Posing Hard Bodies with Vertex Group Parents

As my patrons have seen, I've been working on a proxy model for the Lurker Mindsnipper monster lately.  I had thought that I'd have a good head start on the model, with the intent of remixing my original Lurker model into this new monster... but that didn't really work out.  When I made that model, I wasn't really practicing many techniques to improve model modularity or reusability, so I pretty much remade it from the ground up.  But, that gave me a chance to learn this cool technique that I want to write about today!

Vertex Group Parenting is a technique where an object is parented to a set of vertices, instead of another object.  I use object parenting extensively in my modelling process, where one object's location and rotation is locked to another object.  I most often do this for limbs, so that when I do something like moving the torso, the arms follow it.  Vertex Group Parenting does this same basic thing, except that you pick a set of 3 vertices and the child object locks onto them.  This is really useful if you have a mesh that you're posing with an armature!

An armature lets you bend a mesh, which works great for soft things like arms.  It stretches and distorts the mesh to make the bend happen, and can lead to some funkiness if the bend gets extreme, but is a really useful technique.  That stretching and distorting is awful if you're trying to move around something that's supposed to be hard though, like the carapace of a Lurker or a piece of armor.

Here's an example of what I mean.  Here we see a tube that represents an arm, an elbow pad, and an armature that we want to use to bend that arm.  The easiest way to bend this "elbow" would be to just put everything on the armature by selecting the armor, the arm, and then the armature, hitting ctrl-p and selecting armature deform... with automatic weights.  Once that's done, I can select my armature, hit ctrl-tab to change to Pose Mode and start bending my elbow... and things get funky.

Notice how the spike on the elbow armor isn't staying true and that the whole thing is warped and distorted.  So, that's not really ideal.  I'm sure that I could spend some time messing around with weight paints, or maybe make a dedicated bone to control the piece of armor... but neither of those seem to scale well to me.  Vertex Parenting, on the other hand, is really quick to set up and works quite well!

So, let me start similarly to before; I'm going to select the arm tube and then the armature, and will once again hit ctrl-p and select armature deform... with automatic weights.  Importantly, this time I have not selected the armor pad, as we're going to use Vertex Parenting to let it move with the arm.  To set that up, I select the armor pad and then the arm tube, and press tab to go into Edit Mode.

My next step is to select 3 vertices that will control the orientation of the armor pad.  This is a bit tough to do with the armor pad sitting in the way, but that's easily solved!  Just put your mouse over one of its vertices, then press L to select all linked vertices, then press H to hide them.  Now, select exactly 3 vertices that will control the orientation of the armor pad, and hit ctrl-P and select Make Vertex Parent.  

And that's it!  Now, the tube will bend and distort as it needs to in order to reflect the joint in the armature, but the armor pad will move as a solid object along with its surface!