Join One or Two PolySets using a Logic Operation
Join one or two PolySets using a logic operation.
joinPolys(polysA, polysB=NULL, operation="INT")
This function interfaces with the Clipper library,
specifically version 6.2.1 released 2014-10-31, developed by Angus Johnson.
Prior to 2013-03-23, 'joinPolys' used the General Polygon Clipper library
by Alan Murta at the University of Manchester.
We keep this historic reference to GPC because 'joinPolys' remains faithful
to Murta's definition of a generic polygon, which we describe below.
Murta (2004) defines a generic polygon (or polygon set)
as zero or more disjoint boundaries of arbitrary configuration. He
relates a boundary to a contour, where each may be convex,
concave or self-intersecting. In a PolySet, the polygons associated
with each unique PID loosely correspond to a generic polygon,
as they can represent both inner and outer boundaries. Our use of the
term generic polygon includes the restrictions imposed by a
PolySet. For example, the polygons for a given PID cannot
be arranged arbitrarily.
If 'polysB' is NULL, this function sequentially applies
the logic 'operation' between the generic polygons in 'polysA'.
For example, suppose 'polysA' contains three generic polygons (A, B, C).
The function outputs the PolySet containing ((A op B) op C).
If 'polysB' is not NULL, this function applies the logic 'operation'
between each generic polygon in 'polysA' and each one in 'polysB'.
For example, suppose 'polysA' contains two generic polygons (A, B)
and 'polysB' contains two generic polygons (C, D).
The function's output is the concatenation of
A op C, B op C, A op D, B op D, with PIDs 1 to 4, respectively.
Generally there are n times m comparisons, where n = number of polygons
in 'polysA' and m = number of polygons in 'polysB'.
If 'polysB' contains only one generic polygon, the function
maintains the PIDs from 'polysA'.
It also maintains them when 'polysA' contains only one generic polygon and
the 'operation' is "DIFF" (difference).
Otherwise, if 'polysA' contains only one generic polygon, it maintains
the PIDs from 'polysB'.
If 'polysB' is NULL, the resulting PolySet contains
a single generic polygon (one PID), possibly with several
components (SIDs).
The function recalculates the PID and SID columns.
If 'polysB' is not NULL, the resulting PolySet contains one or more
generic polygons (PIDs), each with possibly several components (SIDs).
The function recalculates the SID column, and depending on the input,
it may recalculate the PID column.
C code: Angus Johnson, Computer Programmer
Implementation: Nicholas M. Boers, Associate Professor – Computer Science
MacEwan University, Edmonton AB
Maintainer: Rowan Haigh, Program Head – Offshore Rockfish
Pacific Biological Station (PBS), Fisheries & Oceans Canada (DFO), Nanaimo BC
locus opus: Institute of Ocean Sciences (IOS), Sidney BC
Last modified Rd: 2021-01-11
Murta, A. (2004) A General Polygon Clipping Library. Accessed: Jul 29, 2004.
Johnson, A. (2014) Clipper – an open source freeware library for clipping and offsetting lines and polygons. Accessed: Oct 31, 2014.
In package PBSmapping:addPolys,
appendPolys,
clipPolys,
closePolys,
fixBound,
fixPOS,
locatePolys,
plotMap,
plotPoints,
thickenPolys,
thinPolys
local(envir=.PBSmapEnv,expr={
oldpar = par(no.readonly=TRUE)
#--- load the data (if using R)
if (!is.null(version$language) && (version$language=="R"))
data(nepacLL,envir=.PBSmapEnv)
### Example 1. Cut a triangle out of Vancouver Island
par(mfrow=c(1,1))
#--- create a triangle to use in clipping
polysB <- data.frame(PID=rep(1, 3), POS=1:3,
X=c(-127.5, -124.5, -125.6), Y = c(49.2, 50.3, 48.6))
#--- intersect nepacLL with the single polygon, and plot the result
plotMap(joinPolys(nepacLL, polysB), col="cyan")
#--- add nepacLL in a different line type to emphasize the intersection
addPolys(nepacLL, border="purple", lty=3, density=0)
box()
### Example 2. Cut Texada and Lasqueti Islands out of Boxes
xlim = list(box1=c(-124.8,-124),box2=c(-124,-123.9))
ylim = list(box1=c(49.4,49.85), box2=c(49.85,49.9))
Xlim = extendrange(xlim); Ylim=extendrange(ylim)
polyA = as.PolySet(data.frame(
PID = rep(1:2,each=4), POS = rep(1:4,2),
X = as.vector(sapply(xlim,function(x){x[c(1,1,2,2)]})),
Y = as.vector(sapply(ylim,function(x){x[c(1,2,2,1)]}))
), projection="LL")
data(nepacLLhigh,envir=.PBSmapEnv)
polyB = nepacLLhigh[is.element(nepacLLhigh$PID,c(736,1912)),]
polyC = joinPolys(polyA, polyB, "DIFF")
par(mfrow=c(2,2),cex=1,mgp=c(2,0.5,0))
plotMap(polyA,col="lightblue",xlim=Xlim,ylim=Ylim)
addPolys(polyB,col="gold");
text(mean(Xlim)-0.05,Ylim-0.04,"Boxes (A,B) and Isles (C,D)")
labs = calcCentroid(polyA)
labs[1,c("X","Y")] = labs[2,c("X","Y")]+c(-0.1,-0.05)
text(labs[,"X"],labs[,"Y"],c("A","B"),font=2)
plotMap(polyC[is.element(polyC$PID,1),],col="pink",xlim=Xlim,ylim=Ylim)
text(mean(Xlim)-0.05,Ylim-0.04,"Box A \"DIFF\" Isle C")
plotMap(polyC[is.element(polyC$PID,3),],col="green",xlim=Xlim,ylim=Ylim)
text(mean(Xlim)-0.05,Ylim-0.04,"Box A \"DIFF\" Isle D")
plotMap(polyC[is.element(polyC$PID,c(1,3)),],col="cyan",xlim=Xlim,ylim=Ylim)
text(mean(Xlim)-0.05,Ylim-0.04,"Box A \"DIFF\" Isles (C,D)")
par(oldpar)
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