Re: Re: [SVG] [Batik] Polygon Area

"Cameron Laird [email protected] [svg-developers]" <[email protected]> Sat, 7 Nov 2015 19:17:44 -0600
Newsgroups	gmane.text.xml.svg.devel
Message-ID	<CAC7gT6gLMhjEdq_9OhmZ-B-jEEXV89esVxx-QESv_fP6iQ7m8g@mail.gmail.com>
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Yes:  yes, this computation remains valid whether the polygon is convex or
not.  HOWEVER, it does *not* give the result one typically wants for
self-intersecting boundaries.

On Sat, Nov 7, 2015 at 9:31 AM, 'Dailey, David P.' [email protected]
[svg-developers] <[email protected]> wrote:

> Wow, Domenico! That is such a neat and concise computation. I didn't know
> it could be so simple. It will take me a while, I think, to figure out why
> it would work. Does it work for concave polygons as well as convex ones?
>
>
>
> cheers
>
> David
>
> ________________________________
> From: [email protected] <[email protected]> on
> behalf of [email protected] [svg-developers] <
> [email protected]>
> Sent: Friday, November 06, 2015 5:26 PM
> To: [email protected]
> Subject: [svg-developers] Re: [SVG] [Batik] Polygon Area
>
>
>
> The code below is JavaScript. Pass X and Y arrays of x and y points
> (clockwise or counterclockwise), and n number of vertices. If you need it
> for a path element the SVG JavaScript binding provides dedicated functions
> like getPointAtLength(), getPathSegAtLength(), etc. to convert the path
> into polygon points.
>
>     function polygon_area (X, Y, n) {
>       var
>         area = 0,
>         i,
>         j = n - 1;
>
>       for (i = 0; i < n; j = i++) {
>         area += (X[j] + X[i]) * (Y[j] - Y[i]);
>       }
>
>       return Math.abs(area / 2);
>     }
>
> Domenico
>
>
>
>
> ------------------------------------
> Posted by: "Dailey, David P." <[email protected]>
> ------------------------------------
>
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<div dir=3D"ltr">Yes: =C2=A0yes, this computation remains valid whether the=
 polygon is convex or not.=C2=A0 HOWEVER, it does *not* give the result one=
 typically wants for self-intersecting boundaries.</div><div class=3D"gmail=
_extra"><br><div class=3D"gmail_quote">On Sat, Nov 7, 2015 at 9:31 AM, &#39=
;Dailey, David P.&#39; <a href=3D"mailto:[email protected]">david.dailey=
@sru.edu</a> [svg-developers] <span dir=3D"ltr">&lt;<a href=3D"mailto:svg-d=
[email protected]" target=3D"_blank">[email protected]=
</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin=
:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Wow, Domenico! Tha=
t is such a neat and concise computation. I didn&#39;t know it could be so =
simple. It will take me a while, I think, to figure out why it would work. =
Does it work for concave polygons as well as convex ones?<br>
<br>
<br>
<br>
cheers<br>
<br>
David<br>
<br>
________________________________<br>
From: <a href=3D"mailto:[email protected]">svg-developers@yaho=
ogroups.com</a> &lt;<a href=3D"mailto:[email protected]">svg-d=
[email protected]</a>&gt; on behalf of <a href=3D"mailto:nst@dotusc=
omus.com">[email protected]</a> [svg-developers] &lt;<a href=3D"mailto:svg=
[email protected]">[email protected]</a>&gt;<br>
Sent: Friday, November 06, 2015 5:26 PM<br>
To: <a href=3D"mailto:[email protected]">svg-developers@yahoog=
roups.com</a><br>
Subject: [svg-developers] Re: [SVG] [Batik] Polygon Area<br>
<span class=3D""><br>
<br>
<br>
The code below is JavaScript. Pass X and Y arrays of x and y points (clockw=
ise or counterclockwise), and n number of vertices. If you need it for a pa=
th element the SVG JavaScript binding provides dedicated functions like get=
PointAtLength(), getPathSegAtLength(), etc. to convert the path into polygo=
n points.<br>
<br>
=C2=A0 =C2=A0 function polygon_area (X, Y, n) {<br>
=C2=A0 =C2=A0 =C2=A0 var<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 area =3D 0,<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 i,<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 j =3D n - 1;<br>
<br>
=C2=A0 =C2=A0 =C2=A0 for (i =3D 0; i &lt; n; j =3D i++) {<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 area +=3D (X[j] + X[i]) * (Y[j] - Y[i]);<br>
=C2=A0 =C2=A0 =C2=A0 }<br>
<br>
=C2=A0 =C2=A0 =C2=A0 return Math.abs(area / 2);<br>
=C2=A0 =C2=A0 }<br>
<br>
Domenico<br>
<br>
<br>
<br>
<br>
</span>------------------------------------<br>
Posted by: &quot;Dailey, David P.&quot; &lt;<a href=3D"mailto:david.dailey@=
sru.edu">[email protected]</a>&gt;<br>
------------------------------------<br>
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