Join One or Two PolySets using a Logic Operation
Join one or two PolySets using a logic operation.
joinPolys(polysA, polysB=NULL, operation="INT")
polysA: PolySet to join.polysB: optional second PolySet with which to join.operation: one of "DIFF", "INT", "UNION", or "XOR", representing difference, intersection, union, and exclusive-or, respectively.This function interfaces with the first version of the Clipper Library, specifically version 6.2.1 released 2014-10-31, developed by Angus Johnson. Angus now offers Clipper 2 Library. 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, Staff Software Engineer
Jobber, Edmonton AB
Maintainer: Rowan Haigh, Program Head -- Offshore Rockfish
Pacific Biological Station (PBS), Fisheries & Oceans Canada (DFO), Nanaimo BC
locus opus: Remote office, Vancouver BC
Last modified Rd: 2024-09-03
In package PBSmapping:
addPolys, appendPolys, clipPolys, closePolys, fixBound, fixPOS, locatePolys, plotMap, plotPoints, thickenPolys, thinPolys
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.
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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