|Color Management Systems
|Color Management Models
The color-managed workflow is fairly straightforward and possesses two major characteristics:
Images are edited in a device-independent color space that is larger than the color space of the output device, such as a computer monitor, a TV screen, film, or a four-color press.
Image files can be saved with profiles that contain information describing the characteristics of the source and output color devices.
These two considerations make a color-managed workflow advantageous. The Image files become portable since they can be repurposed for output on widely differing devices simply by tagging them with different output profiles.
Next, we'll examine the following Color Management Models:
The ICC Color Management Model
In 1993, members of the computer and color publishing industry began working toward a common approach to color management. They formed the International Color Consortium (ICC) in order to establish color standards that would help users achieve reliable and reproducible color throughout the entire reproduction process. They also endorsed an open framework for developing color management systems.
An ICC color management system has three major components:
||A device-independent color space, also known as a Reference Color Space.
||Device profiles that define the color characteristics of a particular device.
||A Color Management Module (CMM) that interprets the device profiles and carries out the instructions on what to do with different device's color gamuts.
One of the first decisions made by the ICC was that color space transformations were the responsibility of the operating system. Placing the responsibility there meant that color management would not have to be replicated in each application while still being available to all applications. Device profiles, which contain information on the color behavior of the various peripherals, provide the data necessary to perform these transformations.
The ICC chose the CIE color model as the device-independent color space for color management. Since device-specific colors from any device can be mapped into a device-independent color space, it's much easier to combine equipment from different vendors into one system and maintain color specifications. Because they are well-defined and reproducible, the CIE color spaces (CIELAB and CIEXYZ) are an excellent language for communicating color information between different systems.
The PostScript Color Management Model
Adobe's Level 2 PostScript language had already implemented much of the device-independent features of ICC color management. The PostScript model achieves device-independent color management by converting RGB to CMYK when printing, rather than at some prior point. It uses the CIEXYZ color space (a cousin of CIELAB) as a reference color space. It also uses a Color Space Array (CSA). which is analogous to an ICC source profile, and a Color Rendering Dictionary, which is the PostScript version of an output profile.
In the PostScript color management model, there are three paths for color-managed printing:
The driver makes the color conversion to the printer's color space. It does this using both source profiles and printer profiles and send converted CMYK data to the printer:
This path is useful for printers without the appropriate color conversion functionality, or in cases where the host platform may have considerably more processing power than the printer.
The driver uses source profiles and a printer profile as above, but, in this case sends the original RGB data to the printer. The source profiles are rolled into color space (CS) descriptions appropriate for the printer and the printer profile is rolled into a color rendering dictionary (CRD). Color conversion is then carried out in the printer:
The resulting page description is device-dependent because it contains a CRD for a specific printer. The same page description sent to a printer needing a different CRD would give incorrect results. The difference from path 1 is that the color conversions have been offloaded to the printer.
The driver sends the data in the source color space. In this case, however, only the source color space descriptions (CS) along with the data are sent to the printer:
The printer profile is not needed since the CRDs (color rendering dictionaries) resident in the printer are used to complete the path from CIEXYZ (the reference color space) to printer device colorants. Note that while all three paths can give identical results, only path 3 is device-independent.
The basic PostScript architecture can also use additional profiles, third-party CMMs, Color Space Arrays, and Color-rendering Dictionaries: