# [R] Faster union of polygons?

Wed Jun 2 07:36:01 CEST 2010

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 <remkoduursma at 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]])
> })
>
> # 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
>
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