Foci of an Ellipse

Dave R

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

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

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

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

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

@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

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

Cheers,

Dave

TIG

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

Dave R

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

TIG

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