When would you use FEM over Grain

[bobbybob]

When would you use FEM over Grain?

The last couple of weeks I have been working with Grains, and got a good understanding of how it works. It seems that you can do pretty much everything with this tool in case of cloth and simulation of object that will deform.

I haven't been using that much time on FEM, and from my perspective it looks like a tool that has a lot of overlaps with the grain tool. It could be nice to get an understanding of where the FEM and Grain tool differs and where to use which, from people that has good experience with both tools.

[StepbyStepVFX]

FEM is a way to represent solids ("discretized" while an object is a continuum of matters) and is widely used for physical accurate simulations of objects that can be deformed or broken, in various fields of science (space, aeronautical, nautical, architecture engineering and so on).

Grain on the other hand is based on particles that interact through constraints or other forces. So it is probably computationally lighter (well, depending on how fine is your FEM discretization of your solid).

Although I never really tried to use grain to deform solids objects (I mean objects that are obviously not like sheets or cloths, that does not have thickness), I would say FEM is to be considered for accurate/physical deformations of solid objects or hulls with thickness (it should keep volumes while deforming and so on), and Grain shall be used more for thin sheets / cloths etc. But once again, never tried to use it for solid ones, but it may gives interesting results more rapidly, although they won't be physically accurates.

Edited September 19, 2018 by StepbyStepVFX

[bobbybob]

@StepbyStepVFX, thanks for your answer.

So this is actually described in the FEM docs, I feel a bit stupid

Quote from documentation

Quote

Finite elements vs. grains

The Particle Based Dynamics grain solver uses a set of constraints between pairs of grains. These constraints express the rest distance between pairs of points, and PBD enforces these constraints to preserve their shape during the simulation. The solver works primarily in terms of point positions. The main advantages of the PDB approach are the consistently fast solve speed and the ability to change point positions directly. You would primarily want to use this for simulating bouncy objects with elastic energy, or background objects.

The finite element solver is based on a physical model, which includes stresses based on strain and volume preservation. The solver works primarily by solving systems of forces and partial derivatives of forces. The main advantages of the finite element approach are the realism of the physically-based simulation and the predictability of the material behavior for varying mesh resolutions and substep sizes. You would primarily want to use this for detailed, physically accurate objects.

 

Edited September 19, 2018 by bobbybob