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Managing Color With ColorSync


Devices and Their Profiles

To assess the way each device interprets color, color scientists and profile developers perform device characterizations. This process, which entails measuring the gamut of a device, yields a color profile for that device. For an overview of profiles, see Profiles .

Device profiles are of paramount importance to any color management system because they characterize the unique color behavior of each device and provide the data needed for color matching and color conversion. Device profiles are used by CMMs that perform the low-level calculations required to match colors from a source device to a destination device.

The ICC defines a device profile class for each of three types of devices:

These classes are described in detail in Profile Classes .

Each device profile class has its own signature. The ColorSync constants for these signatures are described in Profile Class . You can create a device driver for any of the device classes. When you create a profile for your device, you specify the signature in the profile header's profileClass field. For more information on profile headers, see Profile Header

Whether you create a profile for your device or obtain one from a profile vendor, your device driver must provide at least one profile for its device. However, you can provide more than one profile for the same device to characterize different states. Although a printer that your device driver supports may have a number of profiles for different conditions, such as the use of foils or different grades of paper, all of its profiles will use the same profile signature, cmOutputClass .

The Profile Format and Its Cross-Platform Use

Device profiles follow the ICC profile format, an industry standard described in Profiles . You can provide a single profile or a set of profiles that can be used across different operating systems for the device your driver supports. The common profile format specified by the ICC allows end users to transparently move profiles and images with embedded profiles between different operating systems.

Note

The ICC publishes the International Color Consortium Profile Format Specification . To obtain a copy of the specification, visit the ICC Web site at http://www.color.org .

The profile structure is defined as a header followed by a tag table which, in turn, is followed by a series of tagged elements that your device driver can access randomly and individually. Using ColorSync Manager functions, you can read the profile header and modify its contents and you can get and set individual tags and their element data.

ColorSync Profile Format Version Numbers

This document uses "2.x" to refer to ColorSync profiles for ColorSync Manager version 2.0 and greater, as described in ColorSync and ICC Profile Format Version Numbers . Version 2.x profiles require more information and are larger than ColorSync 1.0 profiles, which were originally memory based. Because version 2.x profiles are larger, they are disk-based. The ICC profile format specification defines how profiles can be stored as disk files and how profiles can be embedded in common graphics file formats such as PICT and TIFF. The ColorSync Manager provides the CMProfileLocation data structure to identify the location of a profile. It also provides functions you can use to embed a profile in or extract if from a PICT file, as described in Embedding Profile Information in Pictures .

Storing and Handling Device Profiles

Device profiles reside in the ColorSync Profiles folder, in pictures, or with device drivers. Files that contain profiles store profile data in the data fork and have a file type of 'prof' .

By convention, profiles not embedded in image documents are stored in the ColorSync Profiles folder, as described in Profile Search Locations . You can store your profile files wherever you want, but if you want other drivers or applications to have access to them, you should store them in the ColorSync Profiles folder. Applications that perform soft proofing or gamut checking can use ColorSync Manager routines to search the folder for specific types of profiles to provide a pop-up menu or list to the user. If your profiles are not available, these applications will not be able to include these profiles in menus or lists, and will not be able to color match to your device (unless they provide a profile for your device themselves).

Some applications may place special-purpose profiles for your device in the ColorSync Profiles folder. For this reason, when your device driver itself displays a pop-up menu or list to the user, you should search not only your private profile location storage, if you use one, but also the ColorSync Profiles folder, to make sure that you offer a complete list of available profiles for your device.

The ColorSync Profiles folder can contain both ColorSync 1.0 profiles and version 2.x profiles. However, your device driver will be able to search for only version 2.x profiles. This is because ColorSync Manager 2.x search functions do not acknowledge ColorSync 1.0 profiles. Support for 1.0 profiles may be even more limited in the future. For more information on this and other limitations of the 1.0 profile format, see ColorSync 1.0 Profile Support .

How a Device Driver Uses Profiles

For most ColorSync Manager functions that your device driver calls, you will need to supply references to profiles for both the source device on which the image was created and the destination device for which it is to be color matched and where it will be rendered.

The driver for an input device such as a scanner typically embeds the scanner profile used to create the image in the document containing the image. The driver for a device that displays an existing image on the system's display or a printer device that prints a color-matched image typically extracts the embedded profile that accompanies the image from the document containing the image, and uses that profile as the source profile when matching.

Images created using input devices are commonly color matched using the profile for the input device as the source profile and the system profile for the display as the destination profile. Setting Default Profiles describes how to set the default system profile and other default profiles. Images that are created, depicted, or modified on a display device and that are destined for an output device such as a printer are color matched using the profile for the display as the source profile and the printer's profile as the destination profile.

To use a profile, you must first obtain a reference to the profile. For a description of how to do this, see Obtaining Profile References .


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