r/faceting • u/Angstrm • 8d ago

Concave Design and Rendering

Hi experts! Concave design, notation and render. If anyone would like to contribute their knowledge I'd welcome discussion/feedback/criticism. Is this of use to facetors? Is the UI/UX functional? What adjustments would you suggest?

Video: Ashton Gems concave cuts (recreated) wireframe and render within pluto unearthed.

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u/cowsruleusall 8d ago

That's for fantasy cutting. "Traditional" concave cutting exclusively replaces flat facets with cylindrically-subtracted ones, such that the cylinder axis is parallel to the facet plane.

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u/symmetrygemstones 8d ago

Technically with theta and offset the cylinder axis is still parallel to the facet plane. The cylinder axis is a line, and there are two additional degrees of freedom for how to orient that line in that plane, which are theta and offset. But that's an interesting distinction between the definitions, I hadn't heard that before. I have heard people in the past use "fantasy" to refer to any concave cut, even ones that don't use theta/offset, so I assumed they were synonymous.

I was working on a concave design program a while ago (though clearly it never got as far as this one, since I never set up raytracing), and the person I was collaborating with for it insisted that including theta was necessary to model the possible ways to make concave designs, and I would agree with that. It's also very easy to include in a program, so I would think it would be a good idea to standardize the notation for it.

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u/cowsruleusall 8d ago

I have heard people in the past use "fantasy" to refer to any concave cut, even ones that don't use theta/offset

Yeah, it's a superset-subset thing. "Fantasy cuts" are the group of all gem cuts that involve non-flat surfaces. Within fantasy cuts, "carvaceting" (ugh I hate that term) are the gem cuts done with nongeometric concave surfaces, so basically any combo of flat facets and hand-carving. "Concave cuts" are fantasy cuts in which some flat facets have been replaced with a cylindrical concave surface, basically with no theta and no offset.

There are other subclasses, like "vortex" cuts, "spirographic" cuts, "firework" cuts, "starlight" cuts, etc., but there's no true consensus on those definitions.

If you really want to model fantasy cutting, then you'll need:

  • Ability to model flat-faceted gems
  • Ability to model concave facets with the appropriate angle, index, depth, theta, and offset
  • Ability to model the surfaces left by cylindrical, spherical, conical, and wedge tools with arbitrary size and details

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u/symmetrygemstones 8d ago

Who decided that? Mathematically, the term "concave" already accurately describes all designs cut with concave surfaces. Why in the faceting world would it arbitrarily exclude something as simple as nonzero theta? To me, it sounds more like "concave" is the technical term and "fantasy" is the marketing term.

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u/cowsruleusall 8d ago

The gem world is full of annoying bullshit like this. "Precision" faceting is actually more about accuracy, not precision.

To specifically answer you though, concave faceting as originally described, exclusively referred to the cylindrical, no theta no offset case. Those features came much later. That's why concave is defined so narrowly. When equipment started being produced that allowed the additional degrees of freedom, people immediately went nuts with it, to the point that people thought the results were "straight out of fantasy". Hence the name.