Hybrid Breast Physics

So as promised in the animation's post, here I am explaining the inner workings of the breast physics that's applied there. Warning, there will be a lot of .gifs here!

Let's start off by looking under the hood, if you will. In this .gif, I made the body transparent, which shows the hidden sphere inside (which is just hidden in the final render).

What's important to note here, is that I've combined two methods that I've used in the past (bone-based & full cloth simulation). This particular gif shows off how each method does it's job, while working with the other. 

Bone-based is, and has always been pretty reliable.  When you know how to animate properly, you can get some really nice bounce going just from that.  You can see it in action with the white-grey lines inside the breast. I'm mostly using it to support the cloth, more on that later. In animations with little interaction with the breasts themselves, this is more than enough. A great example of that would be Cheerleader Susan. 

Full cloth on the other hand, is rather unpredictable. You have to simulate it, and you're mostly up to the mercy of the program. There's also a wealth of options and tweaks that heavily impact how it's going to go. Hell, even the shape of the cloth object is important. And I haven't been able to figure out a good solution for that yet. Maybe there isn't one, and it depends on the context in which it'll be used for. Here's an example of me further trying out the limitations of cloth simulation (with nearly identical settings to the main animation). It falls apart pretty quickly, and will need a lot of tweaking for something reasonable to come out of it.

In the end, combining them together was in hindsight extremely simple. In the cloth system, there's an option to assign vertices to retain their shape as if they're never affected by cloth. And if you parent the sphere to the breast bone (the first grey triangle line in this image), the selected vertices (in red) will follow that perfectly. If you pay close attention to the first .gif, you can see those specific vertices in the sphere not changing shape in any way, only position and rotation directly to the bone.

That way, the sphere follows the bone rather nicely, which allows both systems to shine. While the cloth sphere might not act like a breast, the bones help it to appear as one. Bones cover the big movements (like bouncing, jiggling side to side, etc), while the cloth simulation covers the deformation with the hand and the finer jiggle. Which is showcased perfectly in the final few seconds in the animation. The breasts still jiggle thanks to the bones, but when they stop, the cloth sphere still continues for a short while.

All in all, I hope this has been another informative 'in-depth' post about the workings behind the stuff I do. Unfortunately I haven't been able to experiment with new techniques these past few months, but hopefully that'll turn around soon enough.