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Cloth sim with pre-roll breaks
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System Information

openSUSE 12.3 64bits

Blender Version
Broken: 5a8d7af

Short description of error

Cloth sim stops working when using pre-roll

Exact steps for others to reproduce the error
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Event Timeline

Daniel Salazar (zanqdo) raised the priority of this task from to 90.
Daniel Salazar (zanqdo) updated the task description. (Show Details)
Daniel Salazar (zanqdo) edited a custom field.

Afaik, pre roll is more or less broken since many versions - tried it now and then but it never worked. Maybe @Daniel Genrich (genscher) could enlighten us here...

Bastien Montagne (mont29) lowered the priority of this task from 90 to Normal.May 10 2014, 8:41 AM

I tested version 2.57b and it was (sortof) working.
But while I was playing with it, the feature confused me more than it helped me.

My suggestion would be to hide the feature from the UI so old files don't break but I wouldn't suggest using it, even back in 2.57b.

What do you think guys?

I have little experience with cloth sim in general, and i wasn't well aware of this preroll feature, so my opinion may be badly informed.

If this is as broken as it seems i would happily agree to at least hide/disable it for now.

I think such a feature needs to be considered on a wider scale and made available for sims in general. Ton's idea of making simulation timing more independent from the main render/animation timeline could help a lot with this:

  1. set up simulation parameters, colliders etc.
  2. let it "pre-roll" into a stable usable starting state (no frame changes here!)
  3. save that as a cache frame
  4. start the actual animation from this frame onward (load the cache frame on anim start frame, then advance as usual)

The current preroll feature is really complex and tightly coupled to the concept of the "start frame", which is a major limitation afaik.

@Daniel Genrich (genscher) I am for hiding it as it is absolutely useless in its current state!

But away from the non-working state of this feature is this a very important one. All of our simulations here in the studio are made so that they settle down a bit before the actual animation starts. We're doing it by pushing the normal animation a bit back to a later start frame and start the simulation on frame 1.

If Lukas is aware of this feature and he considers it in the future "unifying sim project" then everything is ok but we must not forget it!

Greetings, Thomas

Hey everyone.

Thomas - you are describing a nice workaround for the "preroll" problem.

Lukas - I totally agree with you. Maya got a nice cache system, which I love(d). They were able to edit every cache frame after they baked it. So artists could change physics simulaiton to their needs. Preroll was working exactly as you described it: It would be the "starting" of the simulation in the scene.

I hope we can somehow get this unified, I am all for it. Cloth is suffering badly because of cache stuff.

Sergey Sharybin (sergey) changed the task status from Unknown Status to Unknown Status.Dec 26 2014, 10:03 AM

To me it sounds more like a TODO of general physics refactor. I'm not totally fan of only hiding it from the interface. Either it gets removed all togeteher or we just don't worry that much for now. I'd prefer the latest one.

So thanks for the report, but moving it to TODO now