Compound Rigid Bodies

Rigid bodies can be made of one or more objects, or primitives. For instance, if you assign physical properties to an object in a scene and add it to a Rigid Body Collection, we say that the rigid body has one primitive. However, reactor can also simulate a rigid body made up of more than one primitive. To do this, you need to group objects together using the 3ds Max Design Group menu. You can then add the group to the collection, and the objects in the group then become the primitives that make up the rigid body.

Creating such compound rigid bodies can be useful, as it is much faster to simulate a rigid body made up of convex pieces than to simulate a more complex concave shape. In addition, the primitives that make up a compound rigid body can have different masses, so you can create rigid bodies that have nonuniform mass distribution. For example, you can create a hand axe where the head is heavier than the handle, so that if you toss the axe it spins around its head.

Rigid body as a compound of three objects

You can do this because the Mass property on the Rigid Body Properties dialog is assigned to a specific primitive rather than an entire rigid body, unlike most of the other properties in this dialog. For single-primitive rigid bodies, this distinction is unimportant. However, in the case of a compound rigid body, the group parent is the rigid body and cannot have a Mass value assigned: Mass can be assigned only to its children, the primitives.

Simulation geometry is also defined at a primitive level, so you can combine concave and convex hull geometries in a single rigid body.


To create a compound rigid body:

  1. Arrange the objects to use to make up your compound rigid body.
  2. Select all of the objects.
  3. From the Group menu, choose Group.

    Your objects are now all child objects of a new group parent. You can add this parent object to a Rigid Body Collection.

To edit compound rigid body properties:

  1. With the compound rigid body selected, choose Group Open.
  2. To edit the compound rigid body’s properties, select the parent (dummy) and go to the properties rollout of the utility or open the Rigid Body Properties dialog. You cannot edit the Mass or Simulation Geometry values for a group parent because they are primitive properties.
    TipIt's easier to select a group parent using the Select From Scene dialog with Display Display Children enabled: The group parent is displayed above its members.
  3. To edit one of the rigid body’s primitive’s properties, select the appropriate child and go to the Properties rollout of the utility or open the Rigid Body Properties dialog. You can give this object a Mass value and, if you wish, change its Simulation Geometry setting. Specifying any other rigid body properties for a child object has no effect unless you remove the object from the group.