning to unravel the mysteries of how we perceive color.

    Color management is an engineering discipline that combines color science, computer science, and specific device technologies, such as toner physics for laser printers. Color engineers attempt to implement applications of color science for specific devices within the limitations of the devices' capabilities. The color engineer works to transform the color information received by a specific device (or application) into the native or optimized color space of the device (or application), and then transform those colors in the output, either to media or a digital workflow.

    Exchanging color information between devices requires that color metadata be

    associated with the color content being exchanged. This color metadata describes the color space of the color content. This color space usually provides enough information to estimate what the digital color would be perceived as by the human visual system, though sometimes simple proprietary device color spaces are used (such as generic CMYK) that only describe the percentage of ink, with little tolerance on what color those inks are. The sRGB color space provides the link between the standard digital color counts (for an sRGB device) and how the human visual system perceives these colors. ? 2001, Microsoft Corporation. All rights reserved.

     sRGB Color Management Case Studies 3

    Two basic methods are used to associate color metadata with color content: tightly bound metadata or loosely bound metadata. In either case, exchanging color information always requires two sets of metadata: the source metadata and the destination metadata. For example, whenever an image file is opened, the file format has source color metadata; and the output display has destination metadata. It is important to realize that even when we take no action, we are implicitly implementing a color appearance model and workflow, albeit a flawed one.

    Tightly Bound Metadata

    Tightly bound metadata embeds the color metadata directly into the media content structure or file itself. One example of this is International Color Consortium (ICC) profiles. These profiles provide a description of how device colors relate to the human visual system, although this is an indirect relationship. The ICC profile actually relates the device color information to the profile-tool developer’s human visual system color

    appearance model, and then this proprietary model is related to the ICC’s profile

    connection space (PCS), which is an ideal reflection print under standard graphics arts viewing conditions.

    However, there is no standard accepted color appearance model. This indirection and PCS cause two major difficulties:

    ; Because profile tool developers do not publish their human visual system color

    appearance model assumptions and there is no accepted standard model, it is

    impossible for color engineers to accurately and reliably estimate the original color

    appearance.

    ; Because the ICC has standardized on a high-end graphics arts, paper-centric

    exchange space, this requires all ICC workflows to transform into and out of this

    space even eCommerce and other common workflows that contain little or no

    hardcopy component.

    Despite these major flaws and problems, ICC workflows are useful in professional markets where the workflow is fairly controlled and homogeneous, and where expert resources are available to work though the limitations.

    One issue well addressed by ICC workflows is quality control of semi-variable devices. By using ICC profiles as calibration profiles rather than characterization profiles, day-to-day and machine-to-machine quality drifts can be accounted for reasonably. One factor tempting vendors to implement ICC workflows is the misconception that this allows destination devices or users to receive raw data. In reality, few devices output truly raw information; whatever pseudo-raw information is available requires the destination consumer to have an expert knowledge of the destination device or application and of the source device color space, together with the color engineering expertise to transform from one to the other.

    For repurposing and multi-device workflows, a fundamental flaw of tightly bound data is that if any link in the chain does not properly process and persist the embedded metadata, the entire chain is no longer valid. This places significant burden on all media applications and devices.

    Loosely Bound Metadata

    Loosely bound metadata means that the metadata does not necessary exist in the same file format or data structure with the content. For example, the EXIF file format ? 2001, Microsoft Corporation. All rights reserved.

     sRGB Color Management Case Studies 4

    specification defines all of its content as sRGB and provides a reference to the IEC standard. This allows digital camera vendors to avoid packaging details of the sRGB color space with the image content, while applications or devices receiving EXIF content implicitly know that the sRGB metadata is associated with the content because of the file format specification.

    Windows performance and bandwidth advantages are achieved using implicit sRGB color metadata, because only the content is transmitted and no additional transforms are required. This method has advantages for both hardware and software vendors, including well-defined information, performance, bandwidth, and robustness. The sRGB color space is an open, published international standard (available from http://www.iec.ch). Note, also, that the color-appearance transforms used to create the sRGB ICC profile are published and openly available (http://www.srgb.com).

    sRGB Workflow and Color Space

    The sRGB workflow also has advantages for hardware vendors. For example, vendors can implement conversions to and from sRGB within their devices without having to expose proprietary information about device limitations or technology. (With ICC profiles, competitors can examine a profile and make intelligent conclusions about the device's behavior.)

