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Re: Trying to get a golden spiral from an overhead view of a helix
Posted:
Feb 19, 2005 8:26 AM


Thank you both for your thoughts. The reason I thought that an overhead perspective view of a helix would resemble a logarithmic spiral is that I've seen sidebyside comparisons, for example in National Geographic, of a spiral galaxy and then the DNA double helix from the view I'm describing. The curves appear to be very similar, and I've read that the arms of a spiral galaxy are basically logarithmic spirals. The drawing I have in mind is sort of inspired by that fact. The idea is to have a ton of stylized stars spiraling at the viewer. In 3d, they'd be following a helical path, and in 2d, they would at least come close to forming a golden spiral. That's part of the drawing, anyway.
However, after gleaning what I could from Google searches for "spiral" and "helix," it seems to me that a topdown perspective view of a helix actually gives you a different kind of spiral: a hyperbolic spiral. I'm not positive, and if someone could confirm or deny that it would be a huge help. But the thing about logarithmic spirals is that they continue to circle the origin even as the loops get infinitely farther out. The rules of perspective make this impossible. I'm not sure I can explain why, but I'm pretty sure that the rules of perspective dictate that as the 3d helix comes closer and closer to the viewer, the 2d representation of it comes closer and closer to a ray shooting straight out from the origin. I'm not so hot on perspective either, but I know that weird things start happening when you try drawing stuff that would be outside of the field of vision in real life, and I believe that's what the result would be. And a hyperbolic spiral seems to do that. It's also what I think I saw when I looked down the axis of a helix in my 3d modeling program. Here's MathWorld's entry, by the way:
http://mathworld.wolfram.com/HyperbolicSpiral.html
So I may have to work with that shape, though it's kind of a shame, because I think the golden spiral is especially beautiful. On the other hand, I think that fact about a logarithmic spiral being formed when you look down the central axis of a conical helix could prove helpful. I believe that truly happens only in an orthographic projection, not in perspective, but the perspective drawing would resemble the orthographic projection close to the center. Maybe I can work with a conical helix instead of a cylindrical one. I also think it will be helpful as I continue to experiment to consider only corresponding (similar? not sure what the word would be) points on succesive loops, and the intervals between them.
Thanks again to both of you for helping, and sorry for the long post, but any other insights would be greatly appreciated.
On Fri, 18 Feb 2005 14:47:51 0500, Ed Wall wrote: >Interesting question. Perhaps you might say something in a bit more >detail on how other logarithmic spirals are approximated by helix >projections. Enneper, I think, showed that the projection of a helix >on a cone was a logarithmic spiral, but I'm not sure of the cite. > >Ed Wall > >>Hi, I wonder if someone could help me with a project I'm working on. >>It's a perspective drawing with what I hope are interesting geometric >>underpinnings, but which are a bit beyond me, unfortunately. Here's >>the situation. The viewer or camera or whatever is looking right down >>the central axis of a helix. It's my understanding that if you're >>looking right down the central axis, a 2d representation of the helix >>would approximate a logarithmic spiral. If it's a perspective and not >>an orthographic drawing, that is. I'm going for one logarithmic spiral >>in particular, what I think is called a golden spiral. The one shown >>here: >> >><a href="http://www.levitated.net/daily/levGoldenSpiral.html">http://www.levitated.net/daily/levGoldenSpiral.html</a> >> >>Now, it seems to me that the two properties of the helix that I can >>adjust to make it appear that way are its radius and the distance >>between its loops or coils. (Sorry, part of the problem is that I >>don't really know the vocabulary.) My question is, can anyone help me >>figure out what those two attributes of the helix should be, relative >>to each other, for the view I'm describing to come as close as >>possible to a golden spiral? Would it be the golden ratio or >>something? >> >>Finally, the entry for logarithmic spiral on MathWorld... >> >><a href="http://mathworld.wolfram.com/LogarithmicSpiral.html">http://mathworld.wolfram.com/LogarithmicSpiral.html</a> >> >>...has something about approximating a logarithmic spiral by starting >>with equally spaced rays and drawing a perpendicular from one to the >>next. That would seem to relate, but I just can't get my head around >>it. Thanks so much for any help you can give me!



