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This article describes a procedure for determining the sense of rotation of a closed polygon, using the index numbers of selected extrema vertices in a particular set of inequalities.
English (United States)
This text was extracted from a PDF file.
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Page 1 of 3
Fast Detection of Rotation Sense of a Closed Polygon
This article describes a procedure for determining the sense of rotation of a
closed polygon, using the index numbers of selected extrema vertices in a
particular set of inequalities.
For many methods of breaking up shapes into smaller pieces, the sense of
rotation of the shape has to be found t allow for error-free operation of the
A procedure for detecting the rotation sense of a polygon is known from (*).
It uses the index values (vertex numbers) of three extrema points A (lowest x-
coordinate), B (highest x-coordinate), and C (lowest y-coordinate) in a set of
three pairs of inequalities to determine the orientation of the curve.
Unfortunately, the procedure fails for particular shapes such as that shown in Fig. 1.
To overcome this problem, the following strategy is proposed: The extrema in
both coordinate axis directions are searched. A is the point with the smallest x-
coordinate, and IA its index. Analog B is the one with the largest y-coordinate, C
has the largest x-value, and D the lowest y-coordinate (Fig. 2). If more than one
vertex has the same extremum coordinate, choose the one with the smaller
Let the variable S (for "sense") be a counter initially set to
zero. Then add one to S for each of the following conditions that is
IA < IB
IB < IC
IC < ID
ID < IA
and subtract one from S for each of the following conditions that is
IA > IB
IB > IC
IC > ID
ID > IA
If S is now greater than 0, the figure is clockwise oriented; if S is less than 0,
the figure is counter-clockwise designed.