How hard would it be to compute true filling?

For general discussion of polyhedra, not necessarily Stella-specific.
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metachirality
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How hard would it be to compute true filling?

Post by metachirality » Mon Dec 23, 2019 11:09 pm

True filling is filling regions of a polytope if they can be continuously transformed into a region of odd density. How feasible is this in practice?

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robertw
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Re: How hard would it be to compute true filling?

Post by robertw » Tue Dec 24, 2019 6:09 am

Wouldn't that mean they are already odd density? Can an even-density region ever be transformed to an odd density?

Did you already find "Display->Polygon Filling Options" from the menu? What you want is similar to the Modulo-2 method.

metachirality
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Re: How hard would it be to compute true filling?

Post by metachirality » Tue Dec 24, 2019 3:47 pm

No, I mean if you had a four-shaped thing and transformed it so that it didn't intersect itself, you would've transformed the even density region in the four's intersection into an odd density region.

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Re: How hard would it be to compute true filling?

Post by robertw » Wed Dec 25, 2019 8:08 am

Do you mean something like this, with arms that cross over each other?

Image

And then pull the arms apart so there's no more overlap?

Image

I guess the region of space that was once density 2 is now density 1, but you could simply move the whole object to transform any region of space between odd and even density. So maybe I'm still not sure what you mean.

My way of thinking about density would be as follows. As the shape is transformed, the region 2 density shrinks until it disappears. It can never transform to an odd-density region without faces crossing over it.

Image

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Re: How hard would it be to compute true filling?

Post by metachirality » Wed Dec 25, 2019 9:37 pm

The empty space around the polyhedron is also considered as a region. For orientable polyhedra, this is the same as density filling, for non-orientables its more complicated. For uniform polyhedra I think that binary filling works, but for other polyhedra which are non-orientable but not uniform, it's different.

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Re: How hard would it be to compute true filling?

Post by robertw » Thu Dec 26, 2019 1:58 pm

My understanding is that for orientable polytopes, the density of a region will stay the same however the shape is transformed provided no facets pass over the region. For non-orientables, there is only odd and even density (1 or 0), but still regions will remain odd or even unless a facet passes over them. I suspect also that any even density region in a non-orientable can be transformed to the exterior of the model, but I don't think that's been proven.

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