<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:times new roman, new york, times, serif;font-size:12pt"><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;">Hi! I support cloths development in blender and hope it will be in blender 2.60 road map.<br>thank you all.<br><br><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;">----- Original Message ----<br>From: bjornmose <bjornmose@gmx.net><br>To: bf-blender developers <bf-committers@blender.org><br>Sent: Wednesday, January 9, 2008 8:01:10 PM<br>Subject: [Bf-committers] questions on the implicit solver commit<br><br>
Hi,<br>just wanted to publish that here,<br>because i feel there is a rising divergence in what users think the <br>softbody module is for and what i am working on.<br>Even if the softbody module gets better in faking cloth it has other <br>tasks to cover /* such as make nice wobbeling bellies */ and just <br>because i 've added extensions to cover materials that have a distinct <br>behaviour on push and pull that does not mean the softbody module
claims <br>anything close to true physical simulation, though i think keeping
close <br>to the knowledge some physicists have won't hurt.<br>further more i am guilty for renaming controls to somethings non math <br>people might understand ..<br>i'd better stop now<br><br>tsk<br>why at all? because i spent the time to answer that mail<br><br>BM<br><br><br>Hi,<br>well, soft bodies are not exactly for cloth simulation, but as the name
<br>says for soft jiggeling objects.<br>There is an extra branch in SVN intended to have cloth specialized
code.<br>Most of the papers i read on cloth simulations deal with mass spring <br>sytems.<br>I basicly refer to the Pixar paper by Baraff and Witkins and they state
<br>that very hard springs are needed to get nice looking cloth.<br>Further it is well known that systems with strong elastic forces and <br>high damping lead to stiff sets of ODEs.<br>The general approach is to invoke a implicit solver. Since solving <br>inplicit nonlinear equations is a topic on its own<br>the way mostly followed is a taylor expansion to 1rst order and <br>rearrange the equations.<br>The most simpele variant of this is the so called impicit euler. This
is <br>what i also committed here.<br>What's missing is a smart step size control to go as fast as possible <br>withing a given error limit.<br><br>The 'old' and good (fast) solver for soft bodies is a symplectic <br>adaptation of a 2nd order runge kutta type.<br>Of cause with adaptive step size.<br>Funny enough my experience so far is, because it is very simple it <br>performs faster than the implicit most of the times<br>though it has to do much smaller steps. But as i said in the commit, <br>building up and inverting that huge sparse matrix<br>is quite expensive so, that the benefit of going large steps is
balanced <br>against the extra costs.<br><br>As stated before the softbody shoots in other directions than beeing a <br>perfect cloth solver.<br>But the papers i read made me 'steal' the concepts to widen the
possible <br>use of the softbody engine.<br>Recent event to look into this a bit deeper is the new particle system.
<br>Most of the complaints were about:<br>when softbodies are applied to 'hair' it was too stretchy. That is
why <br>a gave it a try.<br><br>cheers<br>BM<br><br><br>Tomas Björklund schrieb:<br>><br>> Hi!<br>><br>> I saw your post on the blender forum today. I can't say I understand <br>> exactly what you're doing (softbody or cloth or a hybrid of the two).
<br>> What cought my attention was:<br>><br>> "<br>><br>> 1. control on springs needs to be split in pushing and pulling /* <br>> fabric pushes easy but pulls hard */<br>><br>> "<br>> Now I don't now how heavily you are into math but as you probably
have <br>> seen already springs aren't the best way to simulate cloth with. Yes,
<br>> it might be the easiest but not the best. The problems you have <br>> discovered can't be worked around. Since cloth are very stiff you
need <br>> very hard springs and with very hard springs you need short <br>> integration times - thus it becomes very sloooow. Cloth really
doesn't <br>> act springlike if it is not pulled extremely hard so it isn't really <br>> natural do simulate them as strings anyhow.<br>><br>> I have a friend that is right now working on his master thesis which <br>> should be done in a month or two. There he simulates a totally stiff <br>> string (1D-cloth). With stiff I mean it does not change length at
all. <br>> I don't mean it can't bend, it is actually very good at bending. It <br>> can be everything between an overcooked spagetti or an uncooked <br>> spagetti (with realistic two point breaking of the uncooked spagetti <br>> strand when bent too much - try it yourself in the kitchen).<br>><br>> The method he uses is calculations by Descrete Differential Geometry.
<br>> What that actually does is calculate the bending energy of the <br>> curvature of the string. To solve it it is set up as a restrained <br>> system where the pulling/pushing forces are represented as lagrangian
<br>> multipliers. Then he uses an symplectic integrator to calculate the <br>> position of each vertex so that the string doesn't loose energy over <br>> time. This, I *think* is repeated until convergence. At least, <br>> something is repeated until convergence ;-)<br>><br>> The novelty in his thesis is the breaking part, the rest you should
be <br>> able to find papers about on the net, though I have not looked
myself. <br>> If you are interested I would suggest googling for the following <br>> (combined with cloth and string and such terms):<br>><br>> 1. Descrete Differential Geometry<br>> 2. Symplectic integrator<br>><br>> I have followed my friends thesis work over his shoulder so I know
the <br>> basics of it but not closer than that. The work he is doing is for a <br>> company so I am not that thrilled on asking him to reveal the company
<br>> secrets. But what I have disclosed here is of course not secret, it <br>> has been done before. And his thesis will of course be made public <br>> once finished.<br>><br>> If you find some really good papers you might be able to pull it off <br>> integrating that in Blender. Then blender would have a totally <br>> kick-ass cloth system.<br>><br>> Cheers,<br>><br>> Tomas<br>><br><br><br>_______________________________________________<br>Bf-committers mailing list<br><a ymailto="mailto:Bf-committers@blender.org" href="mailto:Bf-committers@blender.org">Bf-committers@blender.org</a><br><a href="http://lists.blender.org/mailman/listinfo/bf-committers" target="_blank">http://lists.blender.org/mailman/listinfo/bf-committers</a><br></div><br></div></div></body></html>