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Nice trick when faceting!

Posted: Thu Jul 10, 2008 7:41 pm
by Woolalan
Sometimes when faceting in Great Stella, I've wanted to have a plane saved in the faceted polyhedron - not just where it cuts a solid volume, but free in space. If you just define a plane in the usual way using the faceting tool, it will have green edges, "only one face, needs another". If you try to define the same face again to make it "double thickness" and get rid of the green edges, Stella doesn't accept it.
However, there is a work/around. If the plane has an even number of edges, like a rectangle, then you can define the contour line, then define a plane consisting of the two shorter sides and two diagonals, and lastly the plane consisting of the two longer sides and two diagonals.
In this way, every line has been used twice and they're no longer green!
"Faceting complete!"
However, Stella won't accept a faceted polyhedron consisting only of infinitely thin planes: somwhere there has to be a closed volume.
Here's an example:
When you create the faceted polyhedron, Stella asks you whether coplanar faces should be blended - you have to answer NO to this, otherwise your planes disappear!
Here's the result:

My latest use for this trick was in using Stella to test the feasability of a three-dimensional construction. Currently I'm working as a volunteer in the Ethiopian highlands, where it's often minus fifteen C at night, so people can get pretty cold. As a woolcraft worker, I've been helping them to work out how to turn their sheep's wool into comfort and income.

That means making spinning wheels, which is tough for people with no wood and no woodcraft. We tried making wheels of grass, like baskets, but we're a bit too high up for that kind of grass so not enough people have the skill.

We've been "importing" plywood for making hand carders, so perhaps we could make wheels of plywood and string?
At 11 000 ft above sea level, Great Stella helped me to check the feasability! This wheel started out as a three-step faceting of a twenty-four sided prism. It works!

Posted: Sun Jul 13, 2008 12:18 pm
by guy
The trick should work for any number of sides above 3. Just divide the polygon into two or more pieces, and then add the whole polygon.

What an amazing application of Stella and facetting! Has anybody made a garment from the yarn yet? And will it be woven or knitted?


Posted: Sun Jul 13, 2008 7:11 pm
by Woolalan
Hi Guy! Thanks for your simplification of my faceting work-around. I think I found my uneccesarily complicated method in a panic situation and didn't realise until your reply that it could be done more simply.
In fact the "more than three sides" isn't a necessary limitation, because in its "Poly" menu Great Stella offers "Subdivide faces", making it possible to do your procedure even when starting with a triangle.

I shall be meeting a group of Highland farmers on Wednesday to guide them in making copies of the spinning wheel. So maybe they'll be spinning by the weekend! The yarn will be used both for knitting and weaving. If you'd like to follow this, you can log into my travel blog at ... 02342&en=1

Thanks also for providing us all with Steel Pillows for when we can't sleep! I expecially enjoyed your presentations of Lost Stellations of the Icosahedron.
Best regards,
Alan Waller/Woolalan