Invert gravity and have device frame as 'walls'?

[MrUltimate]

Hello there!

First off, thanks for the tutorial and the starter. I'm trying to tackle a project and hoping you can help. Essentially I want to have three objects float and collide with each other, bouncing off of the device's frame. I'm wondering if this is achievable through Cannon? Inverting the gravity like so: world.gravity.set(0, 9.82, 0); doesn't need to invert the gravity, and even then, I'd have to have a 'roof' of sorts to have the objects bounce off the top and sides. Can you think of a way to achieve this?

Thanks in advance!