Dirt/sand in Lagoa - noob question

[oz42]

Hi guys, first topic and it's a bit of a noob one, I'm afraid!

I'm trying to create the effect of a crane's bucket scooping up some dirt (dry, granular mud) and I'm having real problems with the bucket's collision.

I'm starting simple and have done a variation of the sample Lagoa scene (Lagoa_Inelastic_01.scn) which just has a cubic volume of Lagoa granular material piling up on a flat surface. When I introduce a bucket model to scoop this up (the bucket is really just a cube open at the front and top at this point!) the particles are scooped in but they react like little bouncy balls! I've played with just about every setting in the Lagoa granular material (e.g. Inelastic Collision, Friction, Flow Rate, etc.) and all the settings in the bucket model's Lagoa Set Collision Data (e.g. Static Friction, No Slip, Body Impulse, etc.) and nothing seems to make a significant difference.

Am I going about this the wrong and should I be using standard ICE particles instead of Lagoa? Or am I just missing a setting within the Lagoa setup that will give the particles more mass? They currently react well to the global gravity (i.e. form a pile) but bounce off the bucket as if they were very light ball bearings!

Thanks in advance if anyone can give me any insight into this one...

[Hirazi Blue]

Could you perhaps post the scene to examine? ;)

[oz42]

Good idea!

here's a quick screen grab. As I say, I'm starting simple (very simple!) and hoping to scale up once I've got the hang of things. Even in this screen grab you can see the particles flying off as the bounce off the bucket.


and here's the scene;
http://www.richardosborn.com/xsi/lagoa/ ... 061211.zip

Thanks for taking a look.

[Hirazi Blue]

Okay, here's what I think you could do (but I'm no expert either):

• Decrease resolution to 0.5 for the "Resolution Per Unit" value in the "Lagoa Emit Grid"
  Disconnect the "Resolution to Substeps" from the "Lagoa Simulate Multiphysics"
  (unsure about this, but I like to control it "by hand")
  Set "Resolution to Substeps" to 2

And for something more dirt-looking: lose the sphere-shape. At least plugin an instance shape into your "Lagoa Emit Grid", preferably with a small group of slightly irregular objects, or at least with something like an "Icosahedron". The look of the sphere obviously does not "cut it", especially if your "Resolution Per Unit" is so high to begin with...
;)

[oz42]

thanks for the reply.

I have decreased the resolution but other than having more, smaller particles, they have much the same 'bouncy' effect. I'll try increasing the substeps and see if that helps.

I know the 'dirt' looks pretty simplistic now but I was hoping to run quick experiments with this simple setup to get some values correct and then add complexity later. However, other than falling due to gravity, the particles don't seem to react as though they have any mass?

[Matic]

Sounds like collisions are imparting energy to your particles, a common problem in sims like this. At a subframe level when an overlap (collision) is detected often an impulse force is imparted to move the particle outside the collision area. If the sim's accuracy is too low this "restitution force" can send things flying.

I assume this is the case with Lagoa, if so there should be a setting somewhere to adjust accuracy and soften collisions. Also if the mass of a particle is low, it is easier to move it with lower forces... Think of a weight falling into a pool of hollow plastic balls versus a pool of billiard balls.

Also, as a general note, the last time I used Lagoa to do some dirt I found that you really want to start out with small particles, at least smaller than what you have in that screenshot. Spheres are fine if you are simulating grains, but look/behave differently when they get bigger than pebbles, so the settings you arrive at won't be nearly as close. If you don't like waiting for the emitter to generate all those "grains" simply emit once and and freeze the pointcloud for a cheap "initial state." I suggest duplicating/hiding your original emission point cloud if you do, in case you need to coax ice to retain certain attributes when you freeze. Hope all that helps some.

[oz42]

Matic,

thank for a very full and enlightening response. I think you're right, some of the particles are caught mid simulation and then when they try to catch up they are imparted with a huge force. I've increased the sub frame sampling and this seems to have improved things a little. I'm still playing with things and hope to post a final version when I get things sorted...