Cloth Sim Broken in 2.81 but works pasted into 2.79, am I doing something wrong?

So, I have a simple scene of a “curtain” opening. The cloth object is literally just a subdivided plane, and it is not even all that dense.

I first made the sim in 2.81, using just a few hooks and a cloth preset. not only does the cloth sim run veeeery slowly (and my comp isn’t bad, i7, 64gig ram, dual 1070s), but it also looks just a little bit off. Acceptable, but off.

But, the real problem comes in when I turn on self-collision. The simulation slows to a creep. Like, one frame of this simple cloth every 10-100 seconds. But more than that, the cloth object collapses in on itself in a matter of frames, just sucking in and imploding. And even though it only takes a handful of frames for this to happen, those frames take FOREVER to calculate. I copied and pasted the relevant pieces into 2.82, and had the exact same outcome.

But, then for some reason, a hunch maybe, I pasted the exact same objects into Blender 2.79 and ran the simulation. Not only did it run MUCH faster, both with self-collision turned on AND off, but it functioned exactly how it was supposed to. No implosion/self-collapse. Just nice, pretty hanging cloth doing what it is supposed to do.

Am I doing something wrong? Not accounting for something in 2.81, even though I am just using presents? I would really love to keep everything within 2.81/2 instead of having to backtrack into old versions of Blender.

This use case is so basic, it makes me more for when I have a more complicated scene, even though I have seen so many great sims done in 2.81 that don’t seem to have these issues at all.

Attaching the 2.81 and 2.79 .blend files, and any advice would be absolutely great. They are set up the exact same. The simulations have not been run or baked to keep file size low, but all that needs to happen is for play to be clicked, and the difference is apparent within the first few frames.BA_DownstairsFireplace35_ChainClothSim_279.blend (1.5 MB) BA_DownstairsFireplace35_ChainClothSim_281.blend (1.6 MB)

I’m not sure really why the 2.8 won’t work, I looked at it and didn’t see anything wrong. I think the self collision is causing problems, when I unchecked it and ran it it seemed to work. I reduced the cache to 50 frames, but it looked good.

Curtain opening is one of the few area where blender cloth sims do really well and have convincing results. The empty hooks add more control but also more complexity, the simplest case I would think of is to create a shape key of the top in an S pattern and shifted over (See image) then keyframe the shape key( (which is evaluated before the cloth sim) and to my eye you get a really convincing curtain opening. And it bakes in like 10 seconds. Have a look if you like, nest of luck!

curtain blend on dropbox

the basis key is just a flat subdivided plane, and this is the single shape key


in motion: