Color Management

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Color Mgmt

Contents

Overview

The objective of a color management system is to reduce the differences between the on-screen colors and final printing, as well as showing colors which are out of gamut, ie. beyond the color range of your selected printer. The caveat is you need to provide a profile of the printer and one for your monitor which is reasonably accurate. For new users, the default settings and descriptions can be quite confusing. Without prior knowledge of the terminology, it is very easy to choose the wrong settings. This can often makes images look worse on screen or print. Then, the first time user simply says enough and disables color management.

In a word don't. Once you understand some basic concepts and know your final print destination, you will be able to predict and control more reliably how your document will look when printed. This is especially helpful when you will be sending files for later reproduction with four color printing or sending hi-resolution PDFs.

The steps to making color management work reliably (in other words, what you see on screen is what you print):

  • Learn a little bit about color management concepts and terminology.
  • Get the profiles setup properly. The most important is getting a good accurate monitor profile. Some profilers do an excellent job at creating monitor profiles. Without a properly created monitor profile, enabling color management will give you unsatisfactory results in printing and inaccurate previews on screen.
  • Embed the image profiles properly in image editing applications where possible.
  • The color management system within Scribus is designed foremost for enabling color managed "soft proofs" of the images within the document. Scribus can also show "out of gamut" warnings for colors which might not be accurately reproduced by printing. (Most printers have a narrower range of colors which can be printed, when compared to most monitors.) It does not embed or alter the profiles within your images. Some image formats, eg. PNG and TIFF, can have an embedded ICM or ICC profile within the file header – also known as tagged, with a color profile. Later on, we will describe how and when it might be appropriate to use littlecms or Photoshop to apply a tag or embed a profile. If a file is tagged, Scribus will work with littlecms to read that profile within the image and use that profile to more accurately display 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 still 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 absolutely sure of the image source.

The littlecms package has developed into a stable mature library. Independent tests vouch for its accuracy and littlecms is used within the printer firmware of some of the most exotic and high end printers available.

What Are Color or Device Profiles?

ICC or ICM profiles are special types of files which describe the color characteristic of a device like a scanner, monitor or printer – basically any device which can create or display manipulate the color of a digital image. There are also color profiles which are known as device independent or working space profiles. These special files can be thought of as translators which are used to convert from one type of color profile to another. An ICC or ICM file is a set of tables which describes the mathematical values by which devices measure and describe color. Fortunately, these are set by open international standards and work the same way on a Mac, Windows PC, Linux or Solaris.

Device Profiles are separate files which describe the way a device creates (scanner or digital camera), displays (monitor) or outputs (printer) 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 similar programs 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.

Why "Soft Proofing"?

With the proper setup of device profiles, littlecms can adjust the colors of your monitor to more accurately represent how your document will actually look when finally printed., This can be doubly helpful if you are sending it to be printed commercially or for service bureau output. Moreover, each image can be individually be assigned a profile in Scribus ( Select image > right-click > Properties) so that it can be properly color managed in document production, such as preparing a plate, a PDF/X-3 or direct to plate technologies. This does not, however, alter the image file internally. Preview profiles assigned to an image are part of the setting retained in a Scribus document or in the creation of a PDF. There are also command line tools in littlecms to embed profiles with tifficc and jpegicc. See the littlecms docs for exact options. You can also embed ICC profiles using Cinepaint, Imagemagick, Krita, GIMP 2.4 along with Adobe Photoshop or Corel Photopaint.

First Steps

Some initial requirements:

  • Have accurately profiled input devices which create the images – scanners, digital cameras, and so on.
  • Most importantly, have an accurately calibrated and profiled monitor.

The first suggestion is to ease up on the eye candy – for example, you might want to switch to or come up with a "vanilla" theme setup. While KDE, Gnome, Windows, and Macs have gorgeous desktops, 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. We know that vibrant, saturated colors will affect the appearance of adjacent colors, so minimizing the contrast with screen features not an actual part of your work will help your eye to better judge color balance for images. For that matter, the same applies for the Gimp or other graphics programs. What we are striving for here is to as accurately as possible mimic the way mixed inks look on paper.

Calibrating is setting the monitor to a known state. Most monitors are set to a default to color temperature of 9300k or Kelvin, which is often too "cold" or bluish for accurate color work. [www.color.org] has a multitude of color specifications for your reference. Most color standards are set to 5000k light temperature or "illuminant". For your monitor, 6500k are recommended, which more closely mimics natural sunlight, as a starting point adjusting for your monitor. At first your eyes will think your monitor has a yellowish cast, but your eyes will soon adjust, especially if the brightness and contrast are set properly. 9300K, which is the default factory setting for most monitors is fine for working with a word processor, but this will wash out colors and color will look less balanced with whites actually showing a bluish cast.

Each of the "working" spaces is 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 and Bruce RGB specifies 6500k and 2.2 gamma, quite common for Intel based PC monitors and are the recommended defaults for users who are editing color critical images.

Printer profiles are highly dependent on the media chosen. Newsprint and un-coated stocks are grayer in appearance, so these profiles will have a narrower "gamut" or color range. They do not to produce the super vivid colors and saturation of coated stock or glossy photographic papers. A single printer could easily have a dozen or more profiles, based just on differences in the paper color and ink absorbency.

