Attractors/repulsors

Effective navigation assistance may be obtained by using attractors or repulsors, where the camera is literally attracted to (or repulsed from) pre-defined positions, regions, or other game objects. Often these positions are defined explicitly using script objects that are placed by the camera designer. Alternatively, offline analysis can be made of the environment to determine automatic placement; in practice this can be a difficult prospect without explicit rules governing their usage.

Both attractors and repulsors function by applying forces to the camera depending on the proximity (and possibly orientation) of the player or camera.

Note that these forces may not be implemented as part of the physical simulation (even if such a facility is available); rather they may simply influence the determination of the desired position or act as hard constraints during the actual camera motion.

The use of proximity as part of the calculation of forces suggests that the attractor or repulsor is spherical in nature, but this is not necessarily appropriate. Cylindrical shapes are frequently the most useful, often with their major axis aligned to the world up-vector.

Simplistic shapes are common because of the relatively low cost in the calculation of the proximity of game objects, and thus the forces required. Spheres, cylinders, and planes are common choices.

One specific example that is often problematic for cameras is navigation through doorways, particularly if the character moves quickly. Other than the problems of ensuring that the door does not physically close either between the camera and its target or on top of the camera, doorways are frequently a problem because they are often confined spaces and there are many opportunities for the player character to move in ways that may cause the camera to be left behind.

Vertical doorways are the most common case encountered, where the door is aligned with the up-axis of the game world. Horizontal doorways have additional challenges especially regarding camera orientation due to the potential for gimbal lock. Additional problems occur when external events may cause the door to close rapidly, leaving the camera behind the closed door (or worse, interpenetrating the door geometry).

To cope with such situations it is generally preferable that the door is held open while the camera is on the opposite side compared to the player, but this is not always possible. Thus, it is often necessary to implement a fail-safe to ensure that the camera is never left on the wrong side of a closed door.

If the camera system is able to track the proximity of the camera to doorways or otherwise be informed of how the camera is positioned and oriented relative to the doorway (e.g., by means of a trigger volume), an axis-based attractor may be used to help guide the camera motion through the doorway.

A proportional attractor can apply forces to the camera dependent upon the camera position relative to the axis of desired motion through the doorway. Care must be taken to ensure that these forces do not impart oscillation and that they are only applied when appropriate.

Considering the general case once again, the direction in which forces are applied by attractors or repulsors is also variable. Common usage would have the forces acting in relation to the physical shape of the attractor or repulsors; for example, a spherical attractor would pull objects toward its origin.

They can be based on a world-relative orientation; a cylindrical repulsor may determine its force from the player character's proximity to the central axis of the cylinder yet apply the repulsion force based on a fixed world orientation.

Attractors and repulsors are sometimes attached to (or form part of) other game objects. Thus, they may vary in their position and effects.

As with any force-based solution, care must be taken to ensure that such objects only apply their forces when appropriate. Stout discusses this in detail. Ensuring that forces do not adversely influence the camera is the most difficult part of their usage.

Cameras do differ somewhat from AI objects in this determination however, since there are additional aesthetic considerations to consider (such as the requirement to ensure smooth motion whenever possible). Thus if the camera incidentally moves past an attractor, it is not necessarily appropriate that it should be influenced. If the camera were moving away from the attractor, for example, we would not wish to apply any forces.

By careful choice of the attractor positions, camera motion can be made to avoid geometry or pass through confined environments without the necessity for complicated hand-scripted camera solutions. Indeed, the attractor or repulsors can be combined with regular game objects (such as doors) to assist camera motion without any additional scripting support.