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Color management with Scribus, an Introduction

For many, color management is a witches brew of black magic and high level theory not seen since university calculus days. Add some relatively obscure terms and the typical user simply rolls their eyes and give up. Moreover, the default dialog boxes within most existing applications are confusing. It is very easy to choose the wrong settings making the images look worse on screen or print. Then the user simply says enough and disables color management.

In a word don't. Once you understand some basic concepts and a simple workflow, you will understand when and how to use color management for making more reliable proofs and final output. Moreover, the type of color management support within Scribus, I believe, is the first for an open source program. Future enhancements will bring even more exact control over color reproduction.

The littlecms package is really remarkable considering the amount of development work done by large companies such as Kodak, Adobe, Agfa and other and the, so far, limited user acceptance. On the littlecms.com site there are two very useful tools which are built for KDE 2: One is a monitor profiler - probably the most critical part for accurate screen previews. The other is a scanner profiler. Both have an easy to use GUI. The defaults of these programs should work fine for Scribus.

As of version 0.5.3, the color management system within Scribus designed with the sole purpose of enabling color managed "soft proofs" of the images within the document. It does not embed or alter the profiles within your images. If an image has an embedded profile or is tagged, with a color profile, Scribus will work with littlecms to read that profile within the image and use that profile to more accurately display and print the colors within an image. If an image does not have a tag but you know the device profile which should be assigned to the image, you can use the CMS settings to preview with the profile temporarily assigned within the document. Caution is advised not to assign the wrong profile unless you are sure of the image source.






Why "soft proofing" ? In a nutshell, with the proper setup of device profiles, littlecms can adjust the colors to represent how your document will actually look when finally printed. Moreover, each image can be individually modified by Scribus to assign profiles in an image, ( Select image > right click > Modify ) so that it can be properly color managed later in document production, such as preparing film, a pdf or direct to plate technologies. This does not however, alter the file internally. Profiles assigned to an image are part of the Scribus document.

Getting good previews from color management depends on these steps from the user:

Having accurately profiled input devices which create the images. Scanners, digital cameras etc. Further on, the specifics are spelled out under tools. Most importantly an accurately calibrated and profiled monitor.

The first suggestion is to ease up on the eye candy. You might want to switch to or come up with a "vanilla" theme setup. Yes, KDE and Gnome can have gorgeous desktops, but Scribus will like plain and simple - no animation, no fancy graphics. For the most accurate color calibration set your desktop to a neutral gray or light color with no gradients or fancy backgrounds. This will help your eye to judge color balance for images. The same applies for the Gimp or other image programs. When I color calibrate monitors for Photoshop with a hardware device, this is the first step I make. What we are striving here is to accurately, as possibly, mimic the the way mixed inks look on paper.

Calibrating is setting the monitor to a known state. Most monitors are set to a default to 9300k, which is often too "cold" for accurate color work. www.color.org has a multitude of color specs for your reference. Most color standards are set to 5000k or 6500k. I suggest 6500k as a starting point. 9300K is fine for working with a word processor, but can wash out colors and make profiling your monitor more difficult.


Using a working space which is appropriate for you. Each of the "working" spaces are based on certain settings for your monitor. Gamma and color temperature of your monitor should match the specs of the working space. For example, Adobe® RGB specifies 6500k and 2.2 gamma, quite common for PC monitors.

Targeting the CMYK device (the printer) properly using a profile, which is appropriate for the paper and device. Printer profiles are highly dependent on the media chosen. Newsprint and un-coated stocks are grayer in appearance so these papers will have a narrower "gamut" or color range, thus they do not to produce the super vivid colors and saturation of coated stock or glossy photographic papers.

Device Profiles - are separate files which describe the way a device uses or displays colors. Users of of Photoshop will be familiar with the choice of Working Profiles or Working Space - which are color profiles not related to a particular device, but to assist in the conversion of color from one device to another. Well known RGB "working spaces" Include sRGB, Adobe® RGB 1998, Colormatch®, Bruce RGB or CIERGB. Users of Photoshop or other color may be wondering if this is a missing feature, but littlecms uses its own internal color conversion process to make the transformation between color spaces. One less setting to worry about!

