Another Breast Physics Post (Patreon)

It's been a couple of months after I did Nicky's Size 5 walk animation, but a recent message of a patron made me realize there's quite a bit to talk about with this project. For the relatively new patrons, I did a similar thing with an older animation which can be found here. Like that one, this won't be a direct tutorial on how to achieve it. But more along the lines of my thoughts and process going into it, as well as any tricks I've used to create the bouncy end result. This is by no means exclusive to 3ds Max, and I'm sure similar results can be achieved in other programs. The three biggest factors are pressured cloth and an alternative to Skin Wrap and partial selection. I'll explain further once we go over it. Without further ado, let's get into it!

The Walk Cycle

The biggest hurdle that kept me from doing breast physics tests sooner was that it required an actual walking animation. I haven't had any proper practice in it and earlier attempts have been pretty mediocre. Fortunately Mixamo has a whole library of animations you can utilize. And you potentially could upload your character on there to directly test these animations, but unfortunately my custom rig doesn't play nicely with proper humanoid setups. I created it with the sole intention to simplify and smooth out the posing process, not with the flexibility of exporting to other software. So instead, I imported the base model, moved and scaled some body parts to better align with Nicky and finally reparented the custom rig to important bones on the imported model. As you can see in the .gif above, it's not too shabby. There is some oddities however, like her left leg snapping too far due to her leg being too short to keep up with the other model. This also gives me the flexibility to add keyframes on top of the current animation, which I did for the arms. Normally they would clip into the breasts, so I had to adjust them a bit to avoid that region which you can see when the arms lag behind the imported model.

The Bounce

Quite similarly to the other animation, I used both bones and simulated cloth to create the proper bounce. However with Nicky here I purely used bones (which have a spring controller, giving them a bit of a sway) for the bra. I used the same method to create the hair swaying. Unfortunately it's really time-consuming to simulate (bra) cloth interacting with another cloth, or in this case the breasts. So I substituted that by using bones to create some sway, ideally give the illusion that the breasts are actually influencing them. The green dummy boxes are indicators of those.

As for the breasts themselves, I basically duplicated the breasts and turned them into cloth objects. In terms of how detailed they should be & which settings, it all really depends on the situation and ultimately boils down to trail & error. I highly advise doing the breast physics in a different file (like I did here) and using Point Cache or some other method to eventually transfer the simulation results over. That way the program doesn't chug as hard and you can iterate faster.

Bringing it all Together

The biggest game-changer for me was Skin Wrap, which I used plenty in this instance. As the name might suggest, it allows you to wrap objects around other objects like a Skin, or basically skinning them. So with my setup, I have an imported animation through Point Cache with all the proper bounce. However it doesn't attach to anything, so I use Skin Wrap to partially wrap it around the breast object that IS properly attached. In Max you can work with stacks of modifiers, of which a fair amount support partial selection. The red vertices in the image follow it 100% which I've selected, and depending on the selection options you can determine how fast that tapers off. As the simulation in the .gif higher up has its issues (particularly the wrinkles forming near the top), that fortunately gets hidden through this partial application of the Skin Wrap. 

So with that all set, I have the final model which has everything properly merged and an increased subdivision which then also has a Skin Wrap on the animated body and breasts. The best thing is that the object wrapping doesn't have to be identical to the one it's wrapping to. That way you can easily add details that the simulation has missed (like proper squish around the edge of the bra), among other things. Besides all that, it was just simulating the tank-top and pants. And because the breasts are already animated, you avoid cloth-on-cloth simulations.

I hope this has been somewhat informative, for those here for the boobs AND the 3D users among us. :D For those starting out with this, I advise starting with simple motions and little to no interaction with the breasts via hand or object. That way you can get a feel for the process without immediately feeling bummed that the interaction's not what you imagined it to be.