Getting the Right Profiles (Or At Least the Best You Can)

Some profiles are "generic" and can be obtained from the device manufacturer, who generate them from a sampling of their units. A growing number of monitors, scanners and certain printer vendors will include this with any software bundled with the device. While these "canned" profiles are rarely a perfect match, this is a good first place to start. Go to the vendors website under drivers and see if there a profile available for your device. Another type of profile is a "generic" press standard profile. These are CMYK profiles which are defined to commercial press industry standards, such as SWOP, FOGRA, ECI and other print standards bodies.

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/share/color/icc directory or you can put them somewhere in your home directory and point to this directory in your Preferences. On Windows XP profiles are stored in Windows\system32\spool\drivers\color, and in Windows 2000 in WINNT\system32\spool\drivers\color. Mac OS X uses /System/Library/ColorSync/Profiles/, /Library/ColorSync/Profiles/ or Users/Username/Library/ColorSync/Profiles/ to store profiles. Color profiles are platform independent, thus files created in or available for Mac OS or Windows are usable in Linux with littlecms as well. It is highly recommended to have the Adobe profiles, if you plan to do any type of cross platform or commercial printing, since most well-setup DTP workstations, as well as most commercial printers will be able to work with these profiles.

Solid colors can be described within RGB (red green blue) or CMYK (Cyan, Magenta, Yellow and K for black – these colors represent the four inks used in process printing or in color ink jets).

Activating Color Management enables color management globally within the document. Scribus will remember the settings from file to file. Note: Saving and closing a file with color management on will slow it down on reopening, as Scribus must not only open the files but in addition littlecms must read and perform the corrections between the profiles. Color conversions make multiple floating point calculations for each color, so be patient. littlecms is extremely stable so far, with only a couple of small bugs for profile handling. There is a second checkbox for simulating the printer on the screen. This tells Scribus and littlecms to do an on-the-fly conversion from the image color space to your monitor profile to simulate your chosen printer's profile. The checkbox to mark colors "out of gamut" will show warnings for colors that might not not print accurately, based on the printer profile you have chosen. Typically, when colors are shown out of gamut, they will print darker, lighter or have a color shift when printing. The last option, "Use Black Point Compensation", is a way to help rendering shadows within color pictures. Experimentation will show if it improves your pictures.

Rendering Intents

The other puzzle for newcomers to color management is rendering intents. Your choice of rendering intents is a way of telling littlecms how you want colors mapped from one color space to another.

  • 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" happens when two or more 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, usually the best choice for photographic images and scans. 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 is a good choice for logos, spot colors, etc. It tends to preserve the amount of or vividness of color, but it can make photos look ugly. If you are working with logos with a specific shade, saturation will bring better color matching, inasmuch as you give more importance to the color than 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 highly controlled lighting conditions with a D50 light box. This often has a "warmer" temperature than more typical viewing conditions. This rendering intent is almost exclusively used when a corporate logo or color must be matched exactly regardless of media. Kodak yellow would be a good example.
  • 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 ink jet 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 more rarely digital cameras) in images being converted to CMYK space with printer profiles very precisely profiled with color measurement devices. This is most likely when someone has spent a lot of time and effort to finely calibrate and profile their equipment. It takes sophisticated (and expensive) color calibration equipment to measure the printer under fairly well controlled conditions.

Final Printing with Scribus

For Scribus users, there are a couple of options for printing with a color-managed intent. When printing, Scribus can optionally apply the printing profile you have chosen in the color management panel. This can be very useful, if you want to simulate a commercial printer profile with your inkjet printer via CUPS.

PostScript Output: This would require having ICC profiles embedded in your images before being placed in Scribus files when outputting a Scribus document either as pure PostScript or as individual EPS files. Scribus uses level 3 PostScript. Level 2 and level 3 PostScript can read and use ICC profiles within an image. Most color PostScript devices will read the embedded profiles and use them to render color within the PostScript using something called a rendering dictionary.

PDF/X-3 and Color Management

Above, we covered the basics of getting color management working and configured properly. Now, let's consider using PDF/X-3 to optimize our output for commercial printing. PDF/X-3 is the latest and most advanced ISO standard for PDF files and Scribus fully supports it.

The principal benefit of PDF/X-3 is the typically more accurate color conversion from RGB to a printer's CMYK output. The accuracy comes from working in RGB colors until very late in the printing process when the printer's own RIP does the RGB to CMYK conversion. The downside is only the latest and most updated RIPs support PDF/X-3, although others may support earlier PDF/X-x standards. As the RIP engine has detailed knowledge of the exact color range and capabilities, it is thought to offer, in most cases, a more accurate conversion. PDF/X-3 does not preclude using CMYK images, but is ideal for maintaining your colors in RGB as long as possible in the process.

The major downsides to working in PDF/X-3 are twofold: You cannot use transparency natively. PDF/X-3 is based on PDF 1.3, which does not support transparency. The second is the limited, but growing number of printers who can support it. If in doubt, inquire.

The other issue is the question which printer profile one should use. There are two strategies: The first is to have an actual ICC profile from your printer for the matching paper type. The second is to use a well-known printing standard as SWOP, ECI or others. Using PDF/X-3 requires cooperation from your printer, but can really improve your print color matching. As always, a short visit to your printer in advance is strongly recommended. If answers are not forthcoming, try another one. Printing is a competitive business and almost always alternatives can be found.

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