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I have modelled a cloth sheet on a washing line that will blow into a Stargate "whoosh" (the vortex that comes out of a Stargate when it first opens).

I would like to have the part of the sheet that intersects the "whoosh" disappear as if it has been disintegrated. I need to delete half the sheet part way through the animation.

Where I want to cut my sheet part way through the animation

I can boolean difference the sheet at frame 0 of the animation, but then half the sheet is missing for the whole animation. I am basically looking for a way to change the mesh half way through the animation.

Is this possible, or can it not be done because the position of the sheet during the animation is going to depend on how the physics bake went?

Edit: Just thinking out loud, could it be done with vertex groups and assigning a transparent material when required? I know enough to describe that but not how to do it!

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  • $\begingroup$ I don't think you can (or I'm quite sure you can't) change the mesh during a cloth simulation... but your idea with the material isn't bad. You don't need vertex groups, just a duplicate material. Assign one to the faces you want to keep, the other to the faces that should disappear. In the second material you combine the original material nodes through a Mix Shader node with a Transparent BSDF shader. Then you can animate the mix factor. $\endgroup$
    – Gordon Brinkmann
    Commented Jul 6, 2022 at 7:39
  • $\begingroup$ you can - with the mask modifier ;) $\endgroup$
    – Chris
    Commented Jul 6, 2022 at 7:55
  • $\begingroup$ @Chris Well, that's actually not really changing the mesh, masking is just a different form of hiding certain parts ;) $\endgroup$
    – Gordon Brinkmann
    Commented Jul 6, 2022 at 8:50

2 Answers 2

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If you just want to delete a part, it is even possible during the cloth animation:

just use the mask modifier, give the part you want to cut off a vertex group with value 0.5

then use this group in the mask modifier and animate the threshold.

enter image description here

possible result:

enter image description here

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  • $\begingroup$ +1, but in your modifier stack the Mask modifier is shown as the last one. This way the cloth simulation treats the masked out part as if it was still there just as if you only made it transparent in the material. If you want collision and self-collision effects to treat the hidden geometry as if it wasn't there, you have to put the Mask modifier before the Cloth modifier. $\endgroup$
    – Gordon Brinkmann
    Commented Jul 6, 2022 at 10:52
  • $\begingroup$ Hmmm... the cloth simulation doesn't really like when you change the mask threshold value during the simulation. $\endgroup$
    – Gordon Brinkmann
    Commented Jul 6, 2022 at 11:14
  • $\begingroup$ Thank you Chris, that worked perfectly, and I learned more about Blender! Awesome! $\endgroup$
    – Casualbob
    Commented Jul 7, 2022 at 6:48
  • $\begingroup$ Glad I could help! $\endgroup$
    – Chris
    Commented Jul 7, 2022 at 6:49
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My first draft used the accepted answer, set vertex groups and change their visibility

On my second draft, after a bit more experience, at the point I wanted to slice / cut / put a hole in the cloth

  • I baked the sim copied the sim object

  • Converted the copy to a mesh at the frame where the "destruction" begins, so the new mesh matched the shape of the sim at that frame

  • Boolean-differenced the change I wanted into the new mesh, putting it into its "ruined" form

  • Hid the old sim (showing the new mesh)

  • Re-cloth-simmed the new mesh, so it actually simulated the cloth from that point, free to animate the new-shaped / tatty / ruined cloth as it would actually behave.

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