    A generic RGB color space is typically defined by three primary limitations: ; The white point describes the visual white of the color space relative to a black-body

    radiator in Kelvin, that is, 6500 Kelvin. Sometimes it is related to the correlated color

    temperature of daylight to a black-body radiator D65 which indicates the

    spectral composition of daylight (sunlight and skylight) most closely correlated to

    6500 Kelvin.

    ; The primaries provide the hue and saturation of the red, green, and blue phosphors

    or colorants, and sometimes the brightness level.

    ; The gamma describes the tone reproduction with respect to luminance of the neutral

    axis.

    Other lesser factors involved in properly defining a color space include viewing conditions, black point, standard observer, and so on. This paper concentrates on just white point, primaries, and gamma.

    Note also that color is a three-dimensional concept and therefore cannot be accurately described or presented in two dimensions. CRTs are shaped like apples with the bulk of the gamut at the top, while printers are shaped like pears with the bulk of the gamut at the bottom. Gamut mapping is the concept of warping one color gamut to fit another, essentially fitting a round peg into a square hole. Some discussions of alternate solutions use two-dimensional illustrations that cover up limitations that would be obvious in three dimensions.

    ? 2001, Microsoft Corporation. All rights reserved.

     sRGB Color Management Case Studies 5

History of sRGB (IEC 69166-2-1)

    HP and Microsoft recognized the need for a common internal color space and also recognized the need to create a common exchange space. This history and motivation for this cooperative work is described in the paper "Standard Default Color Space for the Internet - sRGB." [Stokes, Anderson, Chandrasekar, Motta]

    Several principles that guided that development also provide good guidelines for making design trade-offs:

    ; Respect legacy content and practices.

    ; Ensure compatibility with professional, well-calibrated CRTs and HDTVs. ; Ensure robustness.

    ; Be consistent with the human visual system.

    Most legacy content in RGB relates back to NTSC and PAL television, which is, D65 white point, NTSC primaries, and 2.2 gamma. This includes content ranging from computer graphics to digital photography:

    ; Sony provided studies that proved optimally calibrated CRTs in their native state are

    2.2 gamma and have HDTV primaries. [Katoh and Deguchi, "Reconsideration of CRT Monitor

    Characteristics"]

    ; Charles Poynton showed in HDTV development that the HDTV phosphors were a

    reasonable compromise between CRT manufacturers and within the manufacturer

    tolerances. [Poynton, A Technical Introduction to Digital Video]

    Barco, Sony, and other high-end CRT manufacturers confirmed these results. ; The VRML Color Fidelity Committee confirmed that the sRGB color space provided

    the most robustness with respect to changes in viewing conditions. The Committee

    performed a series of experiments with 3D content created at various gammas

    ranging from 1.0 to 2.5 and then showed this content on displays calibrated to

    various gammas, including 1.0, 1.7, 1.8, and 2.2. Then the experiments varied the

    lighting conditions of the room, from completely dark to simulating a bright office. ? 2001, Microsoft Corporation. All rights reserved.

     sRGB Color Management Case Studies 6

    Members agreed that sRGB was by far the most robust color space with respect to

    being able to retain quality across different display calibrations and viewing

    conditions.

    ; Hewlett-Packard provided studies and computations to prove that gamma 2.2 is very

    close to a perceptually uniform lightness scale for display viewing conditions. The

    mathematics illustrating this can be found on the Hewlett-Packard sRGB web site at

    http://www.srgb.com.

    ; CIE and color researchers confirmed that D65 provided the most robust white point

    for images across viewing conditions. Bruce Fraser and Stephen Johnson provided

    practical confirmation that D65 provided a superior softcopy viewing white point,

    where the print is illuminated with D50 standard graphics arts lighting.

    Taken together, these factors led to the conclusion that sRGB should be defined by a D65 white point, HDTV primaries, and a gamma of 2.2. Any other choices would lead to obvious limitations for both consumer and high-end professional workflows. The gamut and dynamic range limitations are being currently addressed in maintenance of the standard to provide a 10-bit per channel definition with an extended dynamic range and gamut. The need for complementary 1.0 gamma physical sRGB space is also being standardized within IEC currently for use with computer graphics operation. Misconceptions about sRGB

    Bruce Fraser in a MacWEEK article essentially labeled sRGB a cheap PC display space, saying that "it purports to represent the 'average' PC monitor, but … seems to represent a typical $300 15-inch VGA monitor."

