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Filed under: Motion Graphics •

Parenting Skills

Family Trees

Parenting comes in handy for tasks ranging from being moving one layer and having another move with it (such as two lines of text), to creating complex animations where a number of children pirouette around an animating parent. But the real fun comes when you have chains of children.

The first figure on this page shows a multi-jointed robot arm we built by parenting. The smallest appendage was parented to the medium-sized one, which was parented to the largest one, which was parented to the base.

This linkage can be seen by looking under the Parent column in the Time Layout panel below. We can now create simple rotation keyframes for each child, knowing it will automatically stay attached to its parent without having to track it or set position keyframes. Previously, the would have required a chain of nested comps - one per appendage. This makes all sorts of character-style animations significantly easier.

The parenting chain for the robot arm in Figure 1: Each limb is parented to the next larger one in the chain. As a result, we only need to animate their rotations; their positions track automatically.

The golden rule to remember is that life is much easier if you set up the relative positions of the children and parent before you link them together. This is especially true of setting up the anchor points in animations like our robot arm: the anchor point for each arm was set to its pivot point, then the arms were aligned, and then we set up the parenting chain.

Skewing

Parenting usually works like you would intuitively expect. However, there is a nasty gotcha: non-uniform scaling. Remember, the parent casts a reality distortion field over its children. If the parent is non-uniformly scaled - for example, 100% wide but 50% tall - the children will become skewed if they are then rotated on their own, as demonstrated below.

Problems occur when a parent is non-uniformly scaled - i.e. 100% wide but 50% tall - and then a child is rotate. The result is the child being skewed by parent’s scale factors.

It you unlink them in this skewed state, they’ll jump to yet a different size.

The solution to this, and a great tool in general, is the Null Object. A Null is a solid that is invisible. If you link a Null to a parent, and then a child to the Null, the Null acts as a buffer between the two. In the above example, you don’t care if the parent’s non-uniform scale skews the Null, because it’s invisible. As long as you don’t also scale the Null, the child object will be unperturbed. Nulls also make great master parents to group multiple objects to.

In a future column, we’ll introduce the wonderful world of Expressions. At their simplest, they can be thought of as very selective parenting; at their most complex, they allow you program operations you wouldn’t dream of keyframing. Initially daunting, they might be one of the most significant features added to After Effects since it was created..

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