Read/Write polygon and contour information from/to a text file.
read.polyfile(filename, nohole = TRUE)
write.polyfile(poly, filename = "GPCpoly.txt")If nohole is TRUE (the default) read.polyfile
returns an object of class "gpc.poly.nohole". This object has
the hole flag set to FALSE for all contours. If nohole is
FALSE, then an object of class "gpc.poly" is
returned.
write.polyfile does not return anything useful.
the name of the file (a character string) from/to which data should be read/written.
Is this a polygon without holes?
an object of class "gpc.poly"
Roger D. Peng
The text file representation of a polygon is of the following format:
<number of contours>
<number of points in first contour>
x1 y1
x2 y2
...
<number of points in second contour>
x1 y1
x2 y2
...
For example, a data file for a polygon with 2 contours (a four-sided object and a triangle) might look like:
2
4
1.0 1.0
1.0 2.0
3.4 3.21
10 11.2
3
21.0 11.2
22.3 99.2
4.5 5.4
The vertices of the polygon can be ordered either clockwise or counter-clockwise.
If a polygon has contours which are holes, then the format is slightly different. Basically, a flag is set to indicate that a particular contour is a hole. The format is
<number of contours>
<number of points in first contour>
<hole flag>
x1 y1
x2 y2
...
<number of points in second contour>
<hole flag>
x1 y1
x2 y2
...
The hole flag is either 1 to indicate a hole, or 0 for a regular contour. For example, a four-sided polygon with a triangular hole would be written as:
2
3
1
4.0 4.0
6.0 5.0
5.0 6.0
4
0
2.0 1.0
8.0 2.0
7.0 9.0
1.0 7.0
gpc.poly-class, gpc.poly.nohole-class