Color management: Implementation part 1:
Setup, working color space, anatomy of a profile
by Norman Koren

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Thanks to Dennis Wilkins and Jonathan Sachs for excellent suggestions and extensive proofreading.

Carlos E. Mora's Spanish translation:  Gestión del color: Implementación (1ª parte).

Implementing color management

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The colors in the above table are keyed to the simplified image flow diagram, below, as well as the table of color management settings for Picture Window Pro and the detailed image flow diagram in the Workflow summary in Implementation part 2.

By the time you finish this tutorial you'll be intimately acquainted with "the devil in the details," and how it can bite you if you're not careful.

Color management setup

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Picture Window Pro, supports color management with Windows 98, ME, 2000, XP, or later. To enable it, Click File, Color Management... This brings up the dialog box shown on the right. Then set Color Management: to Enabled. The dialog box settings are applied immediately and remain in effect as long as the box is open; you can leave it open as long as you want. When you close the box, you will be asked, Save modified settings? Click Yes to retain the new settings. Click No to revert to the previous settings. You can also load save settings. Merely enabling color management has only one effect. Embedded profiles in image files are recognized. Files without embedded profiles are assumed to be sRGB, and no gamut mappings are performed. To unleash the power of color management, you need to make appropriate dialog box settings, summarized in the table below. Settings that require particular attention are highlighted in red and discussed afterwards. To determine if an image has an embedded ICC profile, right-click on it and select Display Info.

Photoshop color management is more error-prone than Picture Window Pro. Because Photoshop tries to be all things to all people-- in the graphics and printing industries as well as in photography, it has a number of obscure settings that must be set correctly for the monitor image to display correctly.
 

Photoshop 6, 7, CS Click Edit, Color Settings. The box on the right appears. CMYK and Spot settings don't apply to most photographic workflows. Placing the cursor over any of the boxes brings up a description (shown for Adobe RGB (1998)). I recommend starting out with the settings on the right, which you can save and load.   Settings: Can be set to Color Management Off, Custom, or one of several presets, most of which are intended for publishing. Custom turns on color management and allows you to make selections. Turns Color Management on or off. Working Spaces RGB The safest choices are Adobe RGB (1998) and sRGB IEC61966-2.1. Gray Use the settings shown on the right. Color Management Policies, Check boxes Correspond to On Profile Mismatch and Assumed Profile in Picture Window Pro, above. Advanced Mode When checked, the Conversion Options and Advanced Controls sections shown in the box on the right appear. There are also more options for Working Spaces. Use the settings on the right unless you have good reason to change them. Microsoft ICM is also available as an Engine; it probably makes little difference. The two intents of interest to photographers are Perceptual (usually preferred) and Relative Colorimetric.

Differences in image appearance between Photoshop and other programscan be caused by a number of factors; this can be a major source of frustration. Get one setting wrong and Photoshop's color management will bite you; your prints won't match your monitor image. The embedded ICC profile affects the appearance of images in ICC-aware programs. To view it in Photoshop, the Status Bar at the bottom of the window should be displayed. If it isn't, click on Window and check Status Bar. Click on the triangle  in the Status bar and select Document Profile. In Picture Window Pro, right-click on the image and select Display Info. In color-managed workflows it's always good practice to embed ICC profiles in image files. Working Color Space settings are handled differently in Photoshop and Picture Window Pro. In Photoshop, images with no embedded profile are assumed to have the color space specified by Working Spaces: RGB. Changing it immediately changes the appearance of the image. In Picture Window Pro, images with no embedded profile may be converted to the Working Color Space when they are opened, depending on the Assumed File Profile and On Profile Mismatch settings. Images opened with no embedded profile are assumed to have the Windows-default sRGB profile. Changing Working Color Space or Assumed File Profile has no immediate effect on image appearance. Monitor profile.  Photoshop uses the Windows Default monitor profile to gamut-map the image from the working color space to the monitor. To see or change the Default monitor profile, open the Control Panel and click on Display, Settings, Advanced, Color Management. In Picture Window Pro the monitor profile is selected by the user. No gamut mapping takes place if "None" is selected. To obtain the same display as Photoshop, the Windows Default monitor profile should be selected. Soft proofing should be turned off unless you are previewing a specific printer. (Soft proofing works best for 4-color offset printing, where the color gamut is smaller than most CRT monitors. It may not work as well for high quality inkjet printers.) In Photoshop, click on View and make sure Proof Colors is unchecked. In Picture Window Pro, Proofing Profile should be set to "None." Miscellaneous Photoshop Color Settings.  Desaturate Monitor Colors By should be unchecked. Engine should have little effect on image appearance. Choices are Adobe (ACE) and Microsoft ICM. If your image doesn't have an embedded ICC profile, be sure Working Spaces RGB is consistent with the image's color space.

