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Another key concept around hierarchies in animation is the idea of a reference coordinate system. In 3-D you know that you have a grid that has an X-direction or Y-direction and Z-direction. So that is a reference coordinate system, and it's called the World coordinate system. And so, for example, if I grab a Move tool, select an object and move it, I am moving it relative to the World axis. And you can even see that my Move tool is aligned with the axis of the World.
And by the way the current orientation of the World axes is indicated in the lower left-hand corner of each viewport. So that's the World coordinate system. And it's very useful for moving objects around. But 3-D has multiple coordinate systems in it. So when you move or rotate an object you can choose different points of reference around which you can transform that object. So, for example, with rotations, let's say I choose the Rotate tool and I grab this turntable and rotate that around.
You can turn that in some direction. And let's say I want to select the shoulder object. And let's say I want to turn it so that it's going to face downward. Well, if you look closely here now you'll see that my Rotate gizmo is still aligned with the World. Okay, so I am going to my Top view, Alt+W. And let me zoom out a little bit here, and we'll hit G so we can see the grid. What you'll see here is that the Rotate gizmo is still aligned with the World.
And because of that I won't be able to make this thing bend down the way that want it to. Get a little bit closer here. And what I want is for to rotate directly in a diagonal here. Well, it's going in this weird kind of strange, funny direction. I'm not able to make it turn straight the way that I want it to. The Rotate gizmo is aligned with the World, but my object is not. What I want to do here is I want to rotate the object relative to its own Reference coordinate system.
So in other words, I want to rotate it in the space of the object, rather than in the space of the World. So that's where different reference coordinates systems come in. On your main toolbar, right next to the Transform tools you will see a pulldown list that normally says View by default. And that's where I choose the reference coordinate system. And by the way, the reference coordinate system is sticky for each one of the transforms. So if I choose a different reference coordinate system for rotate, that won't change my setting for the Move tool.
So what I want here in this case for animating rotations is the Local coordinate system. So Local coordinates are in the space of the object. So I'll choose Local, and you'll now see that my Rotate gizmo is oriented with the object, not with the World axes. And with my Rotate tool in Local coordinates, I can predictably control the rotations here without having any unpleasant surprises. So I can, for example, select this wrist object here and have that twist around correctly.
I'll undo that, whereas if I was in the default View coordinate system there's no way I could even do that at all. It's just going to do crazy stuff. So almost always when you're animating rotations, you are almost always going to be in Local coordinate system. And now we are back to having the control that we need. So there are several different coordinate systems here. The main ones that you'll work with are View, World and Local. I just want to mention quickly what these are.
If I go over to the Move tool, you'll notice it switches back to View. What is the View coordinate system? Well, essentially View coordinate system is the space of the viewport itself. So you'll see in the Top view my Move gizmo is showing Y pointing upward and X pointing to the right. And we're seeing the same thing down here, which is an indicator of the absolute space of the World. So in this case in the Top view it just so happens that the View coordinate system and the World coordinate system are identical.
But if I go to the Left view here, you'll see that X is pointing to the right and Y is pointing up, but we are having a completely different result than in the World coordinate. So in other words World coordinates are absolute and View coordinates are relative to whatever view you are in. So I then go over to my Front view, I can press the Z key to zoom in. You'll see once again in the Front view, X is always pointing to the right. Y is always pointing upward in the View coordinate system. But that doesn't necessarily line up with the World coordinates.
So essentially the View coordinate system operates in screen space, except if you're in Perspective view. So if you're in a Perspective view, and you've chosen View coordinates, then it's operating in World coordinates. I can choose World coordinates, and now in all views, if I right-click on any one of these views and select my object, you'll see that the Transform gizmo is always aligned with the World coordinates as seen in the lower left-hand corner.
So in general, when moving objects, you're typically using the View coordinate system or maybe the World coordinates, but when rotating, you are almost always going to use Local coordinates.
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