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Managing Color With ColorSync


Monitor Calibration and Profiles

Since ColorSync was first introduced, a common question from end users has been "Where is the ColorSync profile for my monitor?" The answer is that because some monitor manufacturers do not supply ColorSync profiles for their products, purchasers of third-party monitors may not have access to a profile that is specific to their monitor. As a result, they are unable to use ColorSync effectively.

Even when a user has a factory-supplied profile, switching to a different monitor setup can reduce the profile's accuracy. For example, if the user changes the monitor's gamma value and white point, the original profile is no longer useful. The user needs to run a calibration application (if one is available) to generate a new ColorSync profile for the new monitor settings.

Figure 2-3 Monitors & Sound Control Panel for ColorSync 2.5

Starting with version 2.5, ColorSync uses the Monitors & Sound control panel to provide a monitor calibration framework to help users obtain the monitor profiles they need. Figure 2-3 shows the new Monitors & Sound control panel. Note that the list of gamma values (Mac Standard Gamma, uncorrected gamma) has been removed because that function is now part of the calibration process.

Note

If you develop a utility that adjusts gamma or modifies other calibration values, you should modify your software so that it uses the monitor calibration framework to gain system-level support.

Setting a Profile for Each Monitor

Because Monitors & Sound displays a panel for each available monitor, a user can also select, for each monitor, a separate profile from the list of available profiles. When a user sets a profile for a monitor in the Monitors & Sound control panel, ColorSync makes that profile the current system profile, as described in Setting Default Profiles . When your application sets a profile for a monitor, it may also wish to make that profile the system profile. If so, it must set the system profile explicitly by calling CMSetSystemProfile . For information on how to set monitor profiles in your code, see Getting and Setting Monitor Profiles by AVID .

Calibration

Calibration and characterization are related terms, but with important differences. Calibration is the process of setting a device's parameters according to its factory standards. This is also known as linearizing or linearization. Characterization is the process of learning the color character of a monitor so that a profile can be created to describe it. Unlike input and output devices, whose calibration and characterization steps are not the same, a monitor is calibrated and characterized in one step.

The Calibrate button on the Monitors & Sound control panel provides the launching point for monitor calibration. A user can calibrate each monitor and create one or more color profiles for each, based on variations in gamma, white point, and so on. For related information on profiles, see Profiles and Devices and Their Profiles .

AppleVision and Apple ColorSync monitors are self-calibrating, so you will not see a Calibrate button for these monitors, unless there is a third-party calibrator installed in your Extensions folder.

Calibrating a monitor can be a challenging task for a naive user, but Apple Computer supplies a default calibrator that leads the user through a series of calibration steps. Using the default calibrator, even a novice should have a reasonable chance for success.

Note

There are limits to the effectiveness of monitor calibration by users. For example, some monitors, due to age or condition, cannot be calibrated, and a small percentage of the user population is color-blind.

The calibration framework uses a plug-in architecture that is fully accessible to third-party calibration plug-ins. When a user clicks on the Calibrate button, the Monitors & Sound control panel provides a list of all available calibrator plug-ins. To appear in the list, a plug-in must meet the following criteria:

You can find source code for sample monitor calibration plug-ins in the ColorSync 2.5.1 SDK. If you plan to create a monitor calibration plug-in, you should read the next section Video Card Gamma .

Video Card Gamma

Starting with version 2.5, ColorSync supports an optional profile tag for video card gamma, which you specify with the cmVideoCardGammaTag constant. The tag specifies gamma information, stored either as a formula or in table format, to be loaded into the video card when the profile containing the tag is put into use. When you call the function CMSetProfileByAVID and specify a profile that contains a video card gamma tag, ColorSync will extract the tag from the profile and set the video card based on the tag.

IMPORTANT

The function CMSetSystemProfile does not retrieve video card gamma data to set the video card. Only the CMSetProfileByAVID function currently sets video card gamma data.

If you provide monitor calibration software, you should include the video card gamma tag in the profiles you create. For information on the constants and data types you use to work with video card gamma, see Video Card Gamma Constants and Video Card Gamma . You can get more information about AVID values from the Display Manager SDK.


© 1988-1999 Apple Computer, Inc. — (Last Updated 20 Jan 99)