Hierarchies

One of the most useful tools in producing computer animation is the ability to link objects together to form a chain. By linking one object to another, you create a parent-child relationship. Transforms applied to the parent are also transmitted to child objects. A chain is also referred to as a hierarchy.

Left: A disassembled robotic arm is linked into a hierarchy.

Right: The assembled robotic arm uses rotational joints.

You can find the commands to build and manipulate hierarchies in the following places in the interface:

Common Uses for Hierarchies

Parts of a Hierarchy

The relationship between objects linked together in a hierarchy is analogous to a family tree.

Parent

Object that controls one or more children. A parent object is often controlled by another superior parent object. In the following figure, objects 1 and 2 are parent objects.

Child

Object controlled by its parent. A child object can also be a parent to other children. In the following figure, objects 2 and 3 (the support and hub) are children of object 1. Objects 5 (the seats) are children of object 4, the Ferris wheel.

Ancestors

Parent and all of the parent’s parents of a child object. In the following figure, objects 1 and 2 are ancestors of object 3.

The seats of the Ferris wheel are children of the wheel, which is in turn a child of the base and support objects, as shown in the following hierarchy.

Descendants

Children and all of the children’s children of a parent object. In the figures, all the objects are descendants of object 1.

Hierarchy

Collection of all parents and children linked together in a single structure.

Root

Single parent object that is superior to all other objects in the hierarchy. All other objects are descendants of the root object. In the figures, Object 1 is the root.

Subtree

All the descendants of a selected parent. In the figure below, the Rotational Hub, Ferris Wheel, and Seats represent the subtree under the Support object.

Example of a hierarchical structure:

1. Root

2. Leaves

3. Subtree

Branch

Path through the hierarchy from a parent to a single descendant. In the figure above, the Support, Rotational Hub, and Ferris Wheel objects comprise a branch from the root to the leaf objects (the seats).

Leaf

Child object that has no children. The lowest object in a branch. In the figure above, the Seat objects are leaf objects.

Link

Connection between a parent and its child. A link transmits position, rotation, and scale information from parent to child.

Pivot

Defines the local center and coordinate system for each object. You can think of a link as the connection between the pivot of a child object and the pivot of its parent.