Intent

Isolate Model state from Model appearance to minimize impact on controllers when art changes.

Problem

Controllers are often complicated little state machines which interact with Models in very specific ways. It is common for this interaction to change the appearance of the Model, especially in animation controllers. Since art may change frequently (example: more frames are added to an animation) it makes sense to separate the state from the appearance.

Solution

Without an appearance map, a controller is likely to change the "frame" of an animation directly. For example:
if (state == WALKING) {
    model.frame =
        WALK_START_FRAME + WALK_NUM_FRAMES
        * (WALK_DURATION / dt)
    ;
}
In this case, if the animation is changed, the three constants WALK_XXX need to be updated and the game recompiled for the change to take effect.
An appearance map eliminates these constants and replaces them with a lookup. Typically, a table is loaded at game initialize time which encodes the translation from state and delta time ("state") to frame ("appearance").
A game with a built-in editor will probably allow this table to either be edited directly or at least to be re-imported on command.

Structure

Not available at this time.

Issues and Risks

None at this time.

Related Patterns

Appearance Maps translate Model state into an appearance for the View.

Uses and References

Special thanks to Jim Greer.

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Date this article was posted to GameDev.net: 6/19/2001
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Design Patterns