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In the Shift+A Add menu there is an option to add an "Empty".

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I have tried adding various types of them but they don't seem to do anything. They have no other Interaction Modes (Edit mode, Weight Paint mode, etc.) and they don't show up in the render.

What is an empty and what are they useful for?

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If whoever DV'ed sees this I'd love to know why so I can improve. Thanks! – PGmath Jan 29 at 13:39
up vote 25 down vote accepted

An empty is an un-rendered, geometry-less point in space represented by a 3-dimensional symbol of some sort.

There are 8 types of empties, each type differs only in its viewport symbol. The Image type is slightly special in that you can pick any image from your computer to use as its symbol.

What are empties useful for?


Answer, lots! Use an empty any time you want a modifier, object, constraint, particle system, texture, armature, driver, etc. (anything with an object selector box) to do something based on a single point in space.

Though I can't possibly think of, let alone list, every single possible use for an empty, here are a few of the more prevalent examples of uses for an empty. Hopefully by reading this you will get a feel for what they can be used for.

Parenting

Probably the most common use for an empty is to provide a common transformation origin for a group of related/connected objects by parenting them to an empty with Ctrl+P.

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Controlling Modifiers

There are many, many modifiers which can use the location of another object as a reference point for their calculations. Here are a couple of the most common:

Displacement modifier:
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Array modifier:
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Texture mapping

By using the Object mapping coordinates in Cycles you can have the position and transformation of an empty control your texture coordinates. This is most often used to use an empty to position a decal on another object.

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(Here's another example of using an empty's Object coordinates.)

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I also often use an empty as a master object for main scripts when making a game in BGE. It's much easier to remember that an invisible object called 'main' holds them. – Highstaker Jan 29 at 10:40
    
@Highstaker Feel free to add your own answer or edit that into mine. – PGmath Jan 29 at 16:51

My favorite use for an empty is for an invisible thing to track the camera to. Then you can move the empty around and the camera will always point at it. And of course you can animate the empty's location and thereby animate the camera's point of view.

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See also this video which provides a camera rig for realistic camera movement. youtube.com/watch?v=H7CISQFl7kA – lumpynose Feb 2 at 20:03

Empties can also be used with images to set up a background for reference. They can be scaled, rotated and moved so you can position your image anywhere you want. This way you can have multiple background images in the same view.

I usually move them far away so that they only appear in ortogonal view, not perspective.

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