How to simulate this using Bullet the same as RBD Solver?

[magneto]

Hi,

I was following a tutorial and using the shelf buttons for RBD objects the same way as shown but since they now actually create a RigidBodySolver DOP set to Bullet, and not an RBD Solver DOP, I am getting a different behaviour where I thought I was doing something wrong.

Basically it's using the RBD Point Object to copy some objects onto a 3x3 box where they will fall down as separate pieces. RBD Solver DOP does this.

But Bullet simulates this as if the bottom row side of the box is rigid, thus the objects copied on these points acts as a ground where all other objects are falling onto.

Is this normal? If so, how can I make it so it works as if I am using an RBD Solver DOP, but using Bullet?

Here is a picture showing what I mean:

Thanks

[cwhite]

If you could attach an example file that would be very useful - I tried putting together a similar setup with the RBD Point Object and it worked as expected.

Did you look at the guide geometry for the object? That will show you the collision shape that the Bullet Solver is using to represent the objects.

[magneto]

Thanks cwhite. I attached the file.

I found the problem I think but not sure how to fix it. So basically the grill object's collision object is capped at the top, so if I switch the RBD solver to bullet, then the coals do not go through it.

But if I set the solver to RBD, then the goals go through the grill until they actually hit the concave shape of the grill.

Why do they behave differently when the collision geometry is the same?

What do I need to do so Bullet also lets the coals pass through the grill until they actually hit its geometry?

Here is the pic showing the actual geometry and the collision:

rbd.hip

[ikarus]

I found a trick for dealing with concave static geometry is to just prefracture the static geometry enough that the concavity is captured, and load it as a fracture object that's not active.

[Skybar]

Static Object -> Collisions -> Bullet Data -> uncheck "polygons as convex hulls" ?

[cwhite]

Skybar's answer is correct - the default collision shape for Bullet objects is a convex hull, which is why the grill is capped at the top.

If you uncheck the "Polygons As Convex Hulls" setting on the Bullet Data tab of the static object, then a concave collision shape will be used and you'll get the behaviour you want.

Concave collision shapes should only be used when necessary, though, since they will result in slower simulations than convex hull shapes (this is why the convex hull shape is the default setting).

[magneto]

Thanks alot guys, I understand it now. I knew it had to be something simple like that I missed