SCALE 8x: Color management for everyone

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On Sunday at SCALE 8x, Inkscape developer Jon Cruz presented a talk entitled "Why Color Management matters to Open Source and to You," putting the need for color management into real-world terms for the average Linux user, outlining current development work on the subject at the application and toolkit levels, and giving example color-managed workflows for print and web production. Color management is sometimes unfairly characterized as a topic of interest only to print shops and video editors, but as Cruz explained at the top of his talk, anyone who shares digital content wants it to look correct, and everyone who uses more than one device knows how tricky that can be.

"If you have eyes and a display, you need color management"

Color management, broadly speaking, is the automatic transformation of image colors so as to provide a uniformly accurate representation across devices. This includes output-only devices such as televisions and printers, as well as CRT and LCD displays on which editing as well as final output is viewed. The first problem is that every device is capable of generating a different spectrum of colors — different hues, different ranges of white-to-black values, and different degrees of saturation. Collectively, the color capabilities of the device are its gamut, which can be represented by a three-dimensional volume in one of several mathematical color models (or "color spaces").

The second problem is that digital files store the color of each pixel as a numeric triple that may or may not represent coordinates in some specified color space. If the color space to which the file referenced is known, mapping each triple from its stored value into the gamut of the output device is a simple transformation, and the user can visually examine the full range of pixel data. Without that transformation, multiple colors outside the display device's gamut get mapped to the boundaries, causing artifacts and loss of detail, and the entire image can get mapped too dark or too light, misrepresenting the scene.

Although it is clear that graphics professionals need color managed displays and printers, Cruz said, the explosion of user-generated digital content in recent years makes it a problem for everyone.

Home users want to be able to edit video and share it online, knowing that what appears appropriately bright on-screen will not look washed-out or too dark on DVD or YouTube. They also want to drop off family photos at the corner drugstore kiosk and not be disappointed by a red or green cast to the skin-tones. Photo kiosks may be inexpensive per-print, he said, but online vendors like Apple and Google's Picasa are increasingly offering more elaborate services, such as hardbound books, with correspondingly higher prices. Consumers might shrug off paying a few cents for a bad-looking 4x6 print, but getting burned on an expensive book is considerably more aggravating.

Just as importantly, Cruz added, business users need to care about the professionalism of their presentations, both for aesthetic reasons, and because a mis-colored partner logo could accidentally sour the opinion of the executive at the table who recently spent months determining the "perfect shade of puce" to represent the company image. Finally, he said, anyone who sells products online should know that the number one reason for returned consumer purchases is mismatched colors — if the product shots on the web site make the red shirts look orange, the seller is financially at risk for the cost of returns.

In addition to these use cases, Cruz explained that users need color management support in their desktop applications to cope with the variety of different display devices they use over the course of a day. Multiple computers are commonplace, from desktops to laptops to netbooks to hand-held devices, and each have different display characteristics. Laptop screens have noticeably smaller gamuts than desktop LCDs, which are in turn smaller than CRTs, and different also from the displays of consumer HDTVs. Mobile devices, based on different graphics hardware, may not even support full 8-bit-per-channel color. Presenting a consistent display across these platforms cannot be left to chance.

Status report

Fortunately for Linux users, Cruz continued, color management support in Linux is in good shape, although more still needs to be done. Most creative graphics applications support color management already, thanks in large part to the collaborative efforts of the Create project at Freedesktop.org. These include Gimp, Krita, Inkscape, Scribus, Digikam, F-Spot, and Rawstudio, as well as several image viewing utilities.

Enabling users to acquire good ICC profiles (tables measuring the device's attributes against points in a known color space, thus allowing for interpolation of color data) or to build their own is one of the key areas of current color work. Projects like Argyll and Oyranos handle tasks such as precisely measuring monitor color output through hardware colorimeters, creating profiles for printers, scanners, and cameras through color targets, and linking profiles for advanced usage.

A simpler solution aimed at the home user is GNOME Color Manager (GCM); unlike the previous two examples GCM does not attempt to be a complete ICC profile management tool, but focuses on easily enabling users to correctly assign a profile to their monitor. Default profiles are usually available from the manufacturer, either through the web or on the "driver" CDs in the box, and for normal usage they are an excellent first step. Developers from these and several related projects collaborate on common goals in the OpenICC project.

Developers interested in adding color management to their applications should start with LittleCMS, Cruz advised, noting that he personally added Inkscape's color management support in less than one week's time with LittleCMS. LittleCMS is a library that handles the mathematical transformations between color spaces automatically, quickly, and with very little overhead.

Currently, however, one drawback of the Linux color management scene is that most color-aware applications work in isolation from one another, requiring the user to choose display, output, and working ICC profiles in each program — whether through LittleCMS or with in-house routines. Ongoing work to bring color management to a wider range of programs includes adding support to the Cairo vector graphics rendering library, attaching display profiles to X displays, and building color management into GTK+ itself. The latter, in particular, would enable "dumb" applications to automatically be rendered in color-corrected form on the monitor, while still allowing "smart" applications to manage their own color. This is important because graphics and video editing applications need to be able to switch between different profiles for tasks like soft-proofing (simulating a printer's output on-screen by rendering with a different ICC profile) or testing for out-of-gamut color.

To the work!

Finally, Cruz showed several example workflows for print and web graphics, first illustrating potential problem points when working in a non-color-managed environment, then explaining how using a color-aware setup would trap and eliminate the problem.

For web graphics, the example scenario was a simple photo color-correction. Over-correcting the color balance on an improperly-managed monitor easily leads to site visitors seeing a wildly distorted image. In addition, Windows and Macs use different system gamuts, which leads to photos looking either too bright on Macs or too dark on Windows. With a managed workflow, users should target the sRGB color space, previewing the results with Windows, Mac OS X 10.4 and Mac OS X 10.5 profiles (due to changes introduced by Apple in 10.5), as well as mobile devices under different conditions. Because most web site audiences do not have color-corrected displays, he said, not everything is under the designer's control — but if the end user's monitor is broken and the artwork is broken, the problems multiply.

For print graphics, the workflow is more complicated, starting with the fact that — despite the popularity of the term — there is no single, standard "CMYK" color space. All process-color spaces are device-dependent, including common four-ink CMYK printers, CcMmYK photo printers, Hexachrome, and others; there is not even an analogous color space to the "Web safe" sRGB standard. Process color's small gamut makes it very easy to produce poor output when not using color management to edit and proof.

Fortunately, Inkscape and other SVG-capable editing tools can take advantage of the fact that SVG allows different color profiles to be attached to different objects in a drawing. A CMYK profile for the target printer can be used for most of the drawing, with a separate spot-color profile attached to specific objects that need careful attention, and corrective profiles for embedded RGB elements like raster graphics. A test run is always the best idea, Cruz said, but having proofing profiles available on the system saves both money and time.

Conclusion

Color management on Linux has come a long way in the last four years. The application support in the basic graphics suite is good, and for professionals tools like Argyll and Oyranos open the door to complete solutions; as Cruz observed in his talk, the colorimeter hardware that used to cost thousands of dollars and lack support on free operating systems is now cheap and well-supported.

Still, the average desktop Linux distribution does not install in a color-managed state, which is unfortunate. Proper support for transforming pixels from one color space to another is straightforward math that, much like window translucency, smooth widget animation, and audio mixing, should happen without requiring the user to stop and think about it. It is promising that headway is being made on that front as well, with GCM and GTK+; perhaps in a few release cycles Linux will have full color management out-of-the-box.