How can I use PMXE to create new bones in my MMD model? What is Rigging? What are "bones" in PMXE? How do I make eye bones? What does the PMXE append field do? How can I make my MikuMikuDance model easier to use?

If you're rigging a character that you only intend to use yourself, to create custom animations, you don't have to worry about usability very much-- any time there's a  usability problem, you'll easily be able to fix it.  You can load your model full of bizarre bones and poorly named morphs without worrying about it.

If you're rigging a character that you want to use with existing motions, you start to run into some constraints.  You can make a bone that sits between the leg and the pelvis, but no motions are going to take advantage of it-- you'll have to animate that part of the model yourself.  You can make a ton of new morphs, but existing motions won't take advantage of those either.

If you want to make a character intended for wide use, you run into still more constraints.  Do some of your animators prefer animating using semi-standard bones like those IK parents at models' heels?  Are there too many bones for animators to cleanly pick them out when they want to select from the 3D view?  Are there new bones that your animator has to learn, assuming they're even willing?  Are any of the bones or morphs named confusingly, or in a language that your animator doesn't understand well?  Any of these issues are going to lead to problems in the final motion.  Time spent learning your model's ins-and-outs is time that could have been spent on fine-tuning animation.  That's why you want to automate as much of that process as possible.  And that's where you can really go wild with some advanced bone techniques.

Two Important Bone Principles

1) Bones are points.  They should have been called "joints" from the start, but it's too late for that now.  Bones aren't lines.  They don't extend into space.  Those lines leading to the child or offset are meaningless as anything except a visual aid.  Bones are points around which other points (vertices and other bones) rotate.  If you think of them as lines, you're going to run into problems over and over again.  I still find myself thinking of them as lines-- and making mistakes because of that.

2) Bones are sequential.  What does that mean?  It means that they need to be in a particular order. We all know that children need to follow parents, but why?  It's because the position of the child depends on the position of the parent, but not vice versa.  In order to know where the child is, we need to first know where the parent is.  Parent/child relationships are not the only place where this is important, as we'll see.

Let's look at the window...

Hopefully, you're familiar with this window.  If you're like me, you've looked at a lot of those options and wondered exactly how they work.  I'm assuming you're familiar with the basic options, but let's go over a few:

Name.  This is an extremely important field.  If you don't use the right name, existing animations won't use the bone.  Note that adding an English name is very easy, and every high quality model should have both English and Japanese bone names.  If you make a new bone, give it a Japanese name even if you yourself only speak English!  A huge number of MMD users are Japanese, and one of the ways we can bridge the culture gap is through these simple, respectful gestures.  If you need a translation, try https://translate.google.com.  Even if you get it wrong, people will appreciate the effort.

Vis.  What's there to say about Vis?  If it's checked, the bone is visible, if it's not, it's not.  Managing visibility is an important part of reducing clutter in the 3D view.  Unfortunately, hidden bones placed in the display panel won't register properly, and hidden move bones cannot be moved except through numeric input.  So if you need a bone to be adjusted, make it visible, but seriously consider if adding more control is going to be worth the clutter to your animators.  But this tutorial is also going to introduce you to some techniques that you can use to at least move some of that clutter off to the side.

Link to.  What's to be said about this?  Just that it's not very important.  Bones don't need children to function, because they're points, not lines.  Feel free to use the offset setting to create a visual indication of relative rotation, and if relative rotation isn't important for that bone, then feel free to link to bone -1 to reduce visual clutter.

Using the Append Settings

If you can, open up a model in your editor and follow along.  We're going to look at the 両目 bone, which you might be familiar with as the "both-eyes" bone.  This is a great example of a simple thing that a rigger can do to make an animator's life easier.  Let's take a look at it.  To make your life simpler, I'll red-out everything that doesn't affect how this bone functions:

Can it be that nearly nothing about this bone is important to its function?  Apparently the only thing that matters is its name, its order, and the fact that you can see it and rotate it.  How can that move both eyes?

It's because the important stuff isn't in the both-eye bone.  It's in the eye bone.

I guess that when you hire chimeras, you get what you pay for, and Teto's working for hard drive space.  We're looking at the window for the left eye bone.  There's a funny little field in there labelled "Append."  You'll notice that in the parent field is the number of our both-eye bone.  What it's saying is that every time both-eyes rotates, we want this bone to rotate as well.  Since the ratio is 1, for every 1 degree that both-eyes rotates, the left eye rotates 1 degree as well.  This is in addition to any rotation applied directly to the left eye bone.

Let's do some math.  If the ratio is 0.5, then if we rotated both-eyes 90 degrees, left eye would only rotate 45 degrees.  If the ratio is -1, then left eye would rotate 90 degrees, but in the opposite direction.  If the ratio is 4, then left eye won't rotate at all, because 4 times 90 is 360 degrees, which is a full circle!

Take a minute to look at your right eye bone.  You'll see that it has the exact same append settings.  So when both-eyes rotates, both eyes rotate.  It's easy.  It works.  That's why animators use it.

Remember that I said that the order was important?  That's because, in addition to its parent, left eye depends on both-eyes to know exactly how to rotate.  So both-eyes needs to be evaluated earlier than left eye-- it needs to be higher in the bone hierarchy, just as if it was a parent and left eye its child.  (Or you need to change the deform tier, which functions similarly.  Don't worry about that yet.)

Making IK parents

You may have been irritated at how hard it is to select the IK leg bones, because they have to be at the exact same position as the ankle bone.  Let's use the append field to fix that.

Fine.  Position does matter.  Kind of.  We'll get to that.

We've given it the same name as the IK bone.  Why?  Because that way, existing animations will use it.  Otherwise, it will lag behind our center bone.  Of course, we'll need to change our actual IK bones' names to something else.  We've made it a move bone, because that's how IK bones are moved around, but not a rotation bone, because rotation does nothing to IK bones.  And, yes Teto, we've given it a particular position and made sure it's parented to the motherbone.  Position and parent don't affect what the bone does.  But they do affect where the animator sees it.  In this case, we want the animator to see the bone close to where they expect to see an IK bone so they're not wasting their time looking all over the model, and so that the bone more accurately represents the position of the IK bone.

Note that there's one important thing that we haven't done.  We haven't made this an IK bone.  It's not an IK bone.  It's just a move bone.  It doesn't do any fancy IK stuff.

Just like with the both-eyes bone, all of the real action is in another bone.

I hate to say it, but Teto's right.  Even though this bone is hidden, you might consider translating the word "proxy."  If you make a great model, Japanese editors will want to edit it.

Mostly, this is just our old leg IK bone.  We've changed a few things.  We don't need to know anything about IK bones to do what we're doing.

We've made sure that the IK bone isn't evaluated until after our imitation IK bone.  We've made it invisible, so that our animator isn't confused by seeing redundant bones.  We've renamed it, so that existing motions will know not to write to it.  And we've given it a move append from our imitation IK bone.  Otherwise, everything about this bone is the same as it was: its position, its IK settings, its parent and child.

Now, whenever we move our imitation IK bone, the real IK bone will echo it perfectly.  The animator may never even realize that they're not directly moving the IK bone.  All they know is that, on this model, it's easier to select the IK bone than it is on most models.

That's the rigger's curse:

When we do our jobs poorly, we're hated.  When we do our jobs well, nobody ever knows.