Re: observations and improvements for the Octave geometry package

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Re: observations and improvements for the Octave geometry package

Juan Pablo Carbajal
On Tue, Oct 2, 2012 at 4:58 PM, Francesco Potortì <[hidden email]> wrote:

> Please let me know if I should resend this mail elsewhere for archiving
> and publishing, or please do it yourself.
> I recently used the Octave geometry package.  Nice software.  Various
> things follow.
> 1) misbehaviour or bug with workaround
> There is a problem with the area of polygons, which is computed as
> negative if the vertices are counterclockwise.  Before using the
> polygonCentroid function below I had to check for this.  This is how I
> did, which may be useful to others or maybe trigger ideas for a better
> method:
>       a = clipPolygon(c)
>       if (isCounterClockwise(a(1,:),a(2,:),a(3,:)) < 1)
>         a = reversePolygon(a);
>       endif
>       scopecentr = polygonCentroid(a);
> 2) contributed software
> I wrote three small functions that I think could be added to the
> package.  I can write docs and tests for them, if required.  They are
> appended.
> ## Convert a box [minX maxX minY maxY] to a polygon [X Y]
> function poly = box2polygon (box)
>   p([1 4]) = box(1);
>   p([2 3]) = box(2);
>   p([5 6]) = box(3);
>   p([7 8]) = box(4);
>   poly = reshape(p,4,2);
> endfunction
> ## Clip a polygon with a box
> ## Needs pkg 'geometry'.
> function poly = clipPolygon (poly, box)
>   [x y] = oc_polybool(poly,box2polygon(box));
>   poly = [x y];
> endfunction
> ## Centroid of a polygon
> function centr = polygonCentroid (pl)
>   centr = shapecentroid(polygon2shape(pl));
> endfunction
> --
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Thank you very much for your input.

1. didn't you get a warning when using reversed polygons? A reversed
polygon will mean a holw in the future, so functions should handle
correctly, but definitely not ignore the fact. I will see what I can
2. Thank you very much for your functions. I will take a look. If you
do not want to leave this forgoten, add the request, together with
properly formatted files to the Feature request forum
Remember to follow this minimal instructions (more in the future)


M. Sc. Juan Pablo Carbajal
PhD Student
University of Zürich

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