Wired Movies and Sprites

Key Frame Samples and Override Samples

As discussed, a sprite track is defined by one or more key frame samples, each followed by any number of override samples. A key frame sample for a sprite track defines the following aspects of a sprite track:

The number of sprites in the scene and their initial properties.
All of the shared image data to be used by the sprites in the scene, including image data to be used in the subsequent override samples. Because a key frame sample contains the image data for the scene, the key frame sample tends to be larger than its subsequent override samples.

An override sample overrides some aspect of the key frame sample. For example, an override sample might modify the location of sprites defined in the key frame sample. Override samples do not contain any image data, so they can be very small. An override sample can show or hide a sprite defined in the key frame sample, but it cannot define new sprites or remove sprites defined in its key frame sample. An override sample can override any number of properties for any number of sprites. For example, a single override sample might change the layer and location of sprite ID 3, and hide sprite ID 10.

There are two sprite track formats that define how a key frame sample and its subsequent override samples are interpreted. If the current sample is a key frame sample, the key frame sample alone fully describes the current state of the track. If the current sample is an override sample, the current state may differ depending on the sprite track format:

If the sprite track format is kKeyFrameAndSingleOverride , the current state is defined by the most recent key frame sample and the current override sample. This is the default format. The advantage of this format is that it allows for excellent performance during random access. A sprite track that uses this format can play backwards and drop frames smoothly. The disadvantage of this format is that the file size of the track may be larger than a track that uses the other format.
If the sprite track format is kKeyFrameAndAllOverrides , the current state is defined by the most recent key sample and all subsequent override samples, including the current override sample. This format results in a smaller file size. However, you should not use this format if you want your sprite track to play backwards or drop frames smoothly. When you play a movie that contains a sprite track whose format is kKeyFrameAndAllOverrides , you should configure the movie to play all frames.

A sprite track must be authored exclusively with a single format, i.e., either all with kKeyFrameAndSingleOverride or all with kKeyFrameAndAllOverrides .