Exploring joint rotational axis

00:03

Now, let's dig a little deeper into the world of joints.

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Here, you can see we have a series of orientation settings.

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These control how the joint's axis will be created.

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If I enable orient joint to world,

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you see the other options become disabled.

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What this is going to do is create each joint but make its axis match the world axis.

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So the axis you see down here,

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let's create three joints again,

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I'll do them like this. So it represents an arm with the shoulder, elbow and wrist.

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Now, if I select the joints, you can see that the axis is the same as the world axis.

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Usually when joints are created, the axis is automatically adjusted.

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So the joint points to the next one in the chain.

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So having the axis set up like this is no good for our joint chain.

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If I twist the root, you will see that the elbow moves up and down,

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but we need it to remain still and just rotate or twist with the root.

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So we just want it to turn

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and not change position,

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the same with the middle joint or the elbow joint

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rotating. This causes the wrist to move.

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OK. Let's delete those and add in the correct orientations.

01:25

And let's turn this off. Now

01:27

let's create the chain again. But with these default settings,

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you see

01:34

Maya has now adjusted the orientation of each joint for us.

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So the X axis is now pointing towards the next joint.

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What this means is I can now twist the root joint and the elbow joint rotates

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but doesn't move up or down.

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So this is more like how your shoulder would work

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the same with the next joint.

02:00

OK. So let's look at what the settings do.

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The primary axis is the one which will point towards the next joint in the chain,

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the secondary axis points away from the joint.

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Finally,

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the secondary axis world orientation tells Mayer which

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way you want the secondary axis to point.

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It uses the world axis as a guide.

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So setting this to Y means the secondary axis will point up wherever possible.

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You can also use this drop down box to invert the direction if needed.

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Let's build the joints again. So you can see what these do.

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So here you can see we set secondary axis to Y

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and it's pointing up because secondary world axis orientation is also set to Y.

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The primary axis is X just as we specified in the settings.

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The Z axis isn't set because its position is

02:53

purely dictated by the primary and secondary axis.

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What's good is we can change this completely.

03:00

So this time, let's have Y pointing down the joint,

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the secondary axis has automatically jumped over to Z because we can't

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have the primary and secondary axis pointing in the same direction.

03:12

Let's change this to X. Actually.

03:15

Now we need to decide which way we want X to point,

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we can leave this set to Y because we want the X axis to point up.

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Let's create some joints and see what we have.

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So there we now have Y pointing down the joint

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and X is pointing up. Perfect.

03:34

OK. Let's do this one more time.

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So I want Z pointing down the joint this time.

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I want Y to be the secondary axis

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and that should point to the right or along the world's x axis.

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So now we have the X axis pointing up

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and that's because we told the Y axis to try and point to the right

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because the joint is already pointing right. It's trying its best to go there.

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But this is as far as it can go.

04:09

So that's how to set up your rotational axis.