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Wrong time delta for Cloth Fluid Density
Confirmed, NormalPublicKNOWN ISSUE


System Information
Operating system: Antarctic Waterfowl Delight (Linux)
Graphics card: Intel

Blender Version
Broken: 2.93

Short description of error

Instead of imparting the same impulse per volume per time delta, the "Fluid Density" setting for cloth simulations seems to impart the same impulse per volume per simulation step.

The result is that cloths with higher "Quality Steps" will fall more quickly.

Animated GIF:

The two Suzannes have exactly the same cloth sim settings, excepting that the one on the left has 5 for "Quality Steps" and the one on the right has only 1.

A lot of instability and slight accumulated positional variance is of course to be expected with different step sizes. But my understanding is that the physical forces in the sim should be scaled (and most of them are scaled) so they behave consistently with scene time or scene frames, not with simulation steps.

Setting the density to 0.0 does indeed produce much more time-consistent results, even though the quality steps settings are still different:

Exact steps for others to reproduce the error

  • Enable cloth simulation on a solid/manifold mesh object.
  • Set and enable a non-zero fluid density for the sim.
  • Duplicate the object.
  • Give the duplicate's cloth sim a different "Quality Steps" setting, but leave everything else the same.

Or download the attached file:

Event Timeline

Results from trying to nail down causes of the remaining variance in my project:

  • Pressure is correctly time-consistent.
  • The "Wind" force field is correctly time-consistent.

Vertex mass, air viscosity, gravity, etc. are all clearly time-consistent as well, so it looks like Fluid Density is so far the only obvious parameter that's applied per simulation step instead of per scene time.

Richard Antalik (ISS) changed the task status from Needs Triage to Confirmed.Jul 8 2021, 6:20 AM
Hans Goudey (HooglyBoogly) changed the subtype of this task from "Report" to "Known Issue".May 16 2022, 10:54 AM

This sort of inaccuracy is generally to be expected when interacting with complex simulations. However, this could be a result of the pressure simulation not being scaled correctly with the quality steps.

Unfortunately there is no active developer maintaining the cloth simulator at this point who can look into complex issues like this. There are loose plans to replace it in the near future though.