A Contrail is a trail of smoke that forms when a self-propelled projectile emits exhaust. Although contrails are generally produced by such projectiles, they can also be produced by characters or weapons. Contrails can even be placed in a level by the game designer, so some may be present when a level begins.
Whatever their source, contrails are usually produced at standard intervals. When several contrails are emitted, they combine to form a visible smoke trail.
Both the appearance of a contrail and its physical properties depend on the contrail's contrail type. Each contrail of a given contrail type exhibits the same visual and physical properties. A contrail type includes a single animation sequence consisting of any number of frames. When all of the frames have been displayed, the contrail eithar disappears or continues to display its animation frames in an endless loop.
The physical properties of contrails are limited to gravitational acceleration, horizontal deceleration, and draw order. More sophisticated behavior can be achieved through the use of animated scenery objects, which can move along predetermined paths. Contrails, however, have the benefit of being able to appear in a level in large quantities while a game is already in progress.
Information on editing contrail types is available in the Contrail Creation section. Contrails may be added to levels and manipulated as described in the Adding Level Objects and Level Object Manipulation sections.