Color management
The aim of color management ( English colo (u) r management ) is to ensure that a template that was recorded with any input device is reproduced as similarly as possible on any output device .
Basics
A high similarity of colors between the input and output of an image is called color fidelity . In order to achieve this color fidelity, color management systems (CMS, English color management system ) are used, but they can never provide a 100% match.
Use color management systems
- Device-dependent color descriptions (device profiles, English Device Connection Space , DCS)
- Device-independent exchange color spaces ( English Profile Connection Space , PCS).
- See also the differentiation between color model and color space .
The task of a color management system is to convert the device-dependent color descriptions (of the input and output devices) into one another with the aid of the device-independent exchange color space . This ensures that each device in a color management system displays the colors almost identically.
A simple example is the printout of colored documents, which look almost identical with a color management system on the monitor and on the printout:
- ICC profiles are usually used as the device profile .
- The color models involved are often the RGB color model (for digital cameras and monitors) and the CMYK color model (for printers).
- The color spaces involved (RGB and CMYK) are part of the named color models .
- In this case, the device-independent CIELab color space serves as a link between the other color spaces .
In addition to the L * a * b * color space , where the popular content management systems are based, there are other media-neutral color spaces such as L * u * v * , which in contrast to L * a * b rather for measuring light colors used becomes. XYZ and xyY are also physical spaces that have in common that they can represent all colors perceptible to the human eye, i.e. visible light.
For example, color management is widely used in the printing, photography, and advertising industries. The demand for color management solutions is not only increasing among professionals, but also among amateur photographers and ambitious amateurs.
Color profiles
Just as every person perceives colors individually, devices, at least device classes, also have different color spaces in which they register or display colors. Such individuality is conditioned by construction differences and production fluctuations.
Color profiles can reflect the color data of a device class or the individuality of a special device.
The standard format for color profiles was developed by the ICC ( English International Color Consortium ) and standardized internationally in ISO 15076. Each device involved in the conversion (monitor, digital camera, scanner, etc.) needs its own profile. It contains translation tables or calculation parameters that are used to convert the color data from or to the PCS ( profile connection space ). XYZ and LAB are mainly used as PCS .
With regard to their intended use, a distinction is made between input profiles (RGB → PCS), output profiles (PCS → RGB or CMYK) and device link profiles, which allow direct gamut mapping between two CMYK color spaces without the detour via a PCS.
With regard to their internal structure, a distinction is made between matrix profiles and LUT profiles (look up table). Matrix profiles are preferably used for devices whose color behavior is dependent on relatively few influences and is therefore sufficiently good, e.g. B. can be described in the form of a 3 × 3 conversion matrix. The file size of matrix profiles is relatively small (a few kilobytes). LUT profiles are used for devices whose color behavior depends on many factors and is too complex to be described with sufficient accuracy using a simple matrix transformation. LUT profiles can be up to several megabytes in size.
It should be noted that a profile only ever applies to a specific state of the device in question. If, for example, the paper type is changed from white to yellowish paper, the same CMYK values lead to different colors. The same applies to monitors when, for example, the brightness control is turned.
Profiling
The profile creation is based on a color measurement. Colors, the exact color values of which are known, are reproduced by the device (monitor, printer) or measured (scanner) and then compared with the known values. Among other things, this results in the gamut , which describes the ability of a device to reproduce colors. Depending on the device type, profiles are created in different ways. Profiles have to be regenerated regularly because monitors in particular change over time. Manufacturer profiles, for example, are only suitable for the series, but not for the specific device.
scanner
To create a scanner profile, a template with many small, different color fields ( IT8 target ) is used, which was measured by the manufacturer with a spectrophotometer. The scanner reads this template and compares the read color values with the reference values of the target. The ICC profile is now calculated from the differences between these values and the device-specific color space ( RGB color space ) is now linked with a device-independent color space ( L * a * b * color space ) in such a way that the color values read by the scanner can be reproduced with accurate colors.
