Arcane University:Nifskope Prismatic Constraints
Prismatic constraints allow a rigidbody to slide along a fixed line between two points. They are sometimes called rail constraints in other engines as they are alike a train moving along a rail. The prismatic constraints used by the Creation Engine optionally allow the sliding rigidbody to rotate about as it slides, though it is more common to use a fixed rotation.
https://i.imgur.com/5zrFQB0.gif (TODO get this uploaded. I don't have permissions I think)
Contents
Axis Configuration
For A & B each, a prismatic constraint has three properties that control the axis that the target rigidbody moves along:
- Sliding: Controls the axis which the rigidbody slides along.
- Rotation: Lock or constrain the rotation axes allowed for the rigidbody
- Plane: Reference plane for rotation
Bethesda rarely (if ever) uses prismatic constraints in their games, so third-party tooling support is similarly minimal. The Havok > A -> B option usually available to constraints in NifSkope, for instance, does not work, and the B properties must be set manually.
The direction of the prismatic constraint can be inverted by multiplying all properties by -1. This is mostly useful for editor intuition (e.g. make max distance apply to the forward direction instead of backwards) since the constraint position cannot be read in-game without the use of the script extender.
| Property | X Axis Slide | Y Axis Slide | Z Axis Slide |
|---|---|---|---|
| Sliding A & B | (1, 0, 0) | (0, 1, 0) | (0, 0, 1) |
| Rotation A & B | (0, 1, 0) | (0, 0, 1) | (1, 0, 0) |
| Plane A & B | (0, 0, 1) | (1, 0, 0) | (0, 1, 0) |
Disjointed A & B Properties
TODO
Other Properties
Motors
Practical Use
Rigidbodies connected to prismatic constraints can be moved by being pushed by other objects, physics-inducing projectiles (arrows) / magic effects (explosions, shouts), and through the activate drag. The telekinesis spell cannot target havok-enabled rigidbodies unless they can be picked up into the inventory. Barring VR, it is hard to move small objects on prismatic constraints with finesse (partly why Bethesda rarely uses them), which relegates them to be used mostly for larger objects. And while constraints can't be manipulated in-game without the script extender, one option is to create multiple copies of an object with different constraint configurations and disable/enable the ones you need based on script conditions.
Example uses:
- Minecart that can be pushed along a rail (the angle of the track cannot change since prismatic constraints have a fixed axis)
- Hide secret puzzles (no activation prompt) that require the player to notice that a large object can be pushed by happenstance with the player's body into a trigger volume
- Create a platform that starts to sink into lava as the player jumps on it & then returns to its starting position when the player leaves (using a motor with just barely enough force to keep it up under its own weight), or can only be traversed safely one person at a time
