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Sprite Track Media Format

The sprite track media format is hierarchical and based on QT atoms and atom containers. A sprite track is defined by one or more key frame samples, each followed by any number of override samples. A key frame sample and its subsequent override samples define a scene in the sprite track. A key frame sample is a QT atom container that contains atoms defining the sprites in the scene and their initial properties. The override samples are other QT atom containers that contain atoms that modify sprite properties, thereby animating the sprites in the scene. In addition to defining properties for individual sprites, you can also define properties that apply to an entire sprite track.

Figure 3-13 shows the high-level structure of a sprite track key frame sample. Each atom in the atom container is represented by its atom type, atom ID, and, if it is a leaf atom, the type of its data.

The QT atom container contains one child atom for each sprite in the key frame sample. Each sprite atom has a type of kSpriteAtomType. The sprite IDs are numbered from 1 to the number of sprites defined by the key frame sample (numSprites).

Each sprite atom contains leaf atoms that define the properties of the sprite, as shown in Figure 3-14. For example, the kSpritePropertyLayer property defines a sprite’s layer. Each sprite property atom has an atom type that corresponds to the property and an ID of 1.

In addition to the sprite atoms, the QT atom container contains one atom of type kSpriteSharedDataAtomType with an ID of 1. The atoms contained by the shared data atom describe data that is shared by all sprites. The shared data atom contains one atom of type kSpriteImagesContainerAtomType with an ID of 1 (Figure 3-15).

The image container atom contains one atom of type kImageAtomType for each image in the key frame sample. The image atom IDs are numbered from 1 to the number of images (numImages). Each image atom contains a leaf atom that holds the image data (type kSpriteImageDataAtomType) and an optional leaf atom (type kSpriteNameAtomType) that holds the name of the image.

In this section:

Sprite Media Format Atoms
Sprite Media Format Extensions
Sprite Track Property Atoms

Sprite Media Format Atoms

The sprite track’s sample format enables you to store the atoms necessary to describe action lists that are executed in response to QuickTime events. “QT Atom Container Description Key” defines a grammar for constructing valid action sprite samples, which may include complex expressions.

Both key frame samples and override samples support the sprite action atoms. Override samples override actions at the QuickTime event level. In effect, what you do by overriding is to completely replace one event handler and all its actions with another. The sprite track’s kSpriteTrackPropertySampleFormat property has no effect on how actions are performed. The behavior is similar to the default kKeyFrameAndSingleOverride format where, if in a given override sample there is no handler for the event, the key frame’s handler is used, if there is one.

Sprite Media Format Extensions

This section describes some of the atom types and IDs used to extend the sprite track’s media format, thus enabling action sprite capabilities.

A complete description of the grammar for sprite media handler samples, including action sprite extensions, is included in the section “Sprite Media Handler Track Properties QT Atom Container Format.”

Sprite Track Property Atoms

The following constants represent atom types for sprite track properties. These atoms are applied to the whole track, not just to a single sample.

Constant descriptions

kSpriteTrackPropertyHasActions

You must add an atom of this type with its leaf data set to true if you want the movie controller to execute the actions in your sprite track’s media. The atom’s leaf data is of type Boolean. The default value is false, so it is very important to add an atom of this type if you want interactivity to take place.

kSpriteTrackPropertyQTIdleEventsFrequency

You must add an atom of this type if you want the sprites in your sprite track to receive kQTEventIdle QuickTime events. The atom’s leaf data is of type UInt32. The value is the mimimum number of ticks that must pass before the next QTIdle event is sent. Each tick is 1/60th of one second. To specify “Idle as fast as possible,” set the value to 0. The default value is kNoQTIdleEvents, which means don’t send any idle events.

It is possible that for small idle event frequencies, the movie will not be able to keep up, in which case idle events will be sent as fast as possible.

Since sending idle events takes up some time, it is best to specify the largest frequency that produces the results that you desire, or kNoQTIdleEvents if you do not need them.

kSpriteTrackPropertyVisible

You can cause the entire sprite track to be invisible by setting the value of this Boolean property to false. This is useful for using a sprite track as a hidden button track—for example, placing an invisible sprite track over a video track would allow the characters in the video to be clickable. The default value is visible (true).

kSpriteTrackPropertyScaleSpritesToScaleWorld

You can cause each sprite to be rescaled when the sprite track is resized by setting the value of this Boolean property to true. Setting this property can improve the drawing performance and quality of a scaled sprite track. This is particularly useful for sprite images compressed with codecs that are resolution-independent, such as the Curve codec. The default value for this property is false.

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