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 ?

Use Vellum unless it absolutely can't get you realistic enough results. FEM is so incredibly slow compared to Vellum that you could get dozens of iterations of your Vellum simulation done before you'd be finished cooking one FEM test. They're just different approaches to solving similar problems, but one of them comes with a lot more overhead (with the potential of more “realistic” results, but realism is generally not as important as plausibility and directability in VFX).

There is no point to have FEM in that case …

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 (accuracy solver into vellum) and kill fem , it will save time in future by not spending time to hassle with FEM tools.

There's no such thing as an “accuracy solver”. FEM and XPBD (Vellum) are entirely different approaches to soft body dynamics. Vellum is just far more practical for most use cases because it's much faster. There are still going to be edge cases where FEM is more suitable, and so there isn't really a good reason to remove it from Houdini entirely. If you don't want to see it, you can always ophide it.

i know there is no accuracy solver , i meant make vellum more accurate and get rid of fem.
ok , thanks.

If you are adventurous you can always implement Incremental Potential Contact (IPC) [ipc-sim.github.io].

They are comparing it against Houdini's both FEM and Vellum solver:

Like @toadstorm said, there is no “accuracy”. Take, as an example, wool: The way the “fabric” you see is made up from strands of individual threads that support and inhibit each others movement would be insane to “simulate” (and, in reality, it LOOKS like the fabric is behaving like a more or less coherent mass anyway), so “accuracy” isn't really what you are after. What you want to get is “close enough to reality” - or, if you are working in the movie industry, “close enough to what the director wants to see” (cynic remark deleted).

Grain solvers (and to some extent in an over-simplified view Vellum is one of them) are fast and “look good enough” in many cases, especially for simulating cloth-like material, whereas FEM sometimes can be more “flexible” (pun intended) for tricky situations where setting up Vellum restraints or controlling areas of influence would mean investing much more time into fiddling with the scene than just simulating it away in a slower solver.
It's been some time that I have done production-ready “soft body” stuff, but in my experience FEM sometimes took me closer to what I wanted to have than Vellum. Obviously, that can have been because of old habits that I didn't want to break just for getting a job done.

In my experience, more solvers (that behave differently, providing predictably different visual outcome) are better. Especially when their inner workings are as distinguishable as Vellum and FEM.

Marc