Polygonal Line Offset
Given a list of polygonal lines, compute the offset region (guard region, buffer region, morphological dilation) formed by shifting the boundary outwards by a specified distance.
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Data specifying polygons. See Details.
Distance over which the boundary should be shifted.
Spatial resolution for coordinates.
Spatial origin for coordinates.
Tolerance parameters: see Details.
Type of join operation to be performed at each vertex. See Details.
Type of geometrical operation to be performed at the start and end of each line. See Details.
This is part of an interface to the polygon-clipping library
Clipper written by Angus Johnson.
Given a list of polygonal lines
polylineoffset computes the offset region
(also known as the morphological dilation, guard region,
buffer region, etc) obtained by shifting the boundary of
outward by the distance
A represents a polygonal line (broken line)
or a list of broken lines. The format is either
a list containing two components
ygiving the coordinates of successive vertices of the broken line.
list(x,y)structures giving the coordinates of the vertices of several broken lines.
Lines may be self-intersecting and different lines may intersect each other. Note that calculations are performed in integer arithmetic: see below.
jointype determines what happens at the vertices
of each line. See the Examples for illustrations.
jointype="round": a circular arc is generated.
jointype="square": the circular arc is replaced by a single straight line.
jointype="miter": the circular arc is omitted entirely, or replaced by a single straight line.
endtype determines what happens at the beginning
and end of each line. See the Examples for illustrations.
endtype="closedpolygon": ends are joined together (using the
jointypevalue) and the path filled as a polygon.
endtype="closedline": ends are joined together (using the
jointypevalue) and the path is filled as a polyline.
endtype="openbutt": ends are squared off abruptly.
endtype="opensquare": ends are squared off at distance
endtype="openround": ends are replaced by a semicircular arc.
"round" are deprecated; they are
arctol are tolerances.
arctolis the maximum permissible distance between the true circular arc and its discretised approximation.
miterlimit * deltais the maximum permissible displacement between the original vertex and the corresponding offset vertex if the circular arc were to be omitted entirely. The default is
miterlimit=2which is also the minimum value.
Calculations are performed in integer arithmetic
x0,y0 from the coordinates,
eps, and rounding to the nearest integer.
eps is the effective spatial resolution.
The default values ensure reasonable accuracy.
Data specifying polygons, in the same format as
Angus Johnson. Ported to R by Adrian Baddeley Adrian.Baddeley@curtin.edu.au.
Clipper Website: http://www.angusj.com
Vatti, B. (1992) A generic solution to polygon clipping. Communications of the ACM 35 (7) 56–63. http://portal.acm.org/citation.cfm?id=129906
Agoston, M.K. (2005) Computer graphics and geometric modeling: implementation and algorithms. Springer-Verlag. http://books.google.com/books?q=vatti+clipping+agoston
Chen, X. and McMains, S. (2005) Polygon Offsetting by Computing Winding Numbers. Paper no. DETC2005-85513 in Proceedings of IDETC/CIE 2005 (ASME 2005 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference), pp. 565–575 http://www.me.berkeley.edu/~mcmains/pubs/DAC05OffsetPolygon.pdf
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A <- list(list(x=c(4,8,8,2,6), y=c(3,3,8,8,6))) plot(c(0,10),c(0,10), type="n", main="jointype=square, endtype=opensquare", axes=FALSE, xlab="", ylab="") lines(A[], col="grey", lwd=3) C <- polylineoffset(A, 0.5, jointype="square", endtype="opensquare") polygon(C[], lwd=3, border="blue") plot(c(0,10),c(0,10), type="n", main="jointype=round, endtype=openround", axes=FALSE, xlab="", ylab="") lines(A[], col="grey", lwd=3) C <- polylineoffset(A, 0.5, jointype="round", endtype="openround") polygon(C[], lwd=3, border="blue")
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