# Soft-body collision without physics solver?

#1

Hear me out.

As I understand it in my limited knowledge, the role of a physics solver like the nucleus in Maya is to take the position of every physics element on frame X, then compute the next positions for frame X+1, and so on. Everything is time-dependent.

However, it seems there are specific situations where the actions can be resolved without time, and therefore without a solver - for example, basic soft collisions. Say a character wants to grip a rigid object tightly - the deformation of their skin around the object can be found knowing only the depth of the base position inside the object’s collision field, and the repulsive strength of the collision field. Unless you want to model the flexing skin as having mass and inertia in springy motion, there is no need to include time.

Please let me know if I am wrong in any of this, and please let me know if there is a way to achieve this in Maya without creating a full physics solver. Thanks very much.

#2

You can fake stuff like this, but it will be fake.

There’s a hard division between “simulation” and “animation.” Simulations are history dependent – they may be deterministic but the state at any given time is dependent on the state of the previous frame. Animation is arbitrary, so there’s no necessary connection between successive keyframes (although there are some mechanisms, like expressions, which will allow you to set a value now based on the value of something else at a different time).

If the effect you want does not need history, you can definitely make that work; for things like interpenetration fixes that will work fine as long as they are entirely derivable from a current-time position. If you can describe the effect entirely as a function of positions then it’s totally doable.

You’ll definitely find that the instantaneous effects add something, but you’ll also find that many things resist that treatment in subtle ways. The soft body example. again, shows the limitations: you can definitely convert interpenetration into deformation. for example, but things like the wobble that results from a soft body interaction dissipating energy over time won’t work and may appear to be visibly missing. Lag and followthrough are often very visible artifacts, and those are a lot harder to get by without.

TLDR - works for limited cases, but it’s not possible to completely replace simulation with functional deformations.