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.

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 ColorSync Manager Reference.

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 Profiles and Version 2.x Profiles.

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.

Providing Minimum ColorSync Support

The minimum level of ColorSync support you should provide differs depending on the type of device your driver supports.

For a scanner, you should embed the scanner profile used to create the image in the document containing the image; this is also referred to as tagging an image. If you do not tag the image with the profile, you should at least make the profile for the image available so that it can be used for color matching. If you do not provide the scanner profile, an application or driver that attempts to color match the scanned image will use the system profile as the source profile and may produce results inconsistent with the colors of the original image.

For a display device driver or a printer device driver, you must preserve images tagged with a profile by not stripping out picture comments used to embed profiles or by leaving profiles in documents that use other methods to include them. For example, if your driver displays or prints PICT files but does not perform color matching, your driver should not strip out the ColorSync-related picture comments that are used to embed profiles in PICT files, begin and end use of a specific profile, and enable and disable color matching. Even though your driver may not make use of the comments, another display or printer driver or an application may use them.

If you don’t perform color matching but you want to allow other applications to produce images that are color matched for your device, you should provide a device profile to be used as the destination profile. If you provide a profile for your display or printer and place it in the ColorSync Profiles folder, applications that perform color matching can use it to create a color-matched image expressed in the colors of your device’s gamut. A user can then print a color-matched image using the printer your driver supports.

Providing More Extensive ColorSync Support

Instead of relying on an application to color match an image for your printer, your printer driver can color match the image itself before sending it to the printer. To perform color matching, your printer driver must obtain a reference to the source profile. Documents containing images to be printed often contain an embedded profile along with the image. To use the source profile, your printer driver must be able to extract it. If an image is not accompanied by a source profile, the default system profile is used, as described in Setting Default Profiles. In this case, your driver should provide an interface that allows the user to select the rendering intent to be used. Rendering intents are described in Rendering Intents.

You can provide an interface that offers additional features. Your interface can

• allow a user to turn ColorSync color matching on or off before printing

• offer pop-up menus, allowing the user to choose

  • the rendering method to be used in color matching the image (perceptual, colorimetric, or saturation)

  • the color-image quality (normal, draft, or best)

Some of these features are discussed below and in Developing ColorSync-Supportive Applications.

Setting a User-Selected Rendering Intent

The ColorSync Manager supports the four standard rendering intents defined by the ICC—perceptual, relative colorimetric, saturated, and absolute colorimetric. Every profile supports these four intents, which are commonly used to match the colors of a source image to the color gamut of the destination device in the most optimum way for the type of image. These intents are described in detail in Rendering Intents.

If the source profile is embedded with the image, the source profile specifies the rendering intent to be used. However, if the source profile is not available and the system profile must be used as the source profile, you should allow the user to select the rendering intent to be used.

To allow users to choose the rendering intent most appropriate for color matching a graphical image, you can provide a pop-up menu or a dialog box identifying the rendering intent options available. By providing a description of the available rendering intents, you can help a user select the rendering intent that best maintains important aspects of the image.

Color professionals and technically-sophisticated users are likely to be familiar with the ICC terms for rendering intent and the gamut-matching strategies they represent. If your application is aimed at novice users, however, you may prefer to use a simplified terminology based on the typical image content associated with a rendering intent, as described in Table 3-1. For example, you might note the following:

• For perceptual matching, all the colors of a given gamut may be scaled to fit within another gamut. This intent is the best choice for realistic images, such as scanned photographs.

• For saturation matching, the relative saturation of colors is maintained from gamut to gamut. Rendering the image using this intent gives the strongest colors and is the best choice for bar graphs and pie charts, in which the actual color displayed is less important than its vividness.

• For relative colorimetric matching, the colors that fall within the gamuts of both devices are left unchanged. Some colors in both images will be exactly the same, a useful outcome when colors must match quantitatively. This intent is best suited for logos or spot colors.

• For absolute colorimetric matching, a close appearance match may be achieved over most of the tonal range, but if the minimum density of the idealized image is different from that of the output image, the areas of the image that are left blank will be different. Colors that fall within the gamuts of both devices are left unchanged.

After the user selects the intent to be used, you must modify the renderingIntent field of the system profile’s header to reflect the choice. To put the rendering intent chosen by the user in the profile header, follow these steps:

1. Obtain a profile reference to the system profile.

   Identifying the Current System Profile describes how to do this.

2. Get the profile header of the system profile.

   You call the function CMGetProfileHeader, passing the profile reference, to obtain the profile’s header. The function returns the profile header using a union of type CMAppleProfileHeader. You can use this function for both ColorSync 1.0 profiles and version 2.x profiles.

   For a version 2.x profile, you use the data structure CM2Header. For a version 1.0 profile, you use the CMHeader data structure. For more information on profile headers, see ColorSync Manager Reference.

3. Assign the new rendering intent to the header field.

   To assign a rendering intent to the system profile header’s renderingIntent

   enum { cmPerceptual= 0, cmRelativeColorimetric= 1, cmSaturation= 2, cmAbsoluteColorimetric= 3 }; field, use the constants defined by the following enumeration:

   These constants are described in ColorSync Manager Reference.

4. Set the modified profile header of the system profile.

   After you assign the rendering intent, you must replace the header by calling the function CMSetProfileHeader. You can use this function to set a header for a version 1.0 or a version 2.x ColorSync profile. You pass the header using the union CMAppleProfileHeader.

You can now use the system profile to create a color world for the color-matching process. For information on how to create a color world, see Creating a Color World to Use With the General Purpose Functions.

Listing 5-1 includes code that uses the cmSaturation constant to set the rendering intent for a profile.