    This is wrong in that sRGB is consistent with the best professional displays by Barco and Sony as well as with the HDTV standards.

    Other similar published articles have tried to argue that sRGB competes with rather than complements ICC. This is proven erroneous by implementations such as ICM2 and Apple ColorSync that concurrently support both methods.

    Others have attempted to claim sRGB is limited with respect to converting to SWOP CMYK. However, these arguments have instead illustrated the over-simplification in some proprietary gamut-mapping algorithms.

    Most such arguments have come from representatives of graphic arts businesses with products and business models that target the high-end prepress graphics arts industry and high-end digital photography market.

    Over 20 different RGB spaces have been defined including: Adobe 1998, Aldus, Apple, Bruce, Big, ColorMatch, Corbis, Ekta, Joe, ROMM, RIMM, Cine, EBU, NTSC, CIE, HDTV, P22, sRGB, e-sRGB, sRGB64, XsRGB, sRGBX, and scRGB. Two of the more popular and representative are Kodak’s ROMM RGB and Adobe’s 1998 RGB color spaces.

    ROMM RGB

    Kodak has introduced a new color space nearly every two years over the last 12 years, including PhotoYCC, BIG RGB, Professional RGB, CineRGB, and ROMM RGB. ROMM RGB is the latest of the RGB spaces introduced by Kodak. Although it is claimed to be intended for professional photographers, the choice of D50, 1.8 gamma clearly limits its use to the Apple Macintosh-based workflows for high-end presses. Within these ? 2001, Microsoft Corporation. All rights reserved.

     sRGB Color Management Case Studies 7

    limitations, it is a reasonable choice, but this space does not cover most common film and ink-jet gamuts.

    ROMM RGB should not be used to exchange images in Windows-based workflows or workflows for web output.

    Adobe 1998 RGB

    With the release of Photoshop 5.0, Adobe introduced a collection of new color spaces. Unfortunately, several typos occurred in the definitions: what was planned to represent SMPTE-240M was a typo with the NTSC green primary. Ironically, this matched well with the proprietary gamut mapping Adobe uses in Photoshop and provided the appearance of a viable wide-gamut RGB space. The direct result of this typo was the inadvertent creation of the Adobe 1998 RGB color space.

    Wide-gamut RGB spaces or working spaces offer the allure of avoiding loss of information. However, because of ambiguity in round-tripping the space, the real process loses more information than occurs with sRGB processes. Also, the Adobe 1998 RGB color space does not cover most film and ink-jet gamuts.

    The assumption for both ROMM RGB and Adobe 1998 RGB color spaces is that there is no downside to the approach if the space can be made large enough without incurring quantization noise. However, this approach only works well if one vendor controls the workflow. This is because when an image is captured in a digital camera and converted into ROMM RGB, Adobe 1998 RGB, or any wide-gamut space, the gamuts are different so choices must be made about how much of the gamut to fill and how to fill it. When the next user in the workflow wants to transform out of this wide-gamut space for

    example, to print that user must make choices about how much of the wide gamut to transform into the device gamut and where to compromise in the gamut mapping. If the source vendor simply sets its color content within the wide gamut, there is no way to communicate this to the next person in the workflow. The print vendor could then mistakenly compress the entire wide gamut into the printer gamut or take some arbitrary subset and clip values.

    To work around this architectural flaw, Kodak and Adobe as members of the ICC are developing proposals that will require all ICC profiles involved in the workflow to be attached to the media content. This forces the destination vendor to understand the content's color space and also to understand and properly interpret the entire processing history of the color content. Although feasible in the abstract, this formula presents considerable complexity and difficulty.

    Case Studies for sRGB Color Management

    The following case studies help to illustrate that sRGB provides the best combination of factors for almost any workflow.

