QuickTime 4 Reference

Tweening Parameters

An important property of effect parameters is that many can be tweened, and some must be tweened. Tweening is QuickTime's general purpose interpolation mechanism. This allows the value of the parameter to change as the effect executes.

For example, the slide effect built into QuickTime (see "Slide" ) has an angle parameter. This controls the angle from which the second source will slide over the first during the execution of the effect. If this parameter contains a single value, the second source will slide over the first in a straight line from the selected angle. However, if the parameter contains two values, the angle will be interpolated between these values during the execution of the effect. This allows you to specify a curved slide effect.

In fact, any valid tween record can be specified as the parameter value, not just records containing pairs of values. The QuickTime tweening mechanism supports tween records that contain more than two values and that specify the interpolation algorithm used to produce intermediate values. However, the standard parameters dialog box allows only a pair of values to be entered, and the appropriate default interpolator is used. The standard parameter dialog box presents the user with a pair of values for parameters that must be tweened. Parameters that are optionally tweened, such as the angle for the slide effect, are set to a single value by default. In order to set an optionally-tweened parameter to a tweened value, the user must hold down the Option key when selecting the effect in the dialog box.

An application can provide its own user interface for entering multiple tween values for a parameter and choosing an appropriate tweener to perform interpolation, if required.

Note
Effect component authors do not need to write code to handle all the possible combinations of tween record types, as the details of the tween record are handled by using the standard QuickTime tweening APIs. For a complete description of tweening, see Chapter 25, "Tween Components and Native Tween Types."

For more details on specifying which parameter values can contain tween values, see "Parameter Atom Type and ID" . For more details on supporting tweened parameters in your effect component, see "Tweening Parameter Values" .

Refer to "The Parameter Description Format" for a complete description of the possible parameter descriptions you can place in your 'atms' resource.