Foci of an Ellipse

Dave R · 11 Apr 2010, 15:01

This may already exist but I haven't found it. Is there a plugin that, given the perimeter, would locate the foci of an ellipse?

TIG · 11 Apr 2010, 16:05

Here's my version of Didier's ellipse ruby that I have previously adjusted to show the foci, if wanted [extra dialog question, default=No].
I hope Didier doesn't mind me sharing it - I have re-encrypted it so you can't see what he had originally or I've now done... He can publish it as his own if he likes...
It works on a major-axis and minor-axis as before... There is no tool to make an ellipse from a 'perimeter' - I suspect that wasn't quite what you meant ??ellipse.rb

Dave R · 11 Apr 2010, 16:15

Thanks TIG. I'll give that a try. I expect I can make it work for what I need.

What I currently have is an ellipse which I drew by making a circle with the radius equal to half the minor axis length and then scaled in one direction to the other axis. For my use, I thought it would be nice to select the ellipse and run a plugin that would locate the foci. Since I know the minor and major axes, I think I can use your modification of Didier's plugin and construct what I need to know.

Thanks again.

Dave

TIG · 11 Apr 2010, 17:43

Draw [inside a group] over the top of the ellipse you made, picking major and minor axes from it by hand and add foci.
Copy foci and paste in place on top of the original and erase the ellipse group ?

Dave R · 11 Apr 2010, 17:54

That worked for me although based on an online calculator , the foci distance doesn't seem correct. In the attached example, the plugin placed the foci 13.4 apart and the calculator indicated 26.8 as the distance between them.
Ellipse.skp

TIG · 11 Apr 2010, 18:35

@dave r said:

That worked for me although based on an online calculator , the foci distance doesn't seem correct. In the attached example, the plugin placed the foci 13.4 apart and the calculator indicated 26.8 as the distance between them.
[attachment=0:w3x2wdjr]Ellipse.skp[/attachment:w3x2wdjr]

Perhaps we have an error of x2 ?
My typo when editing before encrypting
I have change the original post to suit !!! http://forums.sketchucation.com/viewtopic.php?p=239044#p239044
Please re-download and try again ...

Dave R · 11 Apr 2010, 18:46

Thank you good sir. That makes a big difference. No need to be embarrassed. I couldn't have fixed it.

Cheers,

Dave

TIG · 11 Apr 2010, 19:41

Sometimes my flow of consciousness outstrips my typing [and proof-reading] ability

Dave R · 11 Apr 2010, 19:44

Funny. Mine never does. Usually there's not much flow at all.

TIG · 11 Apr 2010, 20:10

One man's flow is another man's dribble...