Most games involve bodies behaving in ways that defy the laws of physics: floating platforms, elevators, playable characters, etc. This is why kinematic bodies exist: they offer a total control over the body’s trajectory since they are completely immune to forces or impulses (like gravity, contacts, joints).
But this control comes at a price: it is up to the user to take any obstacle into account by running custom collision-detection operations manually and update the trajectory accordingly. This can be very difficult. Detecting obstacles usually rely on ray-casting or shape-casting, used to adjust the trajectory based on the potential contact normals. Often, multiple ray or shape-casts are needed, and the trajectory adjustment code isn’t straightforward.
The Kinematic Character Controller (which we will abbreviate to character controller) is a higher-level tool that will emit the proper ray-casts and shape-casts to adjust the user-defined trajectory based on obstacles. The well-known move-and-slide operation is the main feature of a character controller.
Despite its name, a character controller can also be used for moving objects that are not characters. For example, a character controller may be used to move a platform. In the rest of this guide, we will use the word character to designate whatever you would like to move using the character controller.
Rapier provides a built-in general-purpose character controller implementation. It allows you to easily:
- Stop at obstacles.
- Slide on slopes that are not to steep.
- Climb stairs automatically.
- Walk over small obstacles.
- Interact with moving platforms.
Despite the fact that this built-in character controller is designed to be generic enough to serve as a good starting point for many common use-cases, character-control (especially for the player’s character itself) is often very game-specific. Therefore the builtin character controller may not work perfectly out-of-the-box for all game types.
A new character controller can be created and removed by the physics
Note that the character controller does not store a reference to the rigid-body and collider it controls. Therefore, the
same instance of the
CharacterController class can be used to control different colliders. This can be useful if
you want to apply the same kind of character control settings to multiple characters.
The created character controller can then be used to control the movement of a collider taking into account obstacles on its path. This is done in two steps:
- Given a desired translation, compute the actual translation that we can apply to the collider based on the obstacles.
- Read the result and apply it to the rigid-body or collider (if it isn’t attached to a rigid-body) by setting its position, kinematic velocity, or next kinematic position, depending on the situation.
The recommended way to update the character’s position depends on its representation:
- A collider not attached to any rigid-body: set the collider’s position directly (with
collider.setTranslation) to the corrected movement added to its current position.
- A velocity-based kinematic rigid-body: set its velocity (with
rigidBody.setLinvel) to the computed movement divided by the timestep length.
- A position-based kinematic rigid-body: set its next kinematic position (with
rigidBody.setNextKinematicTranslation) to the corrected movement added to its current position.
The character’s shape may be any shape supported by Rapier. However, it is recommended to either use a cuboid, a ball, or a capsule since they involve less computations and less numerical approximations.
The built-in character controller does not support rotational movement. It only supports translations.
For performance and numerical stability reasons, the character controller will attempt to preserve a small gap between
the character shape and the environment. This small gap is named
offset and acts as a small margin around the character
shape. A good value for this offset is something sufficiently small to make the gap unnoticeable, but sufficiently large
to avoid numerical issues (if the character seems to get stuck inexplicably, try increasing the offset).
Is is not recommended to change the offset after the creation of the character controller.
The up vector instructs the character controller of what direction should be considered vertical. The horizontal plane is the plane orthogonal to this up vector. There are two equivalent ways to evaluate the slope of the floor: by taking the angle between the floor and the horizontal plane (in 2D), or by taking the angle between the up-vector and the normal of the floor (in 2D and 3D). By default, the up vector is the positive y axis, but it can be modified to be any (unit) vector that suits the application.
- Example 2D
- Example 3D
If sliding is enabled, the character can automatically climb slopes if they are not too steep, or slide down slopes if they are too steep. Sliding is configured by the following parameters:
- The max slope climb angle: if the angle between the slope to climb and the horizontal floor is larger than this value, then the character won’t be able to slide up this slope.
- The min slope slide angle: if the angle between the slope and the horizontal floor is smaller than this value, then the vertical component of the character’s movement won’t result in any sliding.
As always in Rapier, angles are specified in radians.
If enabled, the autostep setting allows the character to climb stairs automatically and walk over small obstacles. Autostepping requires the following parameters:
- The maximum height the character can step over. If the vertical movement needed to step over this obstacle is larger than this value, then the character will be stopped by the obstacle.
- The minimum (horizontal) width available on top of the obstacle. If, after the character is teleported on top of the obstacle, it cannot move forward by a distance larger than this minimum width, then the character will just be stopped by the obstacle (without being moved to the top of the obstacle).
- Whether or not autostepping is enabled for dynamic bodies. If it is not enabled for dynamic bodies, the character won’t attempt to automatically step over small dynamic bodies. Disabling this can be useful if we want the character to push these small objects (see collisions) instead of just stepping over them.
The following depicts (top) one configuration where all the autostepping conditions are satisfied, and, (bottom) two configurations where these conditions are not all satisfied (left: because the width of the step is too small, right: because the height of the step is too large):
Autostepping will only activate if the character is touching the floor right before the obstacle. This prevents the player from being teleported on to of a platform while it is in the air.
If enabled, snap-to-ground will force the character to stick to the ground if the following conditions are met simultaneously:
- At the start of the movement, the character touches the ground.
- The movement has no up component (i.e. the character isn’t jumping).
- At the end of the desired movement, the character would be separated from the ground by a distance smaller than the distance provided by the snap-to-ground parameter.
If these conditions are met, the character is automatically teleported down to the ground at the end of its motion. Typical usages of snap-to-ground include going downstairs or remaining in contact with the floor when moving downhill.
It is possible to let the character controller ignore some obstacles. This can be achieved by setting the
optional arguments of the
filterFlags: to exclude whole families of obstacles (e.g. all the colliders attached to dynamic rigid-bodies).
filterGroups: filter based on the colliders collision groups.
filterPredicate: an arbitrary closure to filter-out colliders based on user-defined rules.
As the character moves along its path, it will hit grounds and obstacles before sliding or stepping on them. Knowing what collider was hit on this path, and where the hit took place, can be valuable to apply various logic (custom forces, sound effects, etc.) This is why a set of character collision events are collected during the calculation of its trajectory.
The character collision events are given in chronological order. For example, if, during the resolution of the character motion, the character hits an obstacle A, then slides against it, and then hits another obstacle B. The collision with A will be reported first, and the collision with B will be reported second.
Unless dynamic bodies are filtered-out by the character controller’s filters, they may be hit during the resolution of the character movement. If that happens, these dynamic bodies will generally not react to (i.e. not be pushed by) the character because the character controller’s offset prevents actual contacts from happening.
In these situations forces need to be applied manually to there rigid-bodies. The character controller can apply these forces for you if needed:
Since you are responsible for providing the movement vector to the character controller at each frame, it is up to you to emulate gravity by adding a downward component to that movement vector.