Animating Sculpted Meshes in Blender

Or, How to Apply One of Those Weird Modifiers Which You Never Thought You Would Use


Introduction (Which may be Safely Skipped)

Blender is an extraordinarily complex and beautiful piece of software, and I’m not sure what anyone else would be expecting of it. Of course it is. Aside from all the passion of Ton Roosendaal, numerous contributors have been adding to the project over the years in an effort to create a full-featured multimedia production suite out of what once was a simple 3D model editor.

In fact, sometimes it gets overwhelming. As frequently as I use it, I’m still cracking the surface of a lot of its finer algorithms and tool kits. This article exists to address a common concern among medium-tier modelers.

You see, you can extrusion-model relatively easily with Blender. It was actually the first type of modeling I ever did with it, not long after it was made public license. Of course, this was what, 2002 or 2003, and being a humble American college kid I had only an idea of what I wanted to make, but no idea how to accomplish it. However, the software has expanded to include metaballs, various types of curves, volumes, lattices, probes and even sounds. Weight painting, vertex coloring and grouping, and armature deformation, along with shape keying, has gotten very accessible.

Moreover, it’s introduced a remarkably sophisticated sculpting kit. It’s powerful enough with a mouse, but with a graphics pad (even a cheap one) it’s almost intuitive once you understand the basics.

The actual problem only shows up after you’ve mastered applying armatures to extruded models, and after you’ve sculpted a mesh in fine detail. You have your magnum opus, your masterpiece, your triumphant paragon of a million carefully placed faces, matching precisely what you had in your mind. You understand anatomy, and the rule of thirds; the golden ratio and the laws of balance of form. You move to animate your sculpture, attaching it to a handy metarig and parenting it with automatic weights. And…

It feels like profanity, if it were being spoken in some kind of obscure occult code.

Nothing happens. You get a message, in an almost intentionally uncomfortable shade of orange, warning you that “one or more” bone envelopes could not be calculated. It’s not especially helpful, and you have no idea what went wrong; but your mesh has all ten-thousand-ish vertex groups added and every single one of them is empty.

Don’t despair just yet.

Problems with Automatic Weights

We owe automatic weights a great deal. If they didn’t exist, we would probably never start animating. I personally find that while manual weight painting has come a long way, it’s still rather clunky and something I prefer to avoid; so simply painting in the weights isn’t usually going to do the job.

The most common reasons (and I do mean common) for this bug to occur is having two separate component meshes in an object which intersect in a weird way (or really almost any way), throwing the calculation off; duplicated vertices; and in my experience, malformed normals. Separating meshes, the less common of the two, is relatively simple once you’ve found the offending geometry; but the second one is more automatable.

You can sometimes get away with switching to edit mode, selecting everything, and running the Triangulate operation. This splits every single face into component triangles. You can then select all of those triangles and run Tris to Quads, to convert them into quadrangles. You can also precede this with Merge by Distance (formerly Remove Doubles) to remove any stray duplicate vertices.

Quadrangles (or “quads”) are easily the easiest face type to get along with on an armature; they’re flexible, deformable, and sensible; and I don’t personally like mucking with anything else. However, once you have enough faces on your mesh, this can get inaccurate or incomplete. I’m not concerned about the relatively scattered nature of the quads, I’m more concerned about the fact that sometimes it can’t merge them all and you will still have a few hundred triangles to figure out.

Forget this notion I keep hearing about, that all polygons being broken down into triangles on the graphics card. That’s not how graphics cards work mathematically, it’s only how they work incidentally; and that’s not even necessarily true anymore. A triangle is easily just a truncated quad, and quads are much easier to guide into convincing organic shapes without weird seams showing up.

The other operation is the Remesh modifier, which I still often use; but it comes at a cost. Remesh will scan over your mesh as a volume, and build a new (and perfectly armature-friendly) mesh over it occupying roughly the same volume. (It’s effectively a voxel scan.) It’s very good at getting an animatable mesh; but the problem is that it screws up your topology which you’ve been working so hard on. The second problem is that you can easily exceed a million faces zooming in with it, which is not an easy operation to undo.

There’s a way to apply either of these that will not change your topology at all, and will allow your model, no matter how godlessly non-manifold your vertices may be, to properly deform—using the Transfer Data modifier and a duplicate object.

