Close a PolySet
Close a PolySet of polylines to form polygons.
closePolys (polys)
polys |
PolySet to close. |
Generally, run fixBound before this function. The ranges of a
PolySet's X and Y columns define the boundary.
For each discrete polygon, this function determines if the first and
last points lie on a boundary. If both points lie on the same
boundary, it adds no points. However, if they lie on different
boundaries, it may add one or two corners to the polygon.
When the boundaries are adjacent, one corner will be added as follows:
top boundary + left boundary implies add top-left corner;
top boundary + right boundary implies add top-right corner;
bottom boundary + left boundary implies add bottom-left corner;
bottom boundary + right boundary implies add bottom-right corner.
When the boundaries are opposite, it first adds the corner closest to a starting or ending polygon vertex. This determines a side (left-right or bottom-top) that connects the opposite boundaries. Then, it adds the other corner of that side to close the polygon.
PolySet identical to polys, except for possible
additional corner points.
Nicholas M. Boers, Associate Professor – Computer Science
MacEwan University, Edmonton AB
Last modified Rd: 2013-04-10
local(envir=.PBSmapEnv,expr={
oldpar = par(no.readonly=TRUE)
#--- 4 corners
polys <- data.frame(
PID = c(1, 1, 2, 2, 3, 3, 4, 4),
POS = c(1, 2, 1, 2, 1, 2, 1, 2),
X = c(0, 1, 2, 3, 0, 1, 2, 3),
Y = c(1, 0, 0, 1, 2, 3, 3, 2))
plotPolys(closePolys(polys), col=2)
#--- 2 corners and 1 opposite
polys <- data.frame(
PID = c(1, 1, 2, 2, 3, 3, 3),
POS = c(1, 2, 1, 2, 1, 2, 3),
X = c(0, 1, 0, 1, 5, 6, 1.5),
Y = c(1, 0, 2, 3, 0, 1.5, 3))
plotPolys(closePolys(polys), col=2)
par(oldpar)
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