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I have seen numerous threads on this topic which all have similar attribute as my issue but are not the exact same.

Here is what I did:

  1. Select a cube and cell fracture it
  2. Remove the original cube
  3. Apply rigid body active on the fractured cubes
  4. Animation proceeds as normal, meshes fall down as expected
  5. When collision shape is set to Mesh, instead of Convex Hull as in 4), the fractured cubes explode in every direction

enter image description here

enter image description here

Can someone explain the logic behind this? Isn't Mesh more accurate of a setting than Convex Hull? Convex Hull if i remember correctly, uses a bounding box that only somewhat follows the shape of the mesh it is applied on

I recall reading the explosions happen when the rigid body meshes intersect each other. This is confusing however. Logically, wouldn't convex hull setting have more intersections than Mesh? Mesh would be more accurate and have less intersections, yet it is only Convex Hull that doesn't explode while Mesh does

I've tried everything. Applying scale, setting the centers of all the fractured meshes to their volume again yet nothing seems to work. Why isnt this working with Mesh and how do I get it to work with Mesh?

Link to the file : https://drive.google.com/file/d/1oUC7tagOsGEZtKJIo11IjR4yJwtTQAlF/view?usp=sharing

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  • $\begingroup$ please provide blend file. one reason could be the sensivity ....[1]: i.stack.imgur.com/0vCsT.png $\endgroup$
    – Chris
    Jul 19 at 15:13
  • $\begingroup$ @Chris I have now edited a link to the file in the post. As for sensitivity, what I had tried before didn't quite work well, and the problems still happened $\endgroup$
    – Hash
    Jul 20 at 6:54
  • $\begingroup$ No need for a file. Convex Hull works fine for these things. Mesh usually is more accurate, but with Convex Hull you have the option to activate a Collision Margin under Sensitivity. With Mesh a Margin is activated by default and set to a default value of 0.04, and since your fractured pieces are much closer together they are rejected by each other. $\endgroup$ Jul 20 at 9:41
  • $\begingroup$ @GordonBrinkmann I had tested both Mesh and Convex Hull. Convex Hull has a setting like you said called Collision Margin and Mesh has one simply called Margin. Are these both the same setting functionally? $\endgroup$
    – Hash
    Jul 21 at 10:03
  • $\begingroup$ They work the same, yes. The difference is with Convex Hull you have the option to enable the Collision Margin or leave it disabled. With Mesh you cannot disable the margin, only set it to a low value or 0. $\endgroup$ Jul 21 at 10:25
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Convex hull is not bounding box, but rather “bounding plastic wrap” as it were, and actually produces better results than mesh shape collision most of the time. For mesh shape Collision, Blender apparently has to do some special set up to detect concave areas, and a margin is necessarily put in place around the body. For a convex hull, however, the collision can be right on the surface. Since cell fracture does not produce concave mesh (I think), Convex hull is probably your best option here.

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  • $\begingroup$ But if the margin more closely follows the shape of the mesh in the Mesh setting with Blender doing the extra setup, why is it more error prone then? And if cell fracture does not produce concave meshes, and Mesh shape collision does special setup only to concave areas, then in the case of cell fracture shouldn't Convex Hull and Mesh both behave the same? $\endgroup$
    – Hash
    Jul 20 at 6:58
  • $\begingroup$ @Hash No, because as Irepeat, having to count for concave shapes makes blender have to work a lot harder and the system is a lot less stable. $\endgroup$
    – TheLabCat
    Jul 20 at 11:20
  • $\begingroup$ @Hash i’m not sure why this is the case, but imagine if you were trying to put plastic wrap all around an object: in order to account for the possibility of concave faces when doing that, you would have to use a lot more plastic wrap, and then poke every single place on the object to try to get it to squash in. $\endgroup$
    – TheLabCat
    Jul 20 at 11:21
  • $\begingroup$ But in the case of cell fracture none of the shapes and meshes produced appear to be concave in shape, so why is there an issue when using Mesh here? $\endgroup$
    – Hash
    Jul 21 at 10:04
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    $\begingroup$ @Hash As I said before, when you set the shape to Mesh it automatically sets a margin of 0.04 by default, and your concave pieces are much closer together - that's why they get rejected from each other - if their distance is 0.0 for example but they have a margin of 0.04 around them, then they are inside each others collision boundaries. $\endgroup$ Jul 21 at 10:23

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