    National Geographic Society

    John R. Stokes and Stokes Imaging Servers designed the original National Geographic Society color management system used to digitize the image library for the Society over ten years ago. The author of this paper participated in this design, which followed the common practices at the time and ended up with something almost identical to sRGB. The National Geographic Society experience includes printing and recording film, while providing very high-quality digital images to satisfy many of the best photographers in ? 2001, Microsoft Corporation. All rights reserved.

     sRGB Color Management Case Studies 8

    the world. The originals range from Admiral Perry’s stereoscope images from the North Pole to Japan silk prints from the first U.S. trade with Japan in the 1800s, as well as modern photography.

    The National Geographic Society is pleased with the high color fidelity and reproduction

    quality of the work performed by John R. Stokes and Stokes Imaging Services," said Neil

    Philip, Manager of Picture Editing and Research for the Society's Image Collection. "The

    group of approximately 480,000 images digitized by Stokes continues to be the backbone of

    our search and retrieval system, the vital business link for providing photographs in a timely

    manner to our clients worldwide.

    U.S. Library of Congress

    The U.S. Library of Congress developed a system similar to that used by National Geographic Society, with similar results.

    For background information about the Library of Congress archive project methods and standards, see "Building Digital Collections: Technical Information and Background Papers" on the Library of Congress web site. To quote from this paper:

    The sRGB color space proposed by Hewlett-Packard and Microsoft (or an extended version

    allowing the accommodation of an unlimited gamut and out-of-gamut colors) is a viable color

    space choice for access files. It is sufficiently large to accommodate most photographic

    reproduction intents. Since the first future access to any file will most probably be some kind

    of a monitor using an RGB color space, choosing to keep the access data in the currently

    defined sRGB is a valid solution. Images in sRGB will display reasonably well even on

    uncalibrated monitors. Higher bit-depth per channel would make it possible to communicate

    the predefined rendering intent for each image while leaving enough bit-depth for users to

    modify the image and to map to the intended output device. It would also give a safety factor

    to the archive file if future high-quality output devices require extensive mapping to as yet

    unknown color gamut and gamma. Also, colors that currently fall out of gamut could still be

    accounted for by leaving enough room on both ends of the values scale when defining black

    and white values.

    Phographic Film Study

    A private study by a major photographic vender was performed that determined sRGB was more robust with respect to under- and over-exposed source images than alternatives.

    This study captured a series of outdoor images onto photographic negative film. Each scene was under- and over-exposed up to two stops of exposure in half-stop increments. In addition, a teleradiometer was used to directly capture quantitative absolute tone information for each outdoor scene. These negatives were digitized into sRGB and other color spaces where the primary difference was gamma. These digitized images were then displayed on calibrated displays targeted for each color space. The psychophysical evaluation (statistically valid results based on human observers) proved that:

    ; For displays calibrated to sRGB, the ideally exposed sRGB images looked best. ; For displays calibrated to an alternative color space, the ideally exposed alternative

    color space images looked best.

    ; For displays calibrated to sRGB, the under- and over-exposed sRGB images looked

    best of the under- and over-exposed images.

    ; For displays calibrated to an alternative color space, the under- and over-exposed

    sRGB images looked best of the under- and over-exposed images.

    ? 2001, Microsoft Corporation. All rights reserved.

     sRGB Color Management Case Studies 9

    Windows vs. Apple Color Management

    This section provides answers to some frequently asked questions, comparing the color

    management support in Microsoft Windows and Apple Macintosh.

    How does Microsoft ICM2 compare to color management from Apple?

    Microsoft ICM2 and Apple ColorSync3 are both a set of operating system-level application programming interfaces (APIs) used to manage ICC workflows and applications. About 80 percent of these APIs support nearly identical functionality, with the other 20 percent dealing with operating system-specific issues. ICM was first delivered in the Windows 95 operating system. Microsoft subsequently made major enhancements delivered in ICM2 in Windows 98 and later versions of the Windows family of operating systems. These included significant improvements in performance, quality, flexibility, and usability. ColorSync3 offers visual display calibration; however, this functionality is supported on both Windows and Macintosh platforms by third parties, including Adobe, Praxisoft, and others. In addition to ICC-based color management APIs, Microsoft led the efforts to define sRGB in IEC 61966-2-1, and it has implemented sRGB support in Windows operating systems. When a device or image indicates it uses sRGB, Windows does not need to invoke ICM, leading to improved performance in image-related work. Apple invokes ColorSync3 to interpret sRGB data.

