[Bf-committers] Durian/Hair Troubles

[bjornmose]

Brecht Van Lommel schrieb:
> Hi Jens,
>
> On Tue, May 18, 2010 at 11:45 PM, bjornmose <bjornmose at gmx.net> wrote:
>   
>> I don't know too much about Daniels implementation of the ODE solver.
>> But in general:
>> GC  is known to find a local minimum if:
>>
>> <snip>
>>
>> I hope i could give some hints.
>> If I was not clear enough, don't hesitate to ask.
>> Some times I assume 'simple facts' to be known, ignoring it took me 20+
>> years to sort it out :)
>>     
>
> I looked for larger distances, but on the instable frames, the forces
> were no bigger than other frames, and the initial residual (Ax - b)
> wasn't bigger either. So investigating the other values you suggested
> may be a good idea.  But at this point I'm happy the sim is now stable
> for the scenes I've tested, and I'm hoping that I can just leave the
> integrator code and focus on other problems.
>
>
> There's two problems that I'm looking into, but would also appreciate
> any ideas or help on these.
>
> Currently the scene Campbell posted seems to be simulating stable
> without collision, and the hairs are going through her face. Collision
> is reportedly problematic but I haven't tested it yet, is first thing
> I will do tomorrow. Getting a smooth collision response is important
> here, we don't need the hair to be right on the head, if it stays some
> small distance that's fine, but no jittering.
>   
Again i am not up to date ..
But last observations were  collision is done  asynchronous.
Says unlike SB solver where collision is handled as force, cloth do a 
'ad hoc' move out .
Leads  to  'spikes'  the solver  has to handle  .. shut up here:)

> The second problem, that is actually multiple problems, is that we
> can't get the forces right so that the hair is both moving enough when
> it supposed to while still preserving the original shape when it's
> hardly moving. To preserve the original shape we need to push up
> pin/goal stiffness, damping, etc, which kills all the interesting
> motion. If we don't the hair starts losing shape, which is clearly
> weird for a nearly still shot. For more dynamic sequences when we
> lower those parameters, when the hair comes to rest again it has lost
> it's shape and is just hanging down straight along the z-axis (also
> visible in the trailer when the little dragon is grabbed).
>   
I don't know .. but feel
But, check if cloth code really cares for painted pinning ..
Last time i checked it was 1..0 ... in SB terms either goal 1.0 or 0.0 
but no blending.
Rather guessing than knowing, though.
> It's simple to test, just run the sim with sintel in armature rest
> pose, in the provided scene, her hair starts losing to gravity.
> Tweaking hair weights and parameters can give acceptable results for
> different scenes, and this is what we may end up doing (and are
> already doing), but I would like to get it to do the right thing more
> often.
>
> One way to look at it is that the hair shape sintel has now is
> actually not the right rest pose, but is what we want as the
> equilibrium when gravity is acting on it. So here the rest pose for
> the sim should be such that when we run the sim her hair starts
> falling down into the shape that was specified. Not sure if we can
> actually do anything in practice with that insight, or if that
> mismatch is really at the core of the issue. Another idea is to make
> the pin/goal forces work a bit different, so that they get activated
> more when the hair is moving less or nearer to the rest shape.
>
> Also, bending forces are really not working well, the extra springs
> between every other hair vertex have multiple solutions and can push
> the hair into incorrect shapes once it flips. Ideally angular springs
> would be used, but they seem too complicated to implement in a
> reasonable time. I guess the goal forces will just have to compensate
> for them, or perhaps there's some tweaks possible to get them a bit
> better.
>
> Thanks,
> Brecht.
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