helppyFormex - Support: sr #108602, Offset an object in the same plane


sr #108602: Offset an object in the same plane

Submitter:  None
Submitted:  Mon 23 Jun 2014 09:25:15 AM UTC
Category:  Examples Priority:  5 - Normal
Severity:  3 - Normal Privacy:  Public
Assigned to:  None Originator Email:  -email is unavailable-
Open/Closed:  Open Operating System:  GNU/Linux
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Mon 23 Jun 2014 03:39:25 PM UTC, comment #1: 

I understand that what you want is something like the Formex.shrink() method. But while this one shrinks each triangle with a constant factor with respect to its center, you want to offset the triangle edges with a fixed amount. That of course requires recomputing all vertices as the intersections of translated lines.

In attachment ( you find the Facet class that we used to do just that for our work in cooperation with Anne Bagger some years ago ( The class is more general than you need: our facets could have any number of edges (which is why we could not use the normal Formex class), but it also works well if you only have triangles. There is quite some extra functionality, like detecting diappearing edges (our facets could have some short edges, which disappeared by offsetting the edges inwards). The class will probably (someday, in some form) become part of pyFormex.

The second attachment ( is an example script using the class. It should work well if you put both in the same accessible directory.

Finally, there is an image (shrink.png) that illustrates the results: the black edges are the original triangles, the red ones are those from using Formex.shrink, the blue ones are obtained with the Facet.shrink method.

This has been tested on pyFormex 0.9.1

(file #31602, file #31603,

Benedict Verhegghe <bverheg>
Project Administrator
Mon 23 Jun 2014 09:25:15 AM UTC, original submission:  

I actually discovered and already tried using offset command on TriSurface class. What I really want to achieve is to offest each edge of the triangle and effectively create a new triangle, in the same plane, but slightly smaller. When I apply offset command to entire triangle, what it does, it offsets the triangle in the normal direction: is there a way to tell in which direction to offset? And I assume I have to offset individual edges of my elements and then create a new elements? Thank you.



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Attach Files:

Attached Files
file #31602: added by bverheg (10KiB - text/x-python)
file #31603: added by bverheg (992B - text/x-python)
file #31604:  shrink.png added by bverheg (16KiB - image/png)


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    Follow 3 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2014-06-23 bverheg Attached File- Added, #31602
        Attached File- Added, #31603
        Attached File- Added shrink.png, #31604

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