Working color space

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Without color management the Windows default sRGB color space is assumed. sRGB has a limited color gamut, approximating that of the average computer monitor. When you print on a high quality inkjet printer, which has a larger gamut, you adjust the printer driver settings to take advantage of the device gamut. You might, for example, boost the Saturation setting, as I've described in Printer calibration.

A color space can be associated with an image when it is opened or at any time during an editing session. But it's a good idea to make the association as early as possible in the editing process— during scanning or RAW conversion, if possible. Settings and techniques for performing the association are summarized in the table below.

When an image with an associated color space is saved, the ICC profile of the color space is embedded in the image file-- saved along with it. This doesn't increase the stored image size by much because color space profiles are tiny. In a color managed workflow it's always good pratice to embed ICC profiles in images. Otherwise confusion can result. Photoshop, for example, treats images without embedded profiles differently from Picture Window Pro and other applications. The gamuts of several color spaces are shown in the chart on the right, which has been adapted from Jonathan Sachs' Color Management tutorial. See pp. 10-14, for details. Color spaces can be divided into three categories. Small-gamut spaces have gamuts comparable to CRT monitors. sRGB (gamma = 2.2, white point = 6500K) is the default color space for Windows and the World Wide Web. PAL/SECAM and SMPTE-C are appropriate for video output. sRGB is weak in green and cyan (but not as weak as the perceptually-nonuniform xy chart indicates). If you're working in another color space, you'll need to convert to sRGB for Internet display. Medium-gamut spaces have gamuts somewhat larger than CRTs; comaprable to high quality (more than 4-color) inkjet printers. Adobe RGB (1998) (same as SMPTE-240M) is the best known. Gamma = 2.2; white point = 6500K. Adobe RGB (1998) is the most widely recommended color space when the primary output is high quality inkjet prints.  Most shops that make LightJet prints prefer Adobe RGB (1998) or similar color spaces.

Wide-gamut spaces have gamuts much larger than CRTs or inkjet printers, often comprising most of the colors the eye can see-- and sometimes "colors" the eye can't see. Wide gamut RGB, Universal RGB, CIE RGB, Chrome 2000 D65, and Kodak ProPhoto RGB are examples. The gamut of Chrome 2000 approximates that of transparency film. When colors in wide gamut spaces are squeezed into monitor or printer color spaces using perceptual rendering intent, clipping and color shifts may take place (predictable with Gamutvision image analysis; unpredictable otherwise). Since half or less of the total gamut can be reproduced on monitors or printers, using a wide-gamut space sacrifices bit depth-- something you can ill afford when which working with 24-bit color, which has a bit depth of 8. The solution is to work with 48-bit color. See Bruce Fraser's article, The High-Bit Advantage. Wide-gamut spaces can be tempting; a number of experts recommend them. I'm not among them. There are alternative views: RIMM/ROMM RGB Color Encodings by Spaulding, Woolfe and Giorgianni makes a strong case for wide-gamut spaces (RIMM/ROMM is the same as ProPhoto). 

