Rods and Ropes

Obi 4.0 introduced the ObiRod actor. These may look similar to ropes at first glance, but they're actually quite different both in behavior and intended use. In this page we will briefly discuss when to use ObiRod, and the differences between rods and ropes.

ObiRod

Rods are linear chains of oriented particles. On the other hand, ropes are made of regular particles that only store a position. This notable difference allows rods to maintain a rest shape, and exhibit torsion resistance (wikipedia). Rods are ideal to model springs, thick bars, antennae, etc.

Since rods use oriented particles, you can choose to fix both the position and the orientation of any particle in the rod to constrain the rod rotation. When using ropes, the only way to achieve this is to fix at least two consecutive particles in the rope. Also, pin constraints will constraint both the position and the orientation of any rigidbody attached to a rod particle.

Unlike ropes, rods cannot be torn or dinamically resized. This table sums up the key differences between ropes and rods:

feature	ObiRope	ObiRod
Rest shape	No	Yes
Torsion resistance	No	Yes
Dinamically resizable (ObiCursor)	Yes	No
Tearable	Yes	No

Rods are setup exactly the same way as ropes. The only key difference is that they will honor curve orientations when initializing their rest shape.