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siddd

Dynamic RBD fracture bounces on ground impact

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Hey folks,

I'm having an issue when trying to dynamically fracture a glass tumbler object. Check out the quicktime attached to this post. You can see that the glass hits the floor, the object is fractured, but they immediately bounce back up. What I would expect to happen is have the pieces continue their movement down into the ground, but I can't find a way to get this.

I've tried adjusting the Pre/Post Velocity Bias on the fractureparms node, but that doesn't seem to help.

Does anyone know what params I should be looking at?

Thanks for any help.

Sid

badsmash.mov

badsmash.hip

fixed_tumbler_EXP.obj.zip

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If you turn off the fracturing you can see how your object moves. Even though you have set bounce to zero, the shape of your object in combination with the relatively high initial Angular Velocity will still make it bounce. When it breaks the pieces then will inherit that upward velocity.

My tip would be to set up the motion of your object until you are happy with it, and only then start fracturing stuff.

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I agree with David. You could also animate your gravity and change it to -100 in frame 24 and make them affect only the glass pieces. animate it back when the pieces are no longer flying upwards

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The tumbler is 8.5 x 9.2 x 8.5 units in size but Houdini by default expects units in Meters unless you changed the preferences. That is the issue with this simulation.

You reduced the mass of the object somewhat but did not affect gravity.

Assuming that the units are in inches and Houdini defaults expect meters, the conversion is 39.37 in/m.

Gravity converted to inches:

-9.80665m/s * 39.37in/m = -386.088 in/s

and for mass:

1000kg/m3 > 0.0163871 kg/in3

These figures are orders of magnitude different than the values you set.

Running the sim now gives somewhat expected results.

Now you have to add back bounce to defaults and everything else you changed chasing down the sim including reworking all the fracture parameters.

At this point, it may be better to redo everything setting the Houdini preference for scene scale...

Set the Houdini Options preferences to match the units of the scene that you are trying to simulate PRIOR to adding RBD objects to the scene. This will initialize the parameters to the defaults to correct for scale.

Here's the scene started from scratch entirely using the shelf with the preferences to inches for units and everything at defaults other than the initial velocities you set before with a bit more rotational velocity added. Note that the fracture parameters are at their default assuming meters so you will have to increase the fracture radius but given the way the glass is hitting the ground now, I'd argue that the base is tough so you'd pretty much get this result.

badsmash_jw.hip

In practice, it is better to scale the entire scene to match the default metre units.

Edited by old school
  • Like 2

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Hey everyone, thanks for the replies.

Hudson > I'm going to have a lot of these objects, so I'd like this to work properly.

Skybar, Old School > The motion of my object is as I want without fracturing. When I turn on fracturing, that's when the problem start occurring. I'm expecting the fractured pieces to not bounce upwards - I'd like them to continue into the floor, like in this video:

However, if you look at my quicktime from my first post, the glass fractures and then bounces upwards.

Again, I really appreciate your replies, folks.

Sid

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hey Sid it seems to work with a more sensible scene scale try this...

smash.hip

Edited by TomFX

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Hey Tom!

So, it looks like all you do is set a uniform scale on the tumbler of 0.2, is that right?

Sid

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