Ropes passing trough a point: ObiPinhole

Cable car (rigidbody) constrained using pinholes that allow it to slide freely along a rope.

Pinholes constrain a rope to pass trough a specific point in space, while still allowing the rope to slide trough that point. This is easier to set up, cheaper and considerably more robust that using colliders for the same purpose.

Pinholes also allow for two-way coupling with rigidbodies (just like dynamic attachments) and are equipped with a motor. You can visualize this as a few motorized rubber discs around the border of the hole, holding the rope in place and forcing it to pass trough at a specific target speed. You can limit the force the motor is allowed to exert on the rope - so that it may find it difficult to pull ropes with heavy loads attached to them - and it's also possible to define the friction between the rope and the rubber discs, so the rope may slip.

You can add ObiPinhole components to a ObiRope or ObiRod. Let's take a look at the properties of the ObiPinhole component:

Target

The transform of the object you want the pinhole to attach the rope to.

Coordinate

This is the initial position of the pinhole along the rope, expressed as a length-normalized coordinate: 0 for the start of the rope, 1 for the end.

In order for the coordinate gizmo (and orange wire sphere) to be visible, gizmos must be enabled in the scene view:

It's important to realize that pinholes do not move the rope to the position of the target object, nor the target object to the position of the rope. Moving the rope to the target's position would result in a "kickback" effect once the rope springs back to its original position. Moving the target could cause a whole palette of issues depending on your game logic.

Instead, pinholes are created at the current position of the point defined by the coordinate parameter, relative to the target object. This makes it simple to work with multiple pinholes, and ensures smooth attachments with no initial "jumps" or sudden motions.

Note you can use the particles API to set particle positions at runtime before creating the attachment.

Friction

Friction between the pinhole's motor and the rope, expressed as a percentage: 0 will allow the rope to slide trough the pinhole with no resistance, regardless of the motor's target speed or maximum force. 1 won't allow the rope to slide at all, it will follow the motor's current speed.

Motor target speed

Target relative speed bewteen the pinhole and the rope. Can have any positive or negative value, to move the rope in either direction. Note that the motor may be unable to reach the target speed if its max force is not large enough to move the mass of the rope and everything attached to it.

Motor max force

Maximum force the motor can exert on the rope.

If you want the motor to be fully in control of the rope, set friction to 1 and motor max force to Infinity. This will ensure the motor can deal with any mass no matter how large, and disallows sliding. Then you can precisely control the speed at which the rope passes trough the pinhole using motor target speed.

Character capsule climbing vines using an infinitely strong motorized pinhole with full friction. The player can move along the vines using WASD controls.

Clamp at ends

Determines the behavior of the pinhole when reaching either end of the rope: when enabled, the rope will stop at the pinhole. When disabled, the rope will slide out of the pinhole.

When you cut/tear a rope, both sides of the cut are considered "ends" so the rope will also clamp/slide out pinholes at these areas. Additionally, closed ropes have no ends, so they can slide trough a pinhole indefinitely - or until you cut/tear the rope open.

Clamp at ends enabled: rope can't pass trough pinholes when cut.

Clamp at ends disabled: rope slides out of pinholes when cut.

Break threshold

Amount of force that must be exceeded for the pinhole to break.

Tips and tricks

Here's some helpful tips that will help you use pinholes effectively:

Constraining orientation

Pinholes do not constrain particle orientation: a rope and an object mutually constrained via a single pinhole are free to rotate around as they please. However, there's a trick to leave only one relative rotational degree of freedom: place 2 pinholes close to each other. Since the rope is forced to pass trough both pinholes, rotation can only take place around the imaginary line joining them.

A similar trick is often used with attachments.

A single pinhole would allow the cable car to spin around freely, but 2 pinholes keep it parallel to the cable.

Geometrical friction

Ropes in Obi are represented by a finite amount of particles joined by elements. Because of this, curvature does not change smoothly along a rope. Very sharp corners in the rope can become stuck in a pinhole when forces striving to pull the rope trough the pinhole are not large enough to overcome the corner:

Ropes with low bending compliance and low maximum bending - that is, ropes that cannot bend much - are less likely to be affected by this as they don't usually have sharp corners. However, if your rope must bend easily and may have sharp (close to or less than 90º) corners in it, it may be a good idea to place sphere/capsule colliders around the pinhole to smooth these corners out as they approach it. The rope will then have a much easier time sliding trough the pinhole.

Cube rigidbody attached to the rope using a pinhole. The pinhole is stuck at a very acute angle in the rope, the rope is taut at the right side of the pinhole but the left side has some slack.

Adding a cylinder collider spreads out the rope, widening the angle between consecutive rope elements near the pinhole and allowing it to slide freely.

Be creative

Pinholes allow for lots of use cases: ziplines, motorized cable cars, climbable vines, complex cranes, etc. Experiment with different settings to come up with exciting contraptions.