RGB color spaces are strongly preferred over CMYK for photographic reproduction. All high quality (more than 4-color) inkjet printers must be driven by RGB data; they aren't really CMYK because of the additional colors. CMYK spaces are device-dependent, limited in gamut, and there is no unique representation of colors in CMYK-- different gray component replacement algorithms are appropriate for different printers and intents. CMYK is best left to the 4-color printing industry. Picture Window Pro doesn't support CMYK.

Why not use CIEXYZ or CIELAB as a working color space? They are device-independent, referenced by ICC profiles, and their color gamut includes the entire range of human vision. Two reasons. (1) They are not intuitive. The meaning of each coordinate is more obscure than RGB. (2) The full gamut of human vision encompasses only about 30% of the numerical values in these spaces. (The diagram above only shows x up to 0.8 and y up to 0.9; there's a lot more empty space.) Wide-gamut spaces use about 25%; narrow-gamut spaces such as sRGB are down to 15%. This is equivalent to losing 2 bits of numerical precision. In 24-bit RGB files, which have an 8-bit color depth, only 6 bits would remain-- not enough for good tonal rendition. This isn't a problem for 48-bit color images, which have a 16-bit color depth.

Working color space recommendations. The safest color spaces have gamuts comparable to (or slightly larger than) the preferred output devices. This minimizes color distortion when files are mapped with perceptual rendering intent; it also minimizes the risk of clipping out-of-gamut colors with relative colorimatric intent. sRGB is the default working color space for the Web. Adobe RGB (1998) is widely recommended when the primary output is high quality inkjet printers. There is some unresolved controversy about the use of wide gamut spaces; they might be OK when used with 48-bit color images. If you want to study color spaces, gamut mappings, and rendering intents in depth, I recommend Gamutvision. (As the author of Gamutvision I can't claim to be unbiased.) 

Real-world images and color spaces

In the presentation Metrics for Comparison of Color Encodings (CIE TC8-05) by Kevin Spaulding and Gus Braun of Kodak, available online from colour.org, the authors have created a CIE xy diagram representing the full range of colors available from reflective surfaces. They used a wide variety of sources (graphic arts spot colors, etc.) to assemble this diagram (slide 5 in the presentation). It is instructive to compare their results to the two best known color spaces, sRGB and Adobe RGB (1998).

Real-world reflective colors

Adobe RGB (dotted), sRGB (solid)

Although the eye can see more highly chromatic colors than these (the outer horseshoe), they are extremely rare in nature. The best examples are the vibrant colors of a prism in the sun and some rare irridescent butterflies— colors that can't be reproduced by standard output devices (monitors or printers). Adobe RGB (1998) encompasses most of the reflective colors; only a few reds and cyans are out of gamut, and not by much. The eye is not extremely sensitive to chroma differences in highly saturated colors. So Adobe RGB is a safe, conservative choice for a working color space. sRGB is clearly weaker in greens and cyans. It's the standard color space of Windows and the internet— by far the most convenient, foolproof choice , but Adobe RGB is preferred when the intended output is high quality inkjet prints.

The reader is cautioned that the CIE 1931 xy chromaticity diagram has several well-known limitations. The space occupied by the greens is exaggerated, and the 2D representation has no information on the lightest or darkest colors.

