This is a part of our series of features on the basics of 3D art and animation. You might want to see the previous pieces on 3D art workflows, what a 3D object is made of, how to smooth or harden 3D models and how to make things move in a 3D scene.
If you need the tools to get started, see our guides to the best 3D modelling software and the best laptops for 3D modelling.
Usually, upon creation, a joint’s orientation is set so that it automatically points to its parent in the hierarchy. So, for example, X will be the primary axis and aim at the next joint in the chain.
This is to make animation more predictable, meaning you get a better twist around the joint. Imagine rotating your upper arm while keeping your elbow in place; this works because, in theory, the axis you’re rotating around runs down the humerus bone. There will be times, however, when you may need a different primary axis. This could depend on a number of things, like the game engine you are working on for example, or to help reduce gimbal lock, something we discussed in last month’s issue.
There will also be times when the axes are reset or even changed by accident, so they need to be corrected. Fortunately, editing rotational axes is very easy to do and you have full freedom to configure them any way you like, and at any angle too. Let’s take a look at how you would approach this in a simple, three-joint arm setup.
01. Broken rotations
(Image: © Antony Ward)
The joints in this simple arm setup have been reset to match the world orientation. The issue is, as you rotate the shoulder, the elbow joint moves away from its original location, meaning this isn’t a natural movement and would prove difficult for an animator to work with.
02. Set up your orientations
(Image: © Antony Ward)
What we need to do is reset the orientations, and in most applications there is a simple tool to help do this. Usually there are three options: Primary Axis is the axis that points down the joint, with Secondary Axis pointing away from the joint. Secondary Axis World Orientation is which direction the secondary axis points in.
03. Reset your orientations
(Image: © Antony Ward)
If we use X as the Primary Axis, and Y as both the Secondary Axis and Secondary Axis World Orientation, our axes are now fixed with X now pointing down the joints and Y pointing up.
Now, when we rotate the shoulder, the elbow remains in place. The same goes for the forearm and the wrist.
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This article originally appeared in ImagineFX. Subscribe to ImagineFX to never miss an issue. Print and digital subscriptions are available.
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Since the early 90s, Antony has worked for many of today’s top game and VFX studios. He has written three technical manuals on Game Character Development in Maya, Game Character Development and 3D Modeling in Silo, all of which have been translated into multiple languages. He is the owner of antcgi.com, and regularly produces both written and video tutorials on technical aspects of game development and VFX.