unmerging at contact level instead of body level

[EulalieCoevoet]

When lifting a sub stack in a bigger stack... the contacts that go down to zero force with the single step PGS should become active, and the rigid collection graph should be split in 2 on this "edge".

Or... will this cause a problem in a more complicated jumble, if the top is still rigidly connected to the bottom through different internal constraints!? To discuss!

[paulkry]

On second thought, not sure how internal contacts can simply become external if there is a loop of bodies. Consider the doubleStackUnmergeTest.png, ans suppose we include pinned bodies in the collection, or add another double length bar at the bottom. If you make a set of internal contacts active then they would have zero force as they are between two parts of the same merged collection. Perhaps with additional time steps those zero contact forces would cause other bodies to unmerge in subsequent time steps, but is this a desirable behaviour??

I think this issue should perhaps be tabled, closed, or discussed futher.

[paulkry]

I'm simply going to close this... I don't think it is a real issue. There is a more general problem of unmerging correctly in larger more complicated graphs to discuss later.