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The anatomy of a profile ICC profiles consist of a header and and a set of tags, which contain the bulk of the data. You can examine the contents of profiles with ICC Profile Inspector, which you can download from the ICC Resource Center by clicking on CONTINUE and following the instructions. When you run it, click Browse... to load the profile. The header information and tag table are displayed. Double-click on a tag to see its contents. A typical tag, gXYZ (green primary color), is illustrated on the right. There are three classes of profile, indicated in the Device Class field of the header: Input ('scnr'), Display ('mntr'), and Output ('prtr'). Each has a set of required tags and a set of optional tags. Display profiles are used to define color spaces. Many monitor profiles contain the ://developer.apple.com/techpubs/mac/ACI2.5/WhatsNewColorSync25.6.html">vcgt tag for setting lookup tables when a loader program is run, i.e., for calibrating the monitor. The meaning of the tags is specified in the formidable (126 page) ICC File Format for Color Profiles (Version 4.0.0), which is rich in content and readable if you skip the bureaucratic parts (strong coffee recommended). In most cases the meaning will be obvious from the ICC Profile Inspector display. The basic tag signatures (4-character abbreviations) are desc is the description of the profile, used in PW Pro selection boxes. rXYZ, gXYZ and bXYZ specify the R, G, and B primaries that determine the gamut of the color space or device. wtpt is the white point. The two standard white points are 6500K (D65): X=0.95045, Y=1.0, Z = 1.08905, and 5000K (D50): X=0.96429, Y=1.0, Z=0.82510. Y is always 1.0 and Z varies the most. Used for absolute colorimetric gamut mapping, which is of little interest to photographers. rTRC, gTRC and bTRC are the R, G and B Tone Reproduction Curves that define device or color space gamma in Input and Monitor profiles. An example (gTRC) is shown on the right. Gamma is often indicated on the upper right. If it isn't, it can be calculated from   gamma = -ln(y5)/0.69315 . where x' = x/xmax ,  y' = y/ymax , and y5 = y' at x'=0.5 (the middle of the x-axis) = 0.22 (in the curve on the right). Typical values: y5 = 0.287 for gamma = 1.8, 0.25 for gamma = 2.0; 0.218 for gamma = 2.2. [ This equation can be easily derived from (y/ymax) = (x/xmax)gamma ].   AToBn or BToAn are gamut mapping tables used in printer profiles. A refers to the device; B refers to the profile connection space (PCS); n = 0 for perceptual, 1 for colorimetric or 2 for saturation rendering intent. BToAn tags are used for printing; AToBn are used for proofing (previewing the print). These tables are large. Profiles that contain them can be several hundred kilobytes-- sometimes over a megabyte. TRC tags are omitted in printer profiles. All the printer profiles I've examined have gamt (out-of-gamut) tags, but little information is available about them. The best way to examine the actual performance of printer profiles is with Gamutvision. All monitor profiles have gamma information in TRC tags, and most of them have Lookup table (LUT) loader information in private tags (vcgt or Mtbx). If you calibrate your monitor to gamma = 2.2, the TRC tags should have curves consistent with gamma = 2.2, but sometimes they are inconsistent. But there are many profiles out there incorrect values of gamma (not 2.2 when they're supposed to be). See below. Don't use them! You're better off with sRGB IEC61966-2.1, which is an essentially neutral profile with gamma = 2.2 and R, G, and B primaries close to typical CRT monitors. It's a good idea to check the TRC tags in monitor profiles. Manufacturer-private tags make it difficult to figure out what a profile is supposed to do. An example is vcgt (Video card gamma tag, registered to Apple), widely used in monitor profiles (Adobe, Monaco, etc.) to set video card LUT's. It is not to be found in the ICC specification. Another example: Mtbx, in the monitor profiles created by Adobe Gamma and MonacoEZcolor. Try searching Google and you'll find pages on mountain bikes. Here is what the spec says (p. 3): "Private data tags allow CMM developers to add proprietary value to their profiles. By registering just the tag signature and tag type signature, developers are assured of maintaining their proprietary advantages while maintaining compatibility with this specification. However, the overall philosophy of this format is to maintain an open, cross-platform standard, therefore the use of private tags should be kept to an absolute minimum." And that is how things would be in the best of all possible worlds. Of course a profile's actual performance is more than the sum of its parts. To see how a profile functions with different rendering intents under a variety of conditions, you'll need Gamutvision.

Images and text copyright (C) 2000-2013 by Norman Koren. Norman Koren lives in Boulder, Colorado, where he worked in developing magnetic recording technology for high capacity data storage systems until 2001. Since 2003 most of his time has been devoted to the development of Imatest. He has been involved with photography since 1964.