This is about the laws of physics, the Rigid Body System, and Armatures or ‘Passive’ animated rigid body objects interacting with ‘Active’ rigid body objects...

I use Blender to illustrate the process of track creation (think foot print), and find it well suited to my needs. Both Armatures with mesh ‘feet’, and animated rigid body ‘feet’ are employed to interact with active rigid body ‘sand’ objects.

To see samples go to my website, ‘bctracker.com’ 

(correct site has ‘Stacking Sand’ on banner image. Please forgive the computer-limited simplicity). There are many clips, all made with Blender. A good example is the second clip down on the “Track Features" page, "Test 124

The problem: animated rigid body objects, and armatures seem to be unable to notice resistance to movement from other rigid body objects they contact.

My question: in the real world when a body walks on sand, the foot lands, sinks into the sand until the sand compacts to its own limit, then the foot will sink no more and the laws of physics say the sand starts pushing back.

At this point the body keeps pushing against the sand but the forces are moving the body, not the sand.

When I try to animate this process it seems there’s no way to simulate the real interactions. I mean that if I animate the foot to rotate down into the sand it will rotate exactly as far as I’ve animated it, irrespective of the resistance of the sand, when it should stop once the sand compacts.

One problem from this is that footprints are not as accurate as they can be, because sand movement isn’t allowed to stop after it compacts like it would in the real world. The foot just keeps right on moving it.

Tried the Molecular Particle System add on, which can help, but it does not resolve the problem that animated rigid body objects, and armatures can’t really notice resistance to movement from other rigid body objects they contact.

So is there any work around for this problem?

Would this be something to submit as a feature request to the developers? Thanks!

  • $\begingroup$ What is important? To illustrate the footprint effect (using what ever cheats necessary) or to make a physically correct simulation? $\endgroup$ – Jackdaw Aug 5 at 13:56
  • $\begingroup$ Both ... because these illustrations are for trackers, who by virtue of their discipline must pay attention to fine detail. Also, the website is intended to help instructors of tracking, so because of the instructional component, accuracy will matter. $\endgroup$ – Bob Wise Aug 5 at 17:18
  • $\begingroup$ I haven't done what you are trying to do but I would try to have one rigid body that are animated and then one that gives the print. I would then constrain those together so that the one giving the print could yield to the ground. Does this make sense to you? $\endgroup$ – Jackdaw Aug 5 at 20:56
  • $\begingroup$ Thanks! And it does make sense. Are you thinking constrain them with a rigid body constraint? I’ll try that out and see how it goes. It might help if you went into a little more detail? $\endgroup$ – Bob Wise Aug 5 at 22:50

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