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Cloth and Mask Mesh Keyframed resets cloth simulation from the keypoint frame
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System Information
Intel i7 8750H, 16GB DDR4, GTX1060
Operating system:
Windows 10x64 PRO
Graphics card:

Blender Version
Broken: 2.82 0c57373e2c46
Mesh will animate past the mask KeyFrame Point. But the simulation follows rules of as if the mask didnt clip the centre.
Broken: 2.79 e045fe53f1b0
Works better then 2.79, the mesh will animate until the mask is Keyframed Frame

Short description of error
I am not seeing the MASK Modifier have proper effect of the cloth simulation.

Exact steps for others to reproduce the error
Open Blender

  1. reduce size of cube by 75%
  2. Scale Cube along Y azxis to entire width of viewport
  3. Edit mode and "Control R" to add edges Add 100 edges from centre
  4. using Face Mode editing, Select two Outer Edges only. and create a Vertice Group and assign them 100% shading
  5. select the most using Wireframe view mode, Ctrl B and select only the most centre box and assign to vertice group 2. (this is the part that I want to have removed using MASK)
  6. Set the Animation frames to max 100 frames (not the standard 250) so we are animating 1-100
  7. add a Mask Modifier and ensure its top of modifiers list, add vertice group 2 and invert it.
  8. Add Cloth and pin Vertice Group 1,

now keyframe the Mask Modifier to remove the centre of the of the long cube from frame 50 onwards. so frames 1-49 show the entire cube
frames 50-100 show the cube with the centre missing..

I am expecting that the simulation should....
Cloth should animate the cube to Bow downwards in center, and after frame 50, let go and drop down after the bowing of the cube.

but it using Blender2.82 it seems to follow the path as if the mask has been applied the entire time..

I have a blend file created in 2.79.. but also works in 2.82, to save you rercreating my steps..




Event Timeline

Germano Cavalcante (mano-wii) lowered the priority of this task from Needs Triage by Developer to Needs Information from User.Thu, Nov 7, 4:37 PM

I don't understand what you mean, the animation seems to run as expected.

Is animation supposed to work correctly in the attached file?

shaun (toxsickcity) added a comment.EditedFri, Nov 8, 12:12 AM

Hi Germano Cavalcante

I am looking at how blender cloth is simulating this and it looks is as if the cloth sim is baking two separate simulations then just "copy and paste" the second simulation where the keyframe is. (that is how it looks)

The file I uploaded just isn't showing it enough.. but.
The moment that KeyFrame occur at frame 50.. the distance of the separation of cloth appears too large.

the best way to show off the issue I see, is if you slightly alter my blend file.

  1. Extend animation from 100 Frames to 200 Frames,
  2. Move the 2 keyFramed from 49 and 50 to Frame 99 and 100
  3. Increase weight to 0.5
  4. Increase speed multiply to 1.2
  5. In Stiffness make Structural/Shear/Bending all 15

or Download my New Blend file

this way you can see once simulation playback hits frame 100 you can see the cloth seperation is very big distance from when it was together. (this is showing that cloth simulation is not making a complete simulation but what appears to be two independent simulations)

I think the cloth should act like this Gif

NOTE: I would like to add, that when playing the animation in timeline, the blue line which shows the simulation resets from keyframe.
NOTE: I have not tried to bake the simulation by the bake button, I have been letting it just doing (on the fly) baking. Try baking the simulation using the button, it bakes only half of the simulation and other half appear broken, when playback after baking..

This works by desgin.
The topology has changed and the animation needs to be calculated as if the "new mesh" started the animation from frame 1.
We cannot change current behavior without deep consideration of what needs to be improved.
Currently, it seems, there is no active developer maintaining the cloth physics system (this may change).
You can see what is being planned for the general physics system here T66307: Nodes & Physics Module.

Since it works by design, closing.

For suggestions on how to improve the cloth simulation, please use other channels: