Rigid body inside a sphere

#1

I’ve got a big sphere with smaller objects inside of it, the big sphere is the passive rigid body and the smaller objects all active. My problem is that I can’t get it to behave properly, it seems that the big sphere is acting as if nothing should be inside of it, the small objects will only collide if they come from outside of the big sphere.

I’ve tried inverting the polys as the doc says but the problem still remains. Any ideas?

Thanks.

#2

If you can, make the half sphere a closed surface. The RBDs do not like open surfaces like a simple grid. Also add more triangles and make it an “actual shape” collision object. It is always best to add more triangles using actual shapes when using the RBDs.

Chin

#3

Change the larger, Passive Body sphere’s “Collision Type” from a bounding object to “Actual Shape” and with it’s polygons inverted it will work.

#4

Thanks for the help guys, changing the big sphere to ‘actual shape’ and inverting the poly normals was something that I’d tried and it didn’t work… now I’ve made a thick sphere object and using ‘convex hull’ worked much better.

I experienced quite a lot of popping and collision explosions afterwards however, is a ‘substeps’ value of 80 with adaptive substepping on (at default) considered high?

Cheers.

#5

“changing the big sphere to ‘actual shape’ and inverting the poly normals was something that I’d tried and it didn’t work”

That does work though, I just double checked it yesterday in 2010.

#6

I wouldn’t be surprised if it’s error in user here… I added two spheres, inverted polys on the big one and dropped the small one in the middle. The small one falls for a few frames and then pops to the outside of the sphere.

I’ve saved this simple test out to here, I must be missing something so would be interesting to hear what’s up with this scene.

http://www.mediafire.com/?sharekey=7e79b20e7da2b406111096d429abd3600ad46d5a03fa3ac684bbec0582a7e90e

…but as mentioned, I’ve fixed it with a workaround although the objects inside the sphere don’t glide over the surface very well… they have these really small pops as they get closer to coming to their rest position after having bounced around. Substeps of 200 doesn’t help. :frowning:

#7

Your big enclosed (passive) collision sphere is like an open surface as it technically has no volume. Actual shape collisions are very sensitive to this.

I opened the top and duplicated the sphere. Scaled up the duplicate and merged the spheres (with blend on) to give it volume. Using this new sphere with actual shape now works.

Chin
TIP: Increase triangle count before collision detail for actual shape collision objects. It will be faster.

#8

If you do it the “invert”/actual shape way, it won’t behave properly if the smaller active sphere is using “actual shape” as well, the small one needs to be set to a simple bounding sphere, your scene works if you just change that.

#9

Thanks guys, chinny - that’s actually exactly what I did to get it to work but wanted to make sure I wasn’t doing anything I didn’t have to do.

I actually have to use ‘actual shape’ for what I’m doing but good to know these things you are mentioning… slowly getting there. :slight_smile:

#10

Changing your inside ball to collision sphere would work also, but I guessed you were looking beyond this simple example :slight_smile:

Another tip is when your “actual shape objects” are far away or have few concave parts, use “convex hull” collision type as it is super fast compared to “actual shape”.

Chin

#11

Yeah - the posted example is kinda small in comparison to what I’m doing… :slight_smile: Essentially there’s loads more stuff with odd shapes, but all inside a sphere. ‘Convex hull’ seems to be a good option…

#12

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