Poser cloth sim of quads behaves in odd ways. The principle artifact I see and can produce most easily doesn't have a name so I'm going to give it one.

I call it the "crystal ridge". It happens when a fold occurs along the diagonal of the quads. At some point a small-scale catastrophic collapse occurs in one quad, where it severely bends in half perpendicular to the fold, resulting in a severely non-planar quad. It does this despite using very high fold resistance. It's like fold resistance is ignored when this happens. The chain reaction that follows is like a crystal forming.

This collapse of one quad promotes a similar collapse in the next one, and actually causes the next quad to rise up higher than it should be. In turn, this collapses the next one. Soon a whole diagonal row of quads have all collapsed and risen up, defying gravity and stiffness completely.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)