Duplicate-Remesh-Transfer

The thing to remember about bone groups is that they’re vertex groups with weight gradients; and those will work on anything regardless of topology. So, what we can do is simply duplicate (not instance, by the way—literally duplicate!) our mesh, apply a remesh (or quadrangulation) to our new mesh, and fit it to our armature; then transfer the vertex data over to our unaltered original on the basis of the nearest relative vertex.

Start by duplicating your object with Shift+D, in the 3D View-port space. This is going to be our crash-dummy object which will have its vertex data destroyed; we won’t need it forever. All of our lossy modifications are going to happen on this.

Apply Remesh, and only increase the iterations (or decrease the step size) until the new mesh covers all of the area you expect your armature to deform. Don’t worry about what’s happening to its appearance, we won’t be using it for anything else. When you’re satisfied, apply the Remesh.

Alternatively, you can switch to edit mode and Merge by Distance, Triangulate, and Tris to Quads your vertices, if it works better for you. As always, I encourage experimentation and getting to know your toolkit.

Parent your armature to it, with automatic weights, and our topology-damaged mesh will animate smoothly. Feel free to test this before you continue to the next part.

Switch to your original mesh, and add a new modifier. In the top-left corner of the Add Modifier drop-down, you will find Transfer Data; this is what we want. What Transfer Data does is map data from one object’s vertices, edges, and faces (if specified) to its host with an approximation algorithm. As far as I can tell, it can be used for almost anything.

Data Transfer Modifier

We could ask for a lot of different things from the Data Transfer modifier, including vertex colors, smoothing, even freestyle data; but for now, we’ll just want to check Vertex Data and beneath it, Vertex Groups.

The Ideal Data Transfer set-up

Remember how your vertex groups were automatically populated in your duplicate mesh, which should still (very roughly) approximate your original? What we will be doing here is copying data from it. Click Source and select the name of your duplicated object. This will be our data source.

For this next part, I believe it is very important to ensure that your origin point for your original, and deformed, objects is at exactly the same place. It will save you some time. You could in theory also do this by using local coordinates, but that isn’t the default, and most of us are, after all, on some form of deadline.

We will next want to click on Generate Data Layers. What this does is create the essential layers for our original object from our deformed object. If I’m going to be straightforward here, I have no idea exactly how essential it is, but it does allow you to preview your weight paints and ensure that everything is working well.

For Mapping, I generally recommend Nearest Vertex, and Mix Mode should be Replace, unless you’ve got something weird going on (and every artist who stays busy does, now and then). This determines how the property at the host object’s vertex should be interpolated, from the properties of the old object’s vertices. Feel free to play with it when you’ve got some patience to look at the other options.

If you have a lot of faces in your mesh, and I’m assuming that you do, this will take a moment to complete. Don’t worry, Blender isn’t freezing up, though the process may take up an entire core of your CPU to finish. For each and every vertex, it is scanning a BVH of the other mesh and interpolating between closest matches—in essence you’ve just got a lot of data for it to work through. Go make a coffee or check your mailbox, and come back; you’ll hopefully only need to do it once anyway.

Attaching to the Armature

Almost there.

You’ll want to apply the modifier, too. This will also take a moment. Once the modifier is applied, you can check on Vertex Groups to ensure that all of your bone groups have been transferred, and have been populated in weight paint mode.

Now, we just need to attach the armature; for which you’ll notice all of the bone groups are already properly named. To do this, select the original mesh, and then the armature. Press Control+P, and—and this part is important—do NOT select Automatic Weights. Select Armature Deform, the heading itself, and that’s it. If you select Automatic Weights or any other envelope calculating option, there’s a very good chance that it’s going to ruin your vertex groups, and you will have to start over. Since we’ve already configured our vertex groups, we do not need to do that, and should pointedly avoid it.

Once you’ve done this, you can check your armature in pose mode and ensure that your geometry, no matter how godlessly non-manifold it may be, deforms accordingly. All we needed to do was work around the weight painting constraints, and transfer the resultant data over to our original mesh, which Transfer Data is designed to do.

You can most certainly delete your remeshed copy afterward; you will not need it again.

Conclusion

When presented with a daunting task it’s important to step back and consider what the issue may actually be. In this case, it’s all about calculating vertex weights; and this can easily be done on a remeshed duplicate and cloned over with Transfer Data.

This makes virtually any geometric construction animatable in Blender, without compromising topology or detail.

Published by Michael Macha

I'm a game developer for both mobile and PC. My education is in physics, journalism, and neuroscience. Founder and CEO of Frontier Medicine Entertainment, located in the beautiful city of Santa Fe, New Mexico.

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