Groups VS Heirarchy

Not all, but most programs will have different ways of attaching objects to one another, either as a single grouped object or within an object relationship heirarchy. This has the benefit of being able to move several objects at once or being able to apply several effects, deforers, and/or textures to several objects at once.

Setting up a heirarchy structure involves attaching one or more objects to another object. In this case objects acting as "children" are linked to other objects which act as a "parent". This is typically visualized in a tree structure as seen to the right. It's very similar to the way you've probably seen files arranged within folders within various operating system file explorers.

Groups

Most programs will make a distinction between groups and objects in parent / child hierarchies.

In most cases a "group" of objects simply means that those objects will move, and be affected by modifiers as if they were one object, when they are not.

Some programs (like 3DS Max) require you to remove an object from a group to modify it individually however, making them somewhat tedius to work with. Because of this it can be more convenient to simply define a group by making every object a child of a single "dummy" object. That way you can take advantage of some properties, such as deformers, which are can typically be applied to a parent and propegated to children and also be able to move the entire group just by moving the single "dummy" object.

Hierarchies

So what exactly is a parent child relationship? It might be easiest to note the image to the right. Here we see several "child" objects which share a single "parent" object as visualized by a tree structure.

As you can see, parents can have as many children as they want, but children can only have one parent.

Grouping objects under a common parent object like this has several benefits for movement, visibility, and animation. Let's look at each in more depth.

Moving parented objects

We can move several objects at once and maintain their distance and orientation to one another when we move a parent object.

This is especially beneficial during rotation operations. Moving each object in a set to the left 10 units is easy enough. But what if we want those same objects to rotate around a single point? What's the math involved there?

Let's say we had a set of items we want to rotate around a single point. In this case the location of the parent is crucial because that will be our rotational axis.

The following images illustrate this. In these images the red cubes are children of a single null object (AKA a "dummy" or "locator" One of these terms sure is nicer than the other. ). In this case the properties of the cubes themselves are not being modified. But we have made them into children of a null object which is being modfiied.

The hierarchy used.
The initial position of all of our objects.
The positions after we have rotated the null object (without even touching the red cubes themselves).

Visibility

We can toggle the visibility of several objects at once just by toggling the parent. Take advantage of this to help viewing effeciency when working on a scene. This makes "dummy" parents useful even if you aren't using them to move or add modifiers / effects to the children.

Animation

Animation of objects with independent parts (such as machines / robots) becomes much easier. If you can modify an entire limb at once you can save great. This is something explained further in the animation secion.