Sprite Sample Data

QuickTime Movie File Format Specification, May 1996

Sprite Sample Data

All sprite samples are stored in QT atom structures. The sprite media uses both key frames and differenced frames. The key frames contain all of the sprite's image data and the initial settings for each of the sprite's properties.

A key frame always contains a shared data atom of type 'dflt' . This atom contains data to be shared between the sprites, consisting mainly of image data and sample descriptions. The shared data atom contains a single sprite image container atom, with an atom type value of 'imct' and an ID value of 1.

The sprite image container atom stores one or more sprite image atoms of type 'imag' . Each sprite image atom contains a video sample description immediately followed by the sprite's compressed image data. The sprite image atoms should have ID numbers starting at 1 and counting consecutively upward.

The key frame must also contain definitions for each sprite in atoms of type 'sprt' . Sprite atoms should have ID numbers that start at 1 and count consecutively upward. Each sprite atom contains a list of properties. Four of the properties are required: Image index, Matrix, Layer, and Visibility. Table 0-10 shows all currently defined sprite properties.

Table 10 Sprite properties
Property Name	Property Value	Description
Matrix	1	Specifies a transformation matrix that describes the sprite's location, scaling, and so on.
Layer	5	Contains a 16-bit integer value specifying the layer into which the sprite is to be drawn (lower layers are rendered on top of higher layers)
Visibility	4	Contains a 16-bit integer that controls the sprite's visibility: set to 1 if the sprite is visible; 0 if not
Graphics mode	6	Contains a 32-bit integer specifying the sprite's graphics mode; this value is always followed by a 48-bit RGB value
Image index	100	Contains the atom ID of the sprite's image atom

The frame difference sample differs from the key frame sample in two ways. First, the frame difference sample does not contain a shared data atom. All shared data must appear in the key frame. Second, only those sprite properties that change need to be specified. If none of a sprite's properties change in a given frame, then the sprite does not need an atom in the differenced frame.

The frame difference sample can be used in one of two ways: the frame differences can be combined, as with video key frames, to construct the current frame; or the current frame can be derived by combining only the key frame and the current frame difference.