Double loop strip. How to setup?

Replies to This Discussion

Permalink Reply by Daniel Piker on February 19, 2011 at 12:29pm

Here I started with a surface with the right topology, and pulled those 2 squares where it touches together with springs, while also using bending, length and shear springs.

 

Because the divisions of the initial surface are uneven, you can't use too stiff a bend force initially without it exploding (the current bending model works best when consecutive segments are the same length).

So I put in sliders for all the different stiffnesses, drag and timestep, and you can gradually adjust these as it gets more into shape - it's a little delicate, but does reach a solution.

 

I didn't try it yet, but it would probably be better to add a planarization force to the quads, and this would also give a better guarantee of ending up with a developable surface.

Attachments:

• doublelooppaper.3dm, 229 KB
• doublelooppaper.gh, 78 KB

Permalink Reply by Mårten Nettelbladt on February 19, 2011 at 1:39pm

Wow! Thanks a million Daniel! I'll play with this for a while, and get back...

Permalink Reply by Mårten Nettelbladt on February 19, 2011 at 3:13pm

Thanks again Daniel. It looks like you didn't have to mesh this one, and there's no WeaverBird. If I increase "connection" and decrease "Drag" I get this stable solution (similar to yours I think). I'm not sure what the other sliders do, the surface started to look very funny when I pulled some of them. I think it would be really exciting to see something like you mentioned about planarization force for quads! At the moment, my digitized saw blade (gray surface in the image) and the kangaroo surface (red) seem a little too different. I've attached the rhino file. Hopefully the planarization would push the surface into the "right" place. What do you think?

Attachments:

• MN-doublelooppaper+scan.3dm, 137 KB

Permalink Reply by Daniel Piker on February 20, 2011 at 3:55am

Thanks for doing this testing - interesting to see.

Do you know if this shape is very dependent on the width of the strip ?

 

The quad planarization is already in Kangaroo, and this should ensure developability, but on reflection I think a realistic virtual paper strip is going to need a few new forces;

One would be bending resistance between two faces around an edge (not just 2 edges around a vertex like we have now), dependent on the length of the edge as well as the dihedral angle.

The second would be alignment, to keep the opposite edges of each quad parallel.

Lastly I guess we'd need something to keep the edges a constant distance apart.

Permalink Reply by Mårten Nettelbladt on February 20, 2011 at 3:28pm

I don't think the width plays a huge role. If a surface is very wide the minimum radius of the material may be reached, and that will matter I guess.

It's also worth noting that if a developable ribbon* is to twist, the direction of the curvature must differ from the direction of the surface in some areas, meaning that the ruling lines will not always be perpendical to the edge (consecutive ruling lines don't need to be parallel, but must always be coplanar). I'm not sure how to allow for this in Kangaroo.

 

*Here meaning a surface that unrolls to a straight strip.

Permalink Reply by Daniel Piker on February 20, 2011 at 5:38pm

Here's a definition that uses the planarization force.

It does allow the ruling lines to become non-parallel as you describe.

It produces surfaces which are developable, but not necessarily straight when unrolled flat.

Attachments:

• ribbon.ghx, 261 KB
• ribbon.3dm, 45 KB

Permalink Reply by Mårten Nettelbladt on February 21, 2011 at 12:52am

That looks very nice! 

To make it "straight when unrolled" just make sure angles A+B = 180° = C+D, at every ruling line, as illustrated below. (This is how Tapeworm works)

Permalink Reply by Daniel Piker on February 21, 2011 at 4:24am

Yes, I think an 'angles sum to 180°' force for sets of 4 vertices will be a very nice thing to add.

 

Also a 'sum to 360' force for the angles around an internal vertex should be useful for ensuring developability of folded patterns.

Permalink Reply by PaUla on July 26, 2011 at 5:33am

Dear Daniel, I can not open the definition due of the grasshopper version.

I'm looking at a definition that work with twist and bending (a strip of wood in phyisical models). I'm trying to parametrize the behaviour of twisting and bending a curve. could you give some advice!

Permalink Reply by Mårten Nettelbladt on February 21, 2011 at 4:34pm

That sounds like two very good additions!

I'm planning for a large scale test of this double loop shape using double layers of 2 mm plywood joined to a length of 6 meters...