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Destruction physics bug

Pretty straightforward bug:

alt text

On the right 3 starter content boxes with physics on, on the left 3 starter content boxes converted to destruction meshes with physics on. Everything is placed perfectly in the grid, no touching. This also happen when not awake, it just happens then as soon it is set wake.

This bug exits since 4.8! When I converted my Where Is My Hammer game some time ago to a new version most of the world basically exploded on start because of this bug. All meshes there are destruction meshes and it results in a chain reaction. I have hoped it would be finally fixed but I guess not, Is this on your to fix list and when I can expect it to be fixed? I would just stick with 4.8 but I want to do something with VR and destruction which is pretty much impossible in UE4 currently! :(

Edit: Looks like it is only that extreme when scaled up, those boxes are scaled up x2. With x1 it only happens a tiny bit.

Product Version: UE 4.11 Preview
ettzrtzif.gif (5.1 MB)
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asked Mar 23 '16 at 12:49 PM in Using UE4

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Hi Davision,

This isn't a bug, This is part of how Destructibles collision hulls are generated at the moment.

If you use the CVar PXVIS Collision you will be able to see the collision hulls that are generated. They are not very tight bounding to the box itself and there is no adjustment currently for this, however, IIRC there is a ticket in for this to be addressed when Destruction get it's well-deserved overhaul in the future. The slight movement you see is the destructible now being woken up to simulate physics and the collision hulls push back from intersecting each other.

There are no plans to work on Destruction aside from crashes and regressions in the near future as far as I'm aware though.


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answered Mar 23 '16 at 01:20 PM

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Tim Hobson ♦♦ STAFF
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avatar image Davision Mar 23 '16 at 01:47 PM

Yea, I looked more into. It wasn't this issue which made my world explode. The Enable Impact Damage tickbox is new since 4.8 and it was enabled through the conversion on all my meshes. Before 4.8 the default was Default Impact Damage Depth set to -1 which has the same effect as the tickbox false. This setting can now be not lower then 0. Well I guess it was a combination. The slight push back on start resulted in the Impact Damage getting triggered on all the meshes which then made everything explode. Another new issue I found is that a destruction mesh attached on a destruction mesh results in a strange behaviour where the collision is not welded and can not be set to welded, Which results in every child destruction component moving through collision when the parent object does any physic simulation movement. I made prefabs before where the root component is a destruction component which worked fine pre 4.8, they did just physic simulate independently. So I guess to work around the hull problem issue is to make every asset in the right size and never scale them in the editor or even better make them bigger in the 3d program and then scale them down in UE4 to hopefully get close to zero push back.

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