Bullet Dynamic Fracture Questions

gdkeast

Member
Still learning Bullet Dynamics in LW; two questions.

1. When I fracture an object in Modeler, and bring into Layout, it seems I can see the fracture or sometimes one or two fracture lines. Is there some way to fix this?

2. When I bring a fractured object in (parts body) and set it on a ground plane (static), it collapses automatically. Is there a way to stop that or is that just the way it goes?

Thanks!
 

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1. When I fracture an object in Modeler, and bring into Layout, it seems I can see the fracture or sometimes one or two fracture lines. Is there some way to fix this?

In Modeler:
Lightwave fracture doesn't work very cleanly. First, subdivide the initial object as evenly as possible. After fracture one should delete all 1 point and 2 point polygons, and also all the points that don’t belong to any polygon.

In Layout:
Check “Merge Points” in the bullet properties for the parts object.
Merge Points will attempt to ensure that matching vertices on either side of a fracture have the same position, ensuring the mesh stays ‘air-tight’ while the parts are not broken.

2. When I bring a fractured object in (parts body) and set it on a ground plane (static), it collapses automatically. Is there a way to stop that or is that just the way it goes?

There are several reasons why an object breaks apart without external influence. The Mass (and gravity), the Glue Strength (incl. Breaking Angle and Breaking Distance) and also the Collision Margin.
Apart from that, you can freeze an object until a collision happens (you find the settings in the Activation tab).

ciao
Thomas
 
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Here is an example with much higher geometry resolution (before fracture). Also, I made the Smoothing Threshold much smaller (approx. 20°):

Fracture_smooth_01.gif


ciao
Thomas
 

MonroePoteet

Active member
I'm no expert, but for the visible fractures, I always keep the unfractured object (an option in Fracture), and then do a one- or two-frame dissolve from the unfractured object to the fractured object when the fracturing starts based upon the Bullet Activation.

For delaying the fracture (Bullet Activation), I use two approaches: (a) use the Activate on Last Key option in the Bullet Item Properties=>Activation tab, or (b) use Start Sleeping and a Kinematic Body collision object.

Good luck!
mTp
 

prometheus

OFF TOPIC
Or...



It´s just a hack though, would not account for a fully realistic dynamic scenario, since it´s only set up to have density weight where the map goes, the rest will thus never react to gravity.
 

gdkeast

Member
I've almost got it, but a part of me is also thinking if you can't beat 'em, join 'em. LOL.
 

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prometheus

OFF TOPIC
I'm no expert, but for the visible fractures, I always keep the unfractured object (an option in Fracture), and then do a one- or two-frame dissolve from the unfractured object to the fractured object when the fracturing starts based upon the Bullet Activation.

For delaying the fracture (Bullet Activation), I use two approaches: (a) use the Activate on Last Key option in the Bullet Item Properties=>Activation tab, or (b) use Start Sleeping and a Kinematic Body collision object.

Good luck!
mTp

Yes, me too..

As to be clear on what last key means.
if you set it to activate at last key and just that and play the timeline, it will of course still just collapse directly from keyframe 0 since that is often by default a keyframe insertion there, so all you have to do is to make sure you first of course have your parts selected, move the timeline and insert a keyframe where it should start to collapse based on your own exact idea on when, then it should start to collapse at that point, but you can´t set any other keyframes after that for that object in such case.

And for collapsing based on advent, use start sleeping and kinematics as you said.
 

prometheus

OFF TOPIC
It may be possible to tweak the settings also just with activation time linear speed, angular threshold..without the above mentioned start sleeping or activate on last keyframe, but to always have it active, though I haven´t really researched that, until now.
Extremely tricky to get it right without a slight jitter our bounce, but I am close.
As in the real world, Gravity is always there on this planet affecting everything that isn´t supported by something or resting on something, so a base fond, ground and whatever to hold it up, or other structures is needed, often set to static.

But I think I am very close on finding settings here for that to allow for non collapse, even when always active, but I have to test it all much more, under various cases as well, and various types of fractured pieces, it also depends on frame rate for accuracy, and how close a wall of bricks for instance is to a ground segment that is supposed to support it, it´s mm, micron accuracy maybe.

If it seems to work..then this could mean that I wouldn´t have to manually bother about locating the keyframes when it needs to collapse, or use a kinematic body, it just reacts when something is blowing it away based on force or other dynamic pieces are hitting the parts.

It´s been years I think since I messed around with the bullet dynamics, it feels like that anyway.
 
There are several reasons why an object breaks apart without external influence. The Mass (and gravity), the Glue Strength (incl. Breaking Angle and Breaking Distance) and also the Collision Margin.
Apart from that, you can freeze an object until a collision happens (you find the settings in the Activation tab).

As already mentioned, there are many factors that determine whether an object breaks on its own.

With a high value for “linear damping” (100% works best) and a some “glue strength” you can stop almost every collapse. Of course, it takes a few frames for everything to settle down.
It still depends on the settings already mentioned, but 100% "Linear Damping" stops any linear movement immediately even gravity is powerless against it. But it is no physically correct simulation anymore.

The dilemma with simulations is that on the one hand we want to have a physically correct simulation but on the other hand we want to control and design everything.

To simply set the starting point of the collapse, the activation settings are probably best.

ciao
Thomas
 
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MonroePoteet

Active member
Also remember the "Managed Destruction" technique using a Distance to Object gradient to "dissolve the glue" in the Bullet Parts Body by moving around a Kinematic Body collision object:


A significant shortcoming of this technique is that the glue has to be dissolved long enough to let the individual parts escape their surrounding parts, or they can stick back together in a very unrealistic manner. In other words, the Null may have to move fairly slowly to create the desired effect.

mTp
 
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dpont

Member
Ideally, fracturing should preserve a vertex normal map,
for keeping smoothness along the external surface.
Not obvious because often, especially with smoothing groups
normalmap is stored per vertice, not per point,
like for discontinuous UV.

A compromise for current smoothing,
is possible by copying the normal map of the
unfractured mesh in background (per vertice)
and pasting it with interpolation when needed
to the fractured external mesh per point,
though this is a slow process with a script.

no normal map versus normal map

fractured_no_normalmap_1.gif
fractured_normalmap_2.gif


Denis.
 
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