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Lecture 19: Intro to Physical Simulation (27)
SeanW0823

Is there an explanation for why red springs should be weaker? Wouldn't this result in a structure that is less resistant to out-of-plane bending compared to shearing?

modatberkeley

My understanding is that if red springs are stronger, then the structure will bend out of plane (curve into a basket)

ericyche

Is there a draw back in including all these spring structures in the computational model? Is there a way to improve this without additional computational complexity overhead?

Staffjamesfong1

@SeanW0823 @modatberkeley The red springs are there to encourage the cloth to resist bending. If you make them too strong, then your cloth will not bend at all, and behave more like a sheet of metal.

Sicheng-Pan

How does the added red springs help to resist out of plane bending?

jonathanlu31

I think before without those springs, the sheet could just fold over without encountering any resistance from the springs. But with the red springs, if it tries to fold/bend, either the red springs get stretched or squished, which resists the movement.

michelllepan

I wonder what impact adding the red springs in a different configuration would have, such as connecting them over every two points instead of every other point.

andrewhuang56

That's quite interesting! I'm not sure, but my guess is that the surface might become less resistant (relative to every other) to really small wrinkling since there's now no direct force keeping points that are two apart close together. So I wouldn't be surprised if larger wrinkles/bends were a bit harder to create than without any red springs, there would still be mini-bends at each point. I can't really think of a material that would be well modeled by this, but maybe it's good for something that shrivels up in a weird fashion.

starptr

Others might've found this obvious but I had some trouble gaining intuition for how in-plane springs such as those in red could help resist out-of-plane bending. This is because connecting non-neighboring points is fundamentally different from connecting neighboring points. If you imagine the springs connecting neighboring edges as mechanical axis, you can start to visualize the red springs as the mechanism which stops the sheet from bending into itself.

renaju

I am wondering if the red springs are only added on one side of the sheet instead of both, wouldn't they only resist the sheet from folding over to that side, or at least affect bending behavior to two sides differently? In general, does the mass-spring system have a concept of "sides" since the points masses and springs have no volume?

joeyzhao123

I think we consider the springs as actually the sheet itself? So technically it's on both sides already.

ShrihanSolo

I may be wrong here, but I believe one of the crucial difference between this and the other constraints, is that this is primarily useful as a resistive force while the others are attractive, used to maintain the structure between point masses.

sZwX74

@renaju I don't think we need springs on both sides because as @joeyzhao said the logic is that if the springs attempt to fix the distance between nodes that are two away. Even if we continue to intuit the springs as being on "one side", if the sheet bends to one side, they'll stretch the springs, which will try to shrink back to rest length, and therefore bend the structure back to the plane. If the sheet bends to the other side, the springs will contract but still try to repel back outward to rest length, and therefore bend the sheet the other way.

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