How do I get profiles which are meaningful for my hardware.?

Some profiles are "generic" and can be obtained from the device manufacturer. This type of profile is generated from a sampling of units by a manufacturer. A growing number of monitors, scanners and certain printer vendors will include this with any software bundled with the device. This is a good place to start. Go the vendors website under drivers and see if there a profile available for your device.

Linux Color Tools

Monitor Gamma - Kgamma works very well. There is also a GTKgamma tool, but it currently is unable to save the settings, but it is useful to see how adjusting gamma settings for each red, green or blue channel affects the display. Having Gamma setup properly is an important first step in getting good color balance before trying to creating an accurate profile.

The next step is in accuracy is a custom generated profile created with profiling software, like the Kmonitorprofiler and Kscannerprofiler. Use the Kmonitorprofiler to profile or create a unique file for your monitor. Monitors change their display of color with age and temperature. Identical models will sometimes have noticeable variations. Kmonitorprofiler also has a panel to adjust gamma as a part of its profiling software.

The most precise way to profile a monitor is with a electronic profiling device, basically a very special type of camera which measures color. The software sends known reference colors to the monitor which then takes the results and creates a profile. Linux drivers for common "spyders" or colorimeters are not yet available. Users who have created custom profiles with a "spyder" under Windows, might have good luck using this same profile if the monitor is set up exactly as in Windows AND if the Linux video driver does not make any radical adjustments to the color values of your display. It is worth trying at least! My profile works very well and the colors within Scribus are just about exactly the same in Photoshop 6.0. Quite an achievement by both Scribus and littlecms.

The Scribus Color Management Settings




System Profiles - These drop down boxes show the available profiles on your system. To enable Scribus to use profiles, they should be copied to the /usr/local/share/Scribus/profiles directory. Color profiles, .icm and .icc, are platform independent, thus files created on a Mac or in Windows are useable in Linux with lttlecms in Scribus, as well.

The above screen cap is a good starting point to explain the important parts of littlecms in Scribus. In this case, the images in the document have been created with a mid-range digital camera. The camera's itself performs come color balancing and auto adjusts the output for the sRGB range. So leaving this within sRGB is a good choice. If the images came from a scanner, you would want to select the profile created with profiling software.

Solid colors can be described within RGB or CMYK. In this case, we are using some basic RGB colors, which will be later "soft proofed" in the CMYK color space of the printer, which will be Web Offset on coated paper to ensure rich and vibrant colors.

It is handy to give your device profiles some sort of short hand way of naming. For example the D226500mon.icm monitor profile is a custom profile created with Kmonitorprofiler with 2.2 gamma and 6500k temperature. The D is for daylight. Ambient light can also affect your perception of color, sometimes radically with certain types of artificial light.

Activate Color Management enables color management globally within the document. Scribus will remember the settings from file to file. Note: Saving and closing the file with color management on will slow them on reopening, as Scribus must not only open the files but the littlecms must reading and perform the corrections between the profiles. Color conversions make multiple calculations for each color, so be patient. Testing has shown littlecms has to be extremely stable so far.

Rendering Intents

Perceptual - This rendering intent maps color "smoothly", preserving relationships between similar colors. This prevents "gamut clipping" with its potential loss of detail and "tonal banding" problems. Gamut clipping happen when colors that are different in the input image appear the same when printed. Perceptual rendering intent makes small adjustments throughout the image to preserve color relationships. It sacrifices some precision of colors in order to ensure pleasing results. For photographic images and scans, this is usually the best choice for a default setting.

Perceptual intent will produce the most predictable results when printing from a wide range of image sources, for example, when printing RGB images on CMYK devices, or when trying to match CMYK devices that are radically different from each other. Consider this "foolproof" setting to be best for users who handle the wide variety of images that commonly enter large format printing facilities.

Saturation stands for logos, spot colors, etc. It tends to preserve the amount of color. But it can make photos look ugly. If you were working with logos with a specific shade, saturation will bring better color matching, as far as you give more importance to the color that to the image.

Absolute Colorimetric: When a color is not printable within the "gamut "of the output device, this rendering intent simply prints the closest match. It reproduces in-gamut colors without compromise, as faithfully as possible. This produces the most accurate matching of spot colors. Unfortunately, it can also result in "gamut clipping" where two colors that are different in the original are identical on the print. White points are similarly clipped, then causing color relationship problems in the highlights of images. This type of clipping, and the resultant problems, typically make this rendering difficult to use with anything but spot colors. Some users will be disconcerted with a yellowish cast in their image, but this intent is measured in D50 light box. This often has a "warmer" temperature than more typical viewing conditions.

Relative Colorimetric: When a color is not printable within the gamut of the output device, this rendering intent prints the closest match along with an adjustment that maps white to the paper of the output. This mapping of "white point" prevents the problems of "Absolute Colorimetric" when images except spot colors are concerned. When producing color proofs on RGB inkjet printers, while simulating CMYK printing presses you can use this intent, if you know the intended precise profile. Users of Adobe Press Ready will understand this concept quite well. This approach works well when you have accurate embedded profiles (typically scanner or rarely digital cameras.) in images being converted to CMYK space with precisely profiled printer profiles. This is most likely when someone has spent a lot of time and effort to finely calibrate and profile their equipment, which take time to master.

Final Printing with Scribus

For users of Scribus under the latest versions of Scribus, there are a couple of options for printing with a color managed intent.

Postscript Output - This would require having tagged images before being placed in Scribus files when outputting a Scribus document either: as pure postscript or as individual EPS files. Scribus uses a combination of level 2 and level 3 postscript, depending on the images within the document.

Using the latest AFPL version of Ghostscript. The latest version 7.0x, as of Feb. 2002, include icclib V 2.0, which gives Ghostscript the ability to read and work with icc profiles. The author's experience with the latest versions of Ghostscript on Linux have been problematic with RH 7.2, mostly on account of the default RH printer configuration. While under Win32 on Windows 2000 it has been extremely good, rivaling the latest distillers for reliability.

Using the Ghostscript PDF writer (ps2pdf) driver, one can create a V1.4 PDF file, which should retain , if specified, color profile info within the PDF.

Note: As of version 0.5.4, Scribus includes its first version of its own internal PDF export filter. The color management options for this are not yet complete.

Conclusions

So, by inference, we have simply described a color managed workflow for our document. Take the image from the source, use a good working space for conversions to another color model. Then in our example, Scribus, by checking the simulate printer box, tells littlecms to make a two step conversion so your monitor represents with reasonable accuracy how your images and color will print.

Attempting to incorporate a simple, but effective color management system within Scribus is an ambitious, and the author believes first serious open source attempt to provide the end user with color management. These type of tools are more typically found in high end professional pre-press applications. As this is uncharted territory for open source, your comments and suggestions on this document are most welcome.

Acknowledgments:

Martí Maria - Developer of littlecms, without which this would not be possible.

The program author of Scribus is: Franz.Schmid@altmuehlnet.de

The author of this documentation is Peter Linnell netscribe@mediaone.net (To change in March 2002)

First revision 8/2/02

For further learning:

www.littlecms.com - Home of the littlecms color management system and other useful utilities.

www.color.org

http://www.levien.com/gimp/gcmm.html

http://www.khk.net/color/links.html

http://www.i3a.org/links_color-management.html

http://web.access.net.au/argyll/index.html


Home of icclib V 2.0 - this is a color profile library, not necessary for Scribus, but this has been included in the latest 7.0x AFPL releases of Ghostscript.


Disclaimer;

Adobe, PageMaker, Photoshop, Acrobat and PostScript are registered trademarks of Adobe Systems. All other products mentioned in this file are trademarks of their respective companies.


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