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Issue with rigid body and modifiers dependent on transform
Open, Confirmed, MediumPublic


System Information
Windows 10, GTX1070

Blender Version
Broken: Blender 2.80 Alpha 2 0c8b0771f26
Working: Blender 2.79 ( <- Updated info )

Short description of error
When adding hair to a rigid body, the hair position doesn't update.

Exact steps for others to reproduce the error

  1. Create a new scene
  2. Select the default cube
  3. Open the Physics tab
  4. Select rigid body
  5. Open the Particles tab
  6. Select New
  7. Select Hair
  8. Select Hair Dynamics

Play the animation. The ball will fall, but the hair doesn't follow it.

Have a look at the attached blend - beside the rigid body cube, I've animated a second cubes position to fall, and the hair follows it correctly{F5712545}.



Event Timeline

Note that, if you carry out the above steps and skip 7), the ( non-hair ) particle effects do follow the cube.

Philipp Oeser (lichtwerk) triaged this task as Confirmed, Medium priority.

Can confirm this.
This could just be a drawing issue (e.g. if you go into Particle Editmode the hair is actually in the right spot).

Havent checked code. If this drawing only, I would kindly ask @Clément Foucault (fclem) to have a look, but it might also be one for @Sergey Sharybin (sergey) to check

Another interesting related issue. If you replace step 4) Rigid Body with 4a) Soft Body and optionally 4b) Toggle off Goal, then the hair particles do follow the new physics position of the cube when you press play after step 8)
So, hair seems to work with Soft Body physics updates and not Rigid Body physics updates.

When in particle mode...

  • Hair appears at the position of the rigid body

, but ( if you enable hair dynamics )

  • Hair strands haven't moved, it's like they are showing the default non-simulated hair particle strand shape, rather than the actual simulated shape of the hairs.

In edit mode...

  • Hair doesn't move with the rigid body


  • Hair strands do move as if they are simulated

@Sergey Sharybin (sergey) - I see you just fixed another issue with rigid bodies and particles, could you have a look at this one too?

Would need to have a closer look, but could be caused by the change in the evaluation order of rigid body, similar to T63028.

The issue here is that such setups are causing feedback loop in the system:

  • Hair is evaluated as a part of modifier stack
  • Modifier stack in this case needs to know object's world position
  • Rigid body defines the world position
  • Rigid body needs to know output of the modifier stack to perform proper collisions

This appeared to work in 2.7 better, but was always causing one frame offset during animation. Which isn't really helpful, since it causes self-intersections which are impossible to fix. Additionally, rendering motion blur becomes a huge issue in those configurations.

Hi Sergey,

Thanks for having a look.

Hopefully it can be fixed, think of all the hairy domino simulations that need rendered! :P

Sergey Sharybin (sergey) renamed this task from Issue with adding hair to rigid body to Issue with rigid body and modifiers dependent on transform.

Just tested the release candidate and the bug is still here. Strangely it did appear to work once, with strange results, but after that it went back to only taking into account frame one of the rigid body simulation.