[R] Faster union of polygons?

[Remko Duursma]

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.auguie at 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 <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]])
>> })
>> # 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
>>
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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>