Baptiste, thanks for the tip but this would give me an approximate union, and I really need a (nearly) exact one. I am also not sure how to set the alpha parameter in a non-arbitrary way.
Remko ------------------------------------------------- Remko Duursma Research Lecturer Centre for Plants and the Environment University of Western Sydney Hawkesbury Campus Richmond NSW 2753 Dept of Biological Science Macquarie University North Ryde NSW 2109 Australia Mobile: +61 (0)422 096908 www.remkoduursma.com On Wed, Jun 2, 2010 at 3:36 PM, baptiste auguie <baptiste.aug...@googlemail.com> wrote: > Hi, > > I think you could use a concave hull from the alphahull package, > > http://yihui.name/en/2010/04/alphahull-an-r-package-for-alpha-convex-hull/ > > It may be difficult to find the right parameters if the polygons > differ widely in edge lengths, though. > > > HTH, > > baptiste > > On 2 June 2010 03:53, Remko Duursma <remkoduur...@gmail.com> wrote: >> Dear R-helpers, >> >> thanks for yesterday's speeding-up tip. Here is my next query: >> >> I have lots of polygons (not necessarily convex ones, and they never >> have holes) given by x,y coordinates. >> >> I want to get the polygon that is the union of these polygons. This is >> my current method, but I am hoping there is a faster method (up to >> thousands of polygons, each with ca. 40 xy points). >> >> Example: >> >> library(gpclib) >> >> # A polygon >> leaf <- structure(c(0, 1, 12.9, 16.5, 18.8, 17, 16.8, 15.5, 12.1, 8.2, >> 6.3, 5, 2, 0, -1.5, -4.3, -6.6, -10.3, -14.8, -19.4, -22.2, -23.5, >> -22.2, -17.6, -7.8, 0, 0, -2.4, 2.8, 8.9, 19.9, 33.9, 34.8, 40.4, >> 49.7, 69.2, 77.4, 83.4, 91.4, 99, 92.8, 87.3, 81.2, 71.1, 57.6, >> 45.4, 39.2, 26, 15.6, 5.3, 0.6, 0), .Dim = c(26L, 2L), .Dimnames = list( >> NULL, c("X", "Y"))) >> >> # Lots of polygons: >> releaf <- function(leaf)cbind(leaf[,1]+rnorm(1,0,50),leaf[,2]+rnorm(1,0,50)) >> leaves <- replicate(500, releaf(leaf), simplify=FALSE) >> >> # Make into gpc.poly class: >> leaves <- lapply(leaves, as, "gpc.poly") >> >> # Make union ..... >> system.time({ >> leavesoutline <- union(leaves[[1]], leaves[[2]]) >> for(i in 3:length(leaves))leavesoutline <- union(leavesoutline, leaves[[i]]) >> }) >> # about 1sec here. >> >> # Check it: >> plot(leavesoutline) >> >> >> >> thanks! >> >> Remko >> >> >> ------------------------------------------------- >> Remko Duursma >> Research Lecturer >> >> Centre for Plants and the Environment >> University of Western Sydney >> Hawkesbury Campus >> Richmond NSW 2753 >> >> Dept of Biological Science >> Macquarie University >> North Ryde NSW 2109 >> Australia >> >> Mobile: +61 (0)422 096908 >> www.remkoduursma.com >> >> ______________________________________________ >> R-help@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. >> > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.