2 focus elipse
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Hello all, I just getting started using SketchUp and already seem to have come across a problem. I would like to draw an elipse where I know 2 focus points and one point on the curve. Mechanically generating a this curve is very straight forward, if I were doing it on a piece of wood I'd anchor a piece of string to the two focus points and adjust the length so that a pencil holding the line tite would sit the known point of the curve, then just run the pencil from one end starting on a line intersecting the focus points out to one side and down to the other end (I hope that makes sense, see here for another description).
SketchUp doesn't seem to have a tool for drawing elipses and I haven't been able to find any talk of ways to generate one in this fashion on any of the SketchUp help pages or forums.
Thanks in advance for any tips or pointers that you can pass on to this neophyte,
Greg
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Hi Greg, welcome to the forums.
One simple way of creating an elipse is by drawing a circle, select it and then use the scale tool. You can grab the green boxs of the scale tool to stretch and manipulate to create your elipse.
If you press ctrl whilst moving a side box then you stretch around the center. -
Thanks for the suggestion Dylan, but as I said in my OP, I know the two focus points that I need, and one point on the curve. I don't know what the curve looks like and I can't define a bounding box for it. With the method that you suggest I can define the original focus point as the center of the circle and the point on the curve as the radius, but I don't know as I'm scaling it where the second focus point is. This is critical, I'm trying to define the inside of a concave mirror to reflect light from the first focus point to the second focus point so I need to be able to specify the elipse based on the focus points and the curve point.
Greg
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There is an ellipse script around here somewhere... ah here it is:
http://www.sketchucation.com/forums/scf/viewtopic.php?f=153&t=1529
by Dider Bur.
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Jim, thanks for the post, I had searched for this tool before and had not found it. Unfortunately, while Dider's description of an elipse is exactly what I need, the script doesn't work that way, it uses the 'major' and 'minor' axis lengths.
I guess, even though I don't know Ruby, I'll try to understand Dider's script enough to change it to work the way that I need it to work.
Greg
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@gwlindberg said:
This is critical, I'm trying to define the inside of a concave mirror to reflect light from the first focus point to the second focus point .....
i'm not so sure that sketchup is the best app for critical optic work.. the curves aren't really curves - they're just a bunch of straight lines.. you can lessen the effect by increasing the amount of segments in a curve but still...
in my work, i use SU for complex curves but i'm framing and an 8ball either way doesn't matter too much.. with optics, that could be a different story.. (hubble tele anyone?)
so, if it's critical critical, you might want to use a more exact program.. if it's only sort of critical, then sketchup will be okay...
jeff
[Edit -- 8ball = 1/8"]
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Jeff, thanks for the concern, the output from SU isn't going to be used for grinding a lens. Right now I need to find out the shape of the mirror so that I can figure out what the holding structure is going to look like and if it's going to work out ok. It's just that I need to define the curve from the foci not from a bounding box.
Greg
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I have a suggestion that requires software other than SU: can you draw the ellipse in Illustrator or something similar, export it in dxf format, and then pull it into SU? Would that work for you?
Dennis
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Sorry for the long reply time, got caught up in other stuff.
Jean, the pointer to the tutorial that you left is just what I needed, thanks. I would have thought that it wouldn't be necessary to go through such a rigamarole, but, if it gets the job done.
Thanks again for all of the help,
Greg
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