What is the best choice for soft bodies ? fem vs vellum

[xanter]

Hi !

Vellum mostly was presented as tissue solver rather than common tool for soft body creation.
how to streamline that, what is best for what ?

some people do soft bodies (not cloth but soft shape(any)) with vellum , that creates confusion.

I see FEM is for anything soft/deformable except cloth/tissue in general , so I would do pillow with vellum rather than with FEM

and vellum is for anything that is textile/tissue.

Is that correct ?

[woodenduck]

FEM is for when you have all the time in the world.

Vellum is for when you have to deliver.

;D

[xanter]

I know vellum is faster , but can vellum do anything that FEM does from technical point of view? 

[anim]

you can use both for both
if I oversimplify the answer, FEM is physical and produces more consistent results independently of the geo resolution while Vellum is not and doesn't, but vellum is faster and more flexible in terms of types of constraints and some other stuff, but may be more difficult to handle changing topology than with FEM

also FEM got faster recently so it's always worth to test for the case you have if you need any of the FEM advantages over Vellum

[xanter]

ideally there is no point to have fem in that case ... 

to have something big that stays apart from other tools , you have to have reason for that , because you will have to support that tools.

If you can do all types of softbodies with both FEM and vellum , what is the reason of keeping FEM alive. just add that minor functionality to vellum and kill fem , it will save you lot time in future by not spending time to its development

[anim]

4 hours ago, xanter said:

If you can do all types of softbodies with both FEM and vellum

you can't, as I mentioned. Vellum is not able to handle infinitely stiff materials or doesn't produce the same solve if you change resolution of your geo (it's better in H18, but far from ideal, so building muscle system based on vEllum for example would highly depend on the resolution of the muscles and being aware of their differences between chanracters and extra tweaks per muscle and character, while with FEM you hopefully just get physical result and can rely on that)

also FEM was implemented before vellum, so it needs to be supported for backward compatibility and  would naturally get deprecated in case Vellum is capable of replacing it completely, which is not yet and FEM remains valuable part of Houdini toolset for the time being, it's also being worked on in terms of speed improvements and also newer material models like recently added Neo-Hookean Model which is always nice to see, cause if you have to use FEM at least it can be less painful than before

[xanter]

8 hours ago, anim said:

you can't, as I mentioned. Vellum is not able to handle infinitely stiff materials or doesn't produce the same solve if you change resolution of your geo (it's better in H18, but far from ideal, so building muscle system based on vEllum for example would highly depend on the resolution of the muscles and being aware of their differences between chanracters and extra tweaks per muscle and character, while with FEM you hopefully just get physical result and can rely on that)

also FEM was implemented before vellum, so it needs to be supported for backward compatibility and  would naturally get deprecated in case Vellum is capable of replacing it completely, which is not yet and FEM remains valuable part of Houdini toolset for the time being, it's also being worked on in terms of speed improvements and also newer material models like recently added Neo-Hookean Model which is always nice to see, cause if you have to use FEM at least it can be less painful than before

 

Ok , so I would like to see less tools but with more functionality inside, no one needs additional nodes if it can be avoided.

you've mentioned infinite stiffness , which are infinite stiff materials?

PS 

I thought vellum is also producing resolution independent solve since 18:  "In Houdini 18 vellum improves resolution and scale independence, more robustness, sliding surface constraints, and other constraint improvements."

tha't like above is from "what's new in houdini 18".

 

Edited September 5, 2020 by xanter

[anim]

t's all about evolution of the tools over time and then also keeping them around for backward compatibility for some time as people may rely on them 

things are being figured out new research is being done new solvers are always emerging, so useful things are being added to the software, but it's rarely all super compatible together that you can package it under one solver or system. So as I said once Vellum is mature enough and can handle physical materials FEM may be phased out, but that depends on roadmap for Vellum, so that may not be the intention, maybe MPM, IPC, ... will be implemented, who knows, It's also about giving people the options to test those methods in production and find out what works the best in terms of balance among speed, flexibility, robustness, accuracy, ... because all those methods may look amazing on paper until you actually have to use it

7 hours ago, xanter said:

I thought vellum is also producing resolution independent solve since 18:  "In Houdini 18 vellum improves resolution and scale independence, more robustness, sliding surface constraints, and other constraint improvements."

yeah, improves it, as I mentioned, that doesn't mean it's actually resolution independent

7 hours ago, xanter said:

you've mentioned infinite stiffness , which are infinite stiff materials?

it was a bit of a stretch, but in Vellum no matter now high your stiffness values are (infinite would mean the highest possible float value you can type in) and how high you set constraint iteration or substeps, you will not get above certain level of stiffness if your geo is of higher resolution, like metal or long thin wire that you want to hold horizontally on one end and not expect to bend at all (or just tiny bit realistically) under just gravity force, or stiff rubber object or any stiffer material is much more robustly handled by FEM or MPM at the moment