Polygonflow: clockwise or counter-clockwise?

Jeff Massung massung at gmail.com
Wed Feb 3 08:47:52 CST 2010


Michael,

I assume you are talking about 2D polygons...

Just take a 3D cross product of the first two vector (P3 - P2) x (P2 - P1).
If the resultant vector is coming out of the screen (Z < 0) then the polygon
is counter-clockwise. If it is going into the screen (Z > 0) then the
polygon is clockwise.

http://chortle.ccsu.edu/VectorLessons/vch12/vch12_1.html

That page may be of help if you don't know what the cross product is. Note:
to do the 3D cross product, just use Z=0 for all the points.

HTH,

Jeff M.

On Wed, Feb 3, 2010 at 3:03 AM, Michael Kristensen <
michael-kristensen at dsa-net.dk> wrote:

> HI THERE
>
> Can you figure a smart way to check if the flow of a polygon goes clockwise
> or counter-clockwise?
>
>
> Thanks
> Michael
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