[R] Faster union of polygons?

[Remko Duursma]

Thanks for the tip - this cleans up the code a lot!
Unfortunately, there is no gain in speed.

remko



On Thu, Jun 3, 2010 at 10:46 PM, nikhil kaza <nikhil.list at gmail.com> wrote:
> Reduce might work. Not sure about