
POINTS IN POLYGONName:
Given a set of one or more points, this command returns a tag value (where each row of the tag variable corresponds to the row of the variables containing the x and y coordinates of the points being tested). Each value of the tag variable will contain either a 1, 2, 3, or 4 where
2 = exterior point 3 = edge point 4 = vertex point
<SUBSET/EXPCEPT/FOR qualification> where <xval> is a variable containing the xcoordinates of the data points being checked; <yval> is a variable containing the ycoordinates of the data points being checked; <xpoly> is a variable that will contain the xcoordinats of the polygon; <ypoly> is a variable that will contain the ycoordinats of the polygon; <tag> is a variable (of the same size as <xval> and <yval>) that specifies whether each point is an interior, exterior, edge, or vertex point; and where the <SUBSET/EXCEPT/FOR qualification> is optional.
This algorithm is based on counting the number of ray tracings. A complete description of the algorithm is given on pages 239245 of O'Rourke's book. This algorithm works for both convex and nonconvex polygons.
The <xval> and <yval> points do not need to be in any specific order since they are tested independently of each other.
. Name: poly.dp . Purpose: Demonstrate "points in polygon" routine . Source: Data from O'Rourke (1998), "Computational Geometry in . C", Second Edition. . . Step 1: Read data . read xpoly ypoly 0 0 2 0 2 2 3 1 4 2 5 2 3 3 3 2 2 4 6 3 7 4 7 2 8 5 8 7 7 7 8 8 5 9 6 6 5 7 4 6 4 8 3 7 2 7 3 6 4 4 0 7 2 3 0 2 end of data . read xval yval 0 0 1 0 2 1 1 2 0 2 6 2 2 4 5 4 1 5 3 5 6 5 7 6 8 6 1 7 2 7 3 7 4 7 4 8 5 9 end of data . . Step 2: Plot data . tic mark offset units screen tic mark offset 5 5 presort off line blank solid character circ blank character hw 0.5 0.375 character fill on plot yval xval and plot ypoly xpoly pause . let tag = point in polygon xval yval xpoly ypoly . line bl bl bl bl solid character circ circ circ circ blank character hw 0.5 0.375 all character fill on all char color blue red green cyan black plot yval xval subset tag = 1 and plot yval xval subset tag = 2 and plot yval xval subset tag = 3 and plot yval xval subset tag = 4 and plot ypoly xpoly
Date created: 09/14/2011 