Collide attr and overall collisions clarification

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1

Hi guys,

i’ve been running into an odd one. Probably due to my lack of knowledge ^^

I created a setup for a regular human character. The chest control seems to be causing issues to the sim, most noticeably the neck initial settle at the start and the big unmotivated change on the overall body position/angle as soon as the hip control changes behavior from kinematic to dynamic (hips and chest moving wildly). See gif.

Jumper

Setup is roughly as below:

  • The hips, chest, neck, clavicles and head are all part of the same group and set to local and dynamic.
  • the hips switch from kinematic to dynamic around frame 22-23
  • the chest geo is a big piece that intersects hips, neck, clavicles, upper arms
  • legs, arms and fingers each have their individual group

I tried a lot of different things:

  • collision overlap value for chest, arms, hips, neck to stay at 0
  • collision overlap value for chest, arms, hips, neck changed to 1 to be a custom overlap group
  • collision with self
  • collision with nothing
  • collision off

Out of all the above, the only instance that seems to prevent the sim from breaking is when i switch the chest marker to “collide - off” and i’m not quite sure why and if there’s a better way to do it.

I was under the impression that when the overlap value is set to 0 for the same group elements (such as for clavicles generally), the intersection of those markers is overlooked and overlap allowed. Similarly, if the overlap value is set to the same on different markers they are allowed to overlap/intersect. In my mind this means that having collide off and overlap values matching should give the same result as long as we don’t want to have any interaction with the environment.
However, if a third party (such as a cube) is thrown in the mix and the sim impacts that cube, the resulting behavior should be different as we now have some other element that we want to interact with the existing markers. I believe when the chest collide is off, the cube is ignored and when collide is on, the cube is not ignored as long as the overlap groups of chest and cube are not matching.

Is there a way to retain collisions on the chest in case of external elements for impacts, but also guarantee that the chest does not affect the sim negatively such as in the scenario above?
Additionally, how do groups play a part in collisions with other groups? Am i missing something there as well perhaps?
Is the initial setup (amount of intersecting geo) playing any part in the way the sim understands what is allowed to intersect and what is not allowed to intersect?

Sorry in advance as I might have a bit of a wrong understanding of the whole collision system between markers even though i did follow all available documentation on the blog and available chats on the topic. Thanks for reading till the end! ^^

2

Hey @magicleo88, welcome to the forums! :partying_face:

Then this definitely sounds like a self-collision issue.

Have a look at this method to visualise where there are contacts.

I expect you’ll find some contacts happening inbetween markers in your character, that should give you some idea of which markers are indeed still overlapping.

0 is a special value meaning “respect self-collision”. Meaning that if Self Collide = On then markers in the same group will collide with each other.

The exception to this, globally, everywhere, is markers that are direct neighbours to each other.

So if you’ve got two markers that do overlap - and you would like to permit this - then you’d give them another value. Anything 2 and above. I’ve found that 1 is another special value that isn’t working correctly, so 2 up to 65,000 or so is safe to use.

-1 is the final special value, meaning "collide with everything in the same group, even if Self Collide = False

The rules are:

  1. Neighbours may overlap
  2. Markers in a group with Self Collide = Off may overlap
  3. Markers with the same Overlap Group number may overlap, except -1, 0 and 1.

That’s about all the control you have here. If you have a scenario in mind that requires more control, then this is the place to let me know!

I do touch on overlap groups and self collision in the recent video just published here:

If this still doesn’t solve your case, feel free to upload your scene (or one like it) and I’d be happy to take a look. Just make sure it’s as small as you can possibly make it, 0-5 mb, and .zip it too, to keep our precious backups from exploding. Then you can just drag it into a post here.

PS: If you can make a mp4 instead of a gif, then that would help as we can scrub the video and get some idea of how long it is.

3

Thanks Mark,
i’ll try this out today and let you know!

Your response time is insanely fast, really appreciate it! and keep up the good work!

4

That’s because my eyes are surgically attached to my monitor and this forum.

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Let me know how it goes! :disguised_face:

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5

I just tried changing the overlap group number for chest and upper arms to match and everything seems to be working correctly.

Essentially this is what cleared my doubts.

The rules are:

  1. Neighbours may overlap
  2. Markers in a group with Self Collide = Off may overlap
  3. Markers with the same Overlap Group number may overlap, except -1, 0 and 1.

Based on that i realized that the upper arms and chest did not play nice with each other because not immediate neighbors and their intersections caused all sorts of weirdness happening in the sim.

Thanks again, you are a life saver!

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