[Bf-committers] It's time to get rid of cloth

[Brecht Van Lommel]

To be clear, cloth simulaton does use an implicit solver, including ideas
from the Baraff paper like filtered CG.

For volumetric soft bodies, there was a summer of code project this year.
It uses an implicit solver. However it's not currently in a state that is
good enough to go into master.
https://wiki.blender.org/wiki/User:Mattoverby/GSoC2020/mattoverby_final_report


On Tue, Nov 3, 2020 at 2:19 AM bjornmose via Bf-committers <
bf-committers at blender.org> wrote:

> Well,
>
> my last contribution was ' old man grumbling'
>
> to be more constructive I'd like point to the issues to be focused on:
>
> We do have stiff PDEs ODEs to solve. We know very well, that common
> forward solvers perform very poor under 'stiff' conditions. David Baraff
> proposed using 'implicit backward solvers' some days (~ 10 * 356) ago.
> Even 'numerical recipes' knows stiff differential equations.
>
> How deal with it?
>
> 1. Implement solvers that can do .. expensive but possible in open source.
>
> 2. Think about volume energy constraints ( see 3. )
>
> 3. Think over the model .. a mesh is a mesh .. blender as grown from
> past knows 2D objects in 3D .. a skin of faces. Works very fine if is
> forward driven. But there is no reliable concept if we want to have
> vertex to vertex interaction deep inside. (Think of the cube having
> springs  from (1,1,1) to (-1,-1-1))   The fist thing on the task list
> IMHO should be to add 'edges' that is vertex to vertex connections that
> do not modify the surface but represent physical interaction.
>
>
>
> On 02.11.20 19:07, Brecht Van Lommel via Bf-committers wrote:
> > The paper shows how to make SDF collision more accurate by using the SDF
> > directly rather than sampling it. We have triangle mesh based collision
> in
> > Blender, which in a state-of-the-art implementation would avoid those
> > problems already.
> >
> > The point of switching to SDFs would be performance, at the cost of
> detail
> > loss compared to triangle mesh based methods.
> >
> > On Mon, Nov 2, 2020 at 5:38 PM Aaron Carlisle via Bf-committers <
> > bf-committers at blender.org> wrote:
> >
> >> Hi looking at this thread I wonder if anyone has looked into
> implementing
> >> SDF
> >> based collision detection in Blender. It seems to be quite a bit more
> >> stable
> >> than sample based detection. Here is a recent paper on the topic [1]
> >> <https://mmacklin.com/sdfcontact.pdf>
> >>
> >> 1. https://mmacklin.com/sdfcontact.pdf <
> >> https://mmacklin.com/sdfcontact.pdf>
> >>
> >> On Wed, Oct 21, 2020, 21:56 bjornmose via Bf-committers <
> >> bf-committers at blender.org> wrote:
> >>
> >>> Might it be, that old experienced coders knowing physics and all the
> >>> other things about stiff differential equations turned their back to
> >>> blender development?
> >>>
> >>> May be because lack of time .. may be an ignored patch proposal .. for
> >>> those do not have the time at hand to provide elaborated solutions?
> >>>
> >>> Well ...
> >>>
> >>> bjornmose
> >>>
> >>>
> >>> On 20.10.20 20:35, Joe Eagar via Bf-committers wrote:
> >>>> Brecht, you can avoid the worst numerical instability issues of CCD by
> >>>> adding a small repulsion force when cloth is in close proximity with
> >>>> geometry.  IIRC most commercial cloth sims do this.
> >>>>
> >>>> Ton, I'm sorry I blew up.  Cloth collision is something of a
> >>>> special nightmare for me.
> >>>>
> >>>> Sergej, that's fair.
> >>>> Sebastian, sounds like you're doing good work, just remember that
> >> robust
> >>>> collisions are important :)
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