Pre-defined navigation techniques

Many of the problems associated with camera collision can be avoided by pre-computation of potential camera movement or by using pre-defined assistance based on knowledge of the environment (whether defined explicitly by the camera designer or otherwise automatically pre-calculated).

Camera navigation pre-computation may take many forms, usually at a cost of both memory overhead and time to compute the allowable camera positions, although the latter is not generally as important. Pre-computation typically has an advantage in lower processor overhead during run-time.

Assistance for navigation is often in the form of game script objects placed by designers and therefore tailored to the specific requirements of both game play and environmental concerns. Chapter 6 discusses scripting and its application to camera motion and positioning; however, it is possible to derive navigation assistance information from other game objects than those specifically designed to control camera behavior.

There are a variety of ways in which both types of information may be used to assist camera navigation and collision avoidance. Many of these schemes share common ground with AI or robotic research techniques, and the bibliography outlines some of the relevant papers.

Some of the more common solutions are as follows.

Pre-defined paths

The most common solution for pre-computation is to literally store entire motion paths. Several games have used this effectively since it also allows complete control over camera position (and possibly orientation), irrespective or directly according to player action. Nevertheless, path motion can be somewhat restrictive and could potentially result in the player experiencing a sensation of lack of control.

Conversely, the ability to directly specify the position of the camera is a boon when dealing with motion through a complex environment, especially if the determination of the position can be varied according to player position or action. A full description of path-based camera motion may be found starting with the section Path in Chapter 7.

Path motion behaviors

Although the camera may be constrained by using a path to completely define its motion, the interpretation of how the spline actually constrains the motion offers several alternatives.

• Constrained path camera. A constrained path camera is one that moves precisely along a defined path within the game world. The camera is typically not allowed to leave this path under any circumstance, unless the path is no longer active. This is one of the most typical uses of path cameras, so the camera position may be absolutely controlled to avoid collision or interpenetration with environmental features, and so forth. Non-interactive movies are often driven by this type of camera (as well as stationary or slaved position cameras).
• Follow path camera. In many cases, the motion of a camera on a constrained path may not appear as "organic" or "natural" as required by the game designers, even though the motion along the path may be smooth. However, we may still wish to restrict camera motion in some fashion. A follow path camera provides a mechanism to both restrict camera motion yet allow some variance. In this case, the path only dictates a desired position -- how the camera reaches that desired position can be defined in a number of different ways -- i.e., the camera is not locked directly to the path.

  This may be likened to a "carrot" being dangled in front of the camera. The camera will seek to the location defined by the current evaluation of the path, but is allowed to move in a direct line toward that position (note that this same technique may be applied regardless of the position determination method).
  
  Alternatively, we can use rules to control the distance away from the desired path that the camera is allowed to move; for example, the path now becomes a tube (typically cylindrical) although the cross section may not be circular (e.g., ellipsoidal or rectangular). Indeed, information about the shape of the path volume may be contained within the knots themselves to allow for variance of camera motion along the path (including the radius of the tube).
  
  Another approach is to use a parametric curve with the input parameter as the position of the camera along the path. These methods allow local control over the camera without having multiple paths. When restricting the motion of the camera away from the follow path, it is normally best not to use "hard" constraints as this will likely lead to unwanted motion artifacts. A damped spring or attractor/repulsor may be used to provide a more analog response to the distance restriction.

Pre-defined volumes

Camera motion may also be restricted such that the camera is confined by the extents of a pre-defined volume. The actual definition of the volume is somewhat variable. It may consist of a simple convex shape such as a sphere, cylinder, ellipsoid, or cube. Alternatively it may be derived from a complex collision mesh, much as would other game entities.

Complex shapes, of course, make navigation a more difficult proposition. The desired position within the volume may be defined in a number of different ways. Clearly, we can use the same techniques as mentioned earlier in this chapter regarding player character-relative positions with the additional constraint of remaining within the volume.

This approach is applicable to most of the dynamic determination methods, so that the pre-defined volume constraint is applied after the initial determination has been made. It is typically easier to use fixed dimension volumes, but these could be varied according to game play requirements; for example, the position of the player relative to a reference frame.

In the same manner as path behaviors, directly constraining the camera position may lead to non-continuous motion and often benefits from the use of forces or damped springs to keep the camera within the volume.