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I'm trying to make a hole open up in a primitive, in this case I hooked the vertices of a hole in a flattened cube. The scale animates so that the inner hole opens after a sphere lands on the cube.

Why does the sphere not fall through the hole? Is there a different way I should be doing this? (The image below should be an animated gif) Here's the example of the sphere not falling through the hole.

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    $\begingroup$ Make sure the hole has both physics type as Dynamic and bounds as Mesh $\endgroup$ – Duarte Farrajota Ramos Jul 12 '18 at 18:58
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After setting Rigid Body Collisions to Mesh you need to check Deforming. See my settings below.

enter image description here

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  • $\begingroup$ Yes that did it! So many settings to keep track of :) Can I ask what method you used to animate the opening hole in your example? I'm sure my method is not optimal... $\endgroup$ – Rich Jul 12 '18 at 19:44
  • $\begingroup$ @Rich Shape key. Do you want the .blend? $\endgroup$ – TechTornado Jul 12 '18 at 19:55
  • $\begingroup$ Absolutely! I'm just learning how to use shape keys, so that would be great. $\endgroup$ – Rich Jul 12 '18 at 19:58
  • $\begingroup$ @Rich Here is your link sir. I will be taking it down in 6 hours though (storage issues). $\endgroup$ – TechTornado Jul 12 '18 at 20:06
  • $\begingroup$ @cegaton Here is the permanent version uploaded as you suggested. $\endgroup$ – TechTornado Jul 13 '18 at 0:22
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To explain this further, a "Convex Hull" is an approximation of your shape, NOT the actual shape. For example, the object in the back is a convex hull for the torus. Since it's 'convex' it cannot have holes or indentations - that would be concave:

enter image description here

For physics simulations, the simpler the shape, the faster it is to calculate...that's why there are so many simple, primitive options for the Collision Shape dropdown. If you can get away with an approximate of your mesh then it's quicker to simulate. In my torus example above, if you never care if anything goes through the center hole, the convex hull will work fine. Or, another example, if you had a Suzanne bouncing around, you probably could get away with a way simpler convex collision mesh like this:

enter image description here

However, in your case, you want the actual mesh shape to be the collision, so you need to choose "Mesh" for the collision shape for it to work as you expect. This is slower to calculate for the physics simulation because it needs to check collision with every face of your mesh. Yours is simple enough that you won't likely see any performance hit, but with a more complicated scene you might.

In your case, if you do care about performance, I would make something like two cubes with collision on them that slide apart as the hole opens, rather than using Mesh collision.

Hope this explains in a bit more detail for anyone who is interested!

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Try changing your collision shape on the hole object from Convex Hull to Mesh. That should fix it

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  • $\begingroup$ Still no luck :/ Do you think it might have something to do with the fact that the empty object is controlling the animated hole opening? I'm stumped... $\endgroup$ – Rich Jul 12 '18 at 18:47
  • $\begingroup$ Perhaps you could try using a shape key to open the hole rather than an empty object. This could work better (if it does I'll update my answer). $\endgroup$ – Legoman Jul 12 '18 at 19:04
  • $\begingroup$ Good point! I will also try this method for future use. Thanks! $\endgroup$ – Rich Jul 12 '18 at 19:45

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