Help w/slowing the physics system. "An Underwater Effect?"

Greetings all,

I’m at risk of being doubly annoying here, but I can’t help it.

There’s a related thread regarding time IPO curves and particle physics here.

Here’s my issue: I’m creating a “scary ghost”. This is primarily just to develop my skills in Blender and is being done as a request by my daughters.

  1. I create a sphere, scale it, apply collision.
  2. create a grid mesh above the sphere, apply cloth physics, cloth collision.
  3. bake it to get the cloth to settle on the sphere.
  4. apply the cloth mod to set it.
  5. add a new cloth modifier to the grid->cloth mesh with cloth collision, etc.
  6. add an empty below it, apply a wind field to the empty

This creates a great flowing, billowing effect on the “ghost” but to get it better, I want the effect to flow more slowly…as if the ghost’s fabric is, say, under water.

Any ideas???

Step 7) I bake it again with all the above settings.
8) I apply a time IPO to the cloth to make it flow slower.

kaboom. broke. It wipes the bake. and no physics can be calculated from there on.
I’ve tried applying the time IPO to the empty that provides the wind, it’s just the same.

My question is this: I’m not dis’n Blender, I love it, but what’s the point of even having a Time IPO, if you can’t use it to slow down the physics system?

I’ve looked everywhere for tutorials and help to no avail, does anyone have a better approach to get this same, or similar effect?

The Time IPO has nothing to do with any of the physics simulations after they are baked … IPOs have no relationship to the baked cashed data that a simulation generates .

If you are using cloth you can try and crank up the Air Damp value to something like 5 or even higher to slow down the mesh …

Or alternatively you could render out the animation and use the sequencer to slow down the whole thing .

Thank you for the suggestions Vertex Pusher.

Ya, I realize that the time IPO doesn’t have any relationship to existing baked physics. I was more or less explaining my alarm at the behavior. Most of the time when you make significant changes to the physical system, you have to clear the cache and re-bake. When a time IPO alteration is applied, the physics are not disabled, but are simply ignored. You bake static meshes. If you remove the time IPO, the physics return.

It was frustrating because I spent almost an hour baking the physics in, half expecting that once I applied the time IPO, I would have to again, but I didn’t expect that it would disable everything–the cloth physics, the wind generator, all of it.

I’m using 2.49b today and I noticed a change. Now, when I apply the time IPO to the wind generator, it doesn’t disable the physics in the scene…but it still doesn’t slow time down for the wind generator, it appears to still be ignored. :frowning:

I tried what you suggested, rendering the physics and simply slowing it down post-production but I can’t get other physical systems in the scene to behave appropriately at high-speed (so that it all matches)

I have been experimenting with the cloth air damping, gravity, mass, all of it yields interesting behavior, but not what I’m looking for.

I have tried scaling the “ghost” to an enormous size, so that movement from the physics system is “scaled”. A cloth the size of a skyscraper moves differently than one the size of a bed sheet. it seems to be the closest I can get to the behavior I’m looking for, however the wind noise setting is muted on large scales. I’m ending up with multiple, arbitrarily placed wind generators to get the effect closer.

Again, thank you for your suggestions, I’ll keep at it…wish me luck! :slight_smile:

I’ve been converting cloth sim data to shape keys for a different purpose but it sounds like it could help you as well. After baking the cloth motion, you can then export that object to .mdd format, which is a general vertex-level record of the shape at every frame. You can then import this data to create shapekeys for an object, the IPOs of which you can edit to stretch the time.

Both the .mdd export and the .mmd to shapekey utilities are in the Export and Import menu lists, respectively. There are a few quirks about using them but it doesn’t take much head-scratchin’ to figure them out :D.


Thanks Chipmasque, I never would have thought of that!

I’ll bake the physics at 50 frames a second versus 25 if I want it to flow at half speed. Am I thinking correctly on this?

I’ll then export it, and bring it into my remaining scene as a mesh!

Yer a genius!

I’ll start on that right away…:smiley:

No need to jigger the frame rate unless you want a lot of shapekeys to deal with. My approach would be to bake the sim at 25fps, export the sim to .mdd (which exports every frame of data, btw), then duplicate the cloth object, remove the Cloth modifier from the dupe, and import the .mdd data to shapekeys for that dupe, skipping at least every other frame (a value of 2 in the Step parameter in the import dialog).

The new “shapekeyed” object will then have a key for every frame of cloth sim data but only a specified number will actually have IPO curves, depending on the Step you chose on import. I’ve used step 4 and gotten decent results, btw, which can cut down on the number of shapekey IPO curves you have to handle. You can add an IPO curve to any shapekey in the IPO Editor if you need to.

Since shapekeys automagically interpolate between one and the next if the IPO curves are set up correctly, you don’t really have to have a shapekey for every frame – a lot depends on how exactly close to the cloth sim you need to have your new object move. In come cases I’ve been able to use a “gap” between shapekeys as large as 10 frames, other places I’ve had to use one per frame.

But once the IPO curves are set, they can be manipulated in the IPO Editor like most any other IPO curve, greatly expanding the things you can do with the original cloth motion. I haven’t yet tried applying a Time IPO, but you can scale the shapekey IPOs in the X axis to stretch the time as well. Because of the shapekey interpolation, the new intermediate frames will always be a smooth change from one keyed shape to the next.

EDIT: My brain’s broke I guess. The parameter in the .mdd to shapekey utility is “Key Skip,” not “Step.” Also, it may help to know that shapekeys were once referred to as RVKs (Relative Vertex Keys, iirc) – that’ll help identify the Import utility to choose.

Thanks again! I replied to you while not knowing how it worked and assumed I didn’t have the power to skip keys on import. rendering 50 frames a sec. was what I thought I would need to do to get a fluid motion, not recognizing that its an IPO once it’s imported, and is interpolated between frarmes. It’s really quite powerful.

This knowledge gave me a whirlwind of new ideas. I consider your reply the best tutorial I’ve seen in quite a while, as I have learned something extremely useful today!

Best regards,