    Microsoft’s color management work is based upon open, published international standards, whereas Apple’s work is mostly proprietary, which fosters

    incompatibilities among application and vendor implementations.

    Currently, there is one major difference between the two related systems: ; Apple ColorSync3 architecturally assumes an application-centric view of the

    color-managed work, requiring a single device per device type and significant

    user intervention. This means the target user scenario is a single graphics artist

    working with a single application, such as PhotoShop, and using a single

    capture device, a single display, and a single printer. The user who wants to

    change any of this must manually intervene. This makes cross-group work

    difficult and makes printing from a single source to multiple different color-

    managed printers difficult.

    ; Microsoft ICM2 uses the concept of associated profiles for each individual

    device, instead of forcing the user to manually intervene each time. This allows

    users to easily develop complex, heterogeneous color workflows across a group

    of users and devices that may not be situated in the same geographic area. In its "Designed for Windows" logo program for hardware, Microsoft requires all hardware vendors either to associate an ICC profile with each device or to provide sRGB-compliant devices. To ensure that these profiles are reliable, Microsoft is leading industry efforts to develop color tolerances for printers, displays, and capture devices based on open international standards. For information, see the web site at http://www.microsoft.com/hwdev/color/.

    ? 2001, Microsoft Corporation. All rights reserved.

     sRGB Color Management Case Studies 10

    Isn't Macintosh the industry standard for color management?

    Although some claim the Macintosh to be the leader, actually Adobe PhotoShop and

    Quark Xpress are the leaders, and the operating systems have little impact on these

    high-end applications. PhotoShop5 primarily did not use the operating system color

    management APIs, but instead used Adobe proprietary solutions. With PhotoShop6,

    Adobe allows the use of operating-system color management APIs, but not as the

    default.

    For color spaces, sRGB is the industry standard RGB space. It is the default color

    space for PhotoShop, Windows, PDF, JavaSoft, HTML, SMIL, CSS, PNG, EXIF

    (most digital cameras), Hewlett-Packard color devices, and professional displays

    from Sony and Barco.

    Is ICM2 equal to Macintosh color quality, management, and printing?

    For a business that uses Adobe Photoshop or Quark XPress, its designers will see

    no limitations with respect to color management or quality when switching to

    Windows from the Macintosh. ICM2 offers more flexibility, features, and ease of use

    than does ColorSync3. The level of quality is dependent upon the actual profiles

    provided by device manufacturers, but only Microsoft is working with the industry to

    provide quality tolerances based upon international standards.

    Doesn't the Macintosh have better color capabilities than Windows?

    No. Although Apple offers visual calibration, this capability is available on Windows

    with Adobe Gamma and other utility vendors.

    Is there a feature set comparison between ICM and the Macintosh?

    In general:

    ; ColorSync3 has a built-in visual calibration tool, while Microsoft relies upon third

    parties to supply this (these are available from Adobe and others).

    ; ColorSync3 is designed around the assumption of a single application, while

    ICM2 provides robust support for working across multiple applications, groups,

    locations, and devices.

    ; Microsoft is leading efforts to develop and promote standard methods to ensure

    the color quality of ICC profiles and color devices from vendors.

    ; ICM2 continues to offer more flexibility and features than does ColorSync3.

    Examples include: the ability to associate profiles across applications directly

    with devices that are profiled, the ability to specify media specific profiles with a

    single device at the same time, and support for the standard IEC sRGB ICC

    profile.

    Are there improvements in Windows 98, Windows 2000, and Windows XP?

    Microsoft made major advances in developing ICM2 for Windows 98. Since the

    release of Windows 98, Microsoft has concentrated on improving reliability, quality,

    and performance. Additions in the future will depend upon user requests. Currently,

    one future goal is to achieve seamless integration of multiple media including

    video, drawing, text, 3D graphics, animation, and so on without performance or

    quality degradation.

    References

    Fraser, Bruce, "Out of Gamut: Stop the sRGB bandwagon," MacWEEK;

    http://macweek.zdnet.com/1229/op_fraser.html

    Frey, F. S., & Reilly, J. M. Digital imaging for photographic collections: foundations for technical standards. (1998) Image Permanence Institute, Rochester Institute of

    ? 2001, Microsoft Corporation. All rights reserved.

Report this document

For any questions or suggestions please email
cust-service@docsford.com