Monitors
A colorimeter and suitable software are used to create a monitor profile. The colorimeter is connected to the measurement computer and software and is usually positioned in the middle of the monitor. After the start of the measurement run, the software successively displays colors on the monitor, the exact RGB value of which is known to the software. The color measuring device returns the CIELab value of the actually visible color to the software. After this procedure has been run through for all RGB values, a CIELab value can be assigned to each possible RGB color. Example: The software displays a perfect red = RGB (255,0,0). The measuring device returns that the monitor shows the value CIELab (0.73, 0.26). This means that every value can be translated from RGB to CIELab.
- RGB (255,0,0) = CIELab (0.73, 0.26)
- RGB (254,0,0) = CIELab (0.72, 0.25)
- RGB (253,0,0) = CIELab (0.71, 0.24)
- RGB (252,0,0) = CIELab (0.71, 0.23)
- ...
Not all monitors are able to reproduce the RGB values in full. This means that the same CIELab values are assigned to different RGB values on these monitors.
- RGB (255,0,0) = CIELab (0.70, 0.20)
- RGB (254,0,0) = CIELab (0.70, 0.20)
- RGB (253,0,0) = CIELab (0.70, 0.20)
- RGB (252,0,0) = CIELab (0.70, 0.20)
- ...
These characteristic, device-specific peculiarities of color rendering are the reason why color management is used at all.
printer
Even if all printers ultimately work according to the principle of subtractive color mixing with the printing inks CMYK and possibly also other colors, the vast majority of devices intended for the private and office sector present themselves as RGB devices to the operating system. The required color separation (conversion from RGB to CMYK) is carried out by the driver or the printer hardware without the user having any influence. Only printers for professional purposes, e.g. B. Proof systems or large format printers (mostly controlled by Postscript) appear on the system as real CMYK devices.
ICC profiles for printers are generated by printing out a test chart with many color fields whose color values are known. The L * a * b * values of these color fields are then measured with a spectrophotometer . This creates a relationship between the printed RGB or CMYK data and the visible CIE L * a * b * color values. It is therefore known which color impression (L * a * b * value) is created when a certain ink or toner combination is output on this printer. In a profile creation program, the measured data is brought into a form that corresponds to the specification of the ICC ( International Color Consortium ). Standardized tables are created that allow conversion from RGB or CMYK to the PCS (CIELAB or XYZ) and vice versa. It should be noted that a separate profile must be created for each ink / toner and paper combination in order to obtain predictable and correct printing results.
A second possibility for printer profiling is offered by the software of higher quality scanners from different manufacturers using standard profiling. Assuming a scanner is available, no additional hardware is required to carry out ICC profiling of the printer.
implementation
If the profiles for the input and output device are available, the color descriptions can be implemented with the help of the color management module (CMM). The color management module is the color calculator that reads the values from the tables (color profile) and makes adjustments if necessary.
In this way, pixel by pixel, a conversion of RGB data into CIELab color values and finally into CMYK values for the relevant input and output device can be achieved.
With the help of color management it is possible to combine any devices with each other and still get the best possible result (depending on the capabilities of the output device). The procedure described here is standard in printing practice today.
See also
literature
- Bruce Fraser, Chris Murphy, Fred Bunting: Real World Color Management . 2nd Edition. Peachpit Press, 2004, ISBN 0-321-26722-2 .
- Rolf Gierling: Color Management . 3. Edition. MITP-Verlag, Bonn 2006, ISBN 3-8266-1626-X .
- Jan-Peter Homann: Digital color management . 3. Edition. Springer, Heidelberg 2005, ISBN 3-540-66274-X .
- Andreas Kunert: Color management in digital photography . 2nd edition, MITP-Verlag, ISBN 3-8266-1645-6 .
- Hansl Loos: Color measurement - basics of colorimetry . Verlag Beruf und Schule, Itzehoe 1989, ISBN 3-88013-380-8 .
- Christian Piskulla: PDF / X and color management. Cleverprinting-Verlag , Holle 2016
- Kurt Schläpfer: Colorimetry in the graphic industry . 3. Edition. UGRA Verlag, 2002, ISBN 3-9520403-1-2 .