WIP Update #8: Soft body animation (Patreon)

Hey all, this is the first in what will likely be a series of tutorial/write up posts about my process for doing techanim and dynamics. Today I'll be covering one approach I use for soft body interactions: layered deformations! I think it's a good place to start, as it covers a lot of the technical knowledge that some folks might be confused about. The other place one could start is with animation principles, which are equally important, so I plan to do another tutorial on those.  

When you're doing a technical task it's important to know what all of your tools are, so when you're deforming a character mesh you should keep in mind a list of all the different deformation methods you could use to solve a particular problem. I can't do a comprehensive breakdown of all the possibilities here (another post, perhaps), but the most relavent techniques in character animation are armatures, blendshapes and lattices. There are also things like simulations, curves, displacement etc, and these will have niche uses but today we'll focus on the main three. I'm also not going to break down exactly how each deformer works, as I'd be here forever and that information can be found elsewhere.

The key is to know what kind of movement each technique is good at handling, and use it only to create that result. Trying to use an armature to do the job of a lattice, for example, will take ten times longer and look half as good. It's also worth noting I'll be using blender terminology here but all these techniques can be used in any full-featured software package, they have analogues in maya and 3ds max, etc.  

First, let's look at the motion we're starting with:

Not great, eh? The breasts just phase straight through the bar. This is the stage of the animation where you need to think carefully about exactly what result you're trying to acheive, and plan out which techniques to use based on which kinds of deformation you need for that result. Here, we need the breasts to move up and down as they get stuck on the bar and then slide underneath, we need them to respond to being pushed around by the bar by changing shape, and we need the bar to press an immediate indentation into the breasts, to sell how soft they are.

The first tool any animator reaches for is the armature. Armatures are great because once a body part is bound to a bone, you can interpolate and manipulate that bone in a lot of elaborate ways to achieve 90% of what any artist needs. In this case we're using translation, rotation and scaling of the breast to move it around as it interacts with the bar.

Side note, in this case we already have bones that can freely transform the breasts, but this may not always be the case. Not having the bones you need is usually when you send an irate slack message to the rigging artist responsible for this garbage (in this case I would be shouting at myself, hopethetically), but you're not totally SOL. Sometimes you can add bones yourself, sometimes shape keys can bridge the gap, and sometimes you can use things like hooks and empties to sort of approximate a hack armature. Where there's a will, there's a way.

That's better! Now the breasts visibly respond to the bar! note, the animation here is quite simple and there is still obvious clipping. This is fine! I could sit for ages and make a complex animation with lots of keyframes trying to get the right movement but I would be shooting myself in the foot. The point of the technique is we only use the armature for the animation armatures are best at: translating parts of a model smoothly and consistantly. We get the timing we want, we check our arcs and move on. The rest can be handled better with other techniques.

Techniques like shape keys! Shape keys are excellent because you have very fine control of the exact shape of a mesh, and can move parts of a form relative to one another in whichever arbitrary way you please. However, they can't do the complex interpolations possible with armatures - each shape key just interpolates linearly between A and B. Therefore they should be treated as such: design only shape keys that do exactly one thing, and only use them to do that specifc thing. They are highly specialized, where armatures are highly generalized.

Here, we use shape keys to deform the breast under the bar:

The effect of this step is subtle but important. For the result to be believable the mass of whatever we're deforming must be preserved at all times. The different parts of the breast now expand and contract as they are pressed and released by the bar, and also as neighboring sections contract, as the mass has to go somewhere. In this case the effect was achieved with just two shape keys: one for the top of the breast and one for the bottom. Each shapekey only has three or four keyframes of animation.

We can also use this technique to redistribute mass in other ways. Notice now that the top of the breast expands slightly as the bar pushes the mass up, until it slips under the bar, and there is a very slight bounce and settle in the breast as the mass that was held back overshoots subtly. We don't want to over-exaggerate this effect here as dash's breasts are meant to be perky, but you can imagine on a character with much larger breasts may see more of this.

Now we can complete the effect with the use of a lattice. Lattices are an even more specialized tool, very useful when you want a wave, depression or other deformation to move across a surface, as the deformation is more of a displacement than a transformation.

So now we can see that satisfying indentation in her breasts as the bar passes across them. Really, this is the perfect use of a lattice and this effect could not be achieved by any other means. Think for a minute about how hard this would be to do with, say, armature deform.

What we're doing here is essentially passes, but instead of say doing a pass on the arms or the fingers, we're doing passes for each deformation technique we're using. This way we don't get confused and forget what deformer is doing what, and end up trying to sculpt the impression of the bar into a shape key or move the mass of the breast around with an armature. It's all about keeping things separate, handle the steps one at a time and you'll be shocked at how much you can get done!

I hope that helped, or was at least interesting! Ciao!