## Draw line tagent

Please use this forum to post feedback and suggestions related to QCAD version 3

Moderator: andrew

plgarcia
Junior Member
Posts: 16
Joined: Sat May 19, 2012 8:24 am

### Draw line tagent

Hello,
It would be great to be able to draw tangents with ellips and not only with circle.
This would be really usefull for isometric projections as a circle is projected as a ellipse.

It is not a problem for cylindres projection as the tangent needed goes thew the reference point of the ellipse, but for a cone it is not the case.

regards

andrew
Posts: 4270
Joined: Fri Mar 30, 2007 6:07 am

### Re: Draw line tagent

Tangents from points to ellipses should be possible with QCAD 3. Tangents between two ellipses are currently not supported.

plgarcia
Junior Member
Posts: 16
Joined: Sat May 19, 2012 8:24 am

### Re: Draw line tagent

Thank you for this information. As I was trying to draw a trunc of a cone I was also trying to have a tangent of two ellipses. But if I project the top of the cone I should be able to contruct this tangent.

But I found now an other strange thing. I can construct a tangent to an ellips result of projection. But if the line starts at the end of a line that is the result of a projection, (the heigh of the cone) it does not work. Either the tangent goes to the center of the ellips or goes in an orthogonal direction or do not draw at all. It happens that this start point is also on the small axis of the ellips by construction.

So I construct an other point on this axis and got the same problem.

I did the same with an ellips directly drawn as (not a projection) and got the same problem.

The tangent to an ellips does not work if the start point is on an axis of the ellips.

Regards
Last edited by plgarcia on Sat May 19, 2012 2:25 pm, edited 1 time in total.

Clive
Moderator / Drawing Help
Posts: 1207
Joined: Thu Aug 25, 2011 9:28 pm
Location: UK

### Re: Draw line tangent

I`ve added a 'workaround' tip which I have used successfully in the past.This may be useful until tangents between ellipses are supported.

See here:
viewtopic.php?f=47&t=2004

plgarcia
Junior Member
Posts: 16
Joined: Sat May 19, 2012 8:24 am

### Re: Draw line tagent

Thank you, seems to be great.
For the cone I think it is better to solve the bug I found.
Regards
Pascal

Clive
Moderator / Drawing Help
Posts: 1207
Joined: Thu Aug 25, 2011 9:28 pm
Location: UK

### Re: Draw line tagent

To add support for drawing tangents between ellipses.

plgarcia
Junior Member
Posts: 16
Joined: Sat May 19, 2012 8:24 am

### Re: Draw line tagent

That is great but there is a bug with the tangent between a point and an ellipse when the point is right on an axis of the ellipse.

I attatch a small test file. On this example there is no way to draw the tangent between the point A by contruction on the axis of the ellipse, and the ellipse. But you can draw the tangent going thow any other point not on the axis.

Regards
Attachments
testTangentElipse.dxf

Clive
Moderator / Drawing Help
Posts: 1207
Joined: Thu Aug 25, 2011 9:28 pm
Location: UK

### Re: Draw line tagent

Hi
I see what you mean, the only workaround solutions for now are:
Attachments
tangent problem.png (37.75 KiB) Viewed 6781 times
tangent problem 3.png (44.47 KiB) Viewed 6781 times

plgarcia
Junior Member
Posts: 16
Joined: Sat May 19, 2012 8:24 am

### Re: Draw line tagent

Thank you for this workaround, it works.

Forget my problem, it is a bug that should be fixed. And it is very reproducable. The funny thing is that depending on the test I have had 3 different behaviours but all wrong. I beleave that there is an error in a formula : a limit case (a division by 0 for example) or a test (< instead of <=). I made some more tests and it seems it hapens only for the second axis and not for the first one.

Hope that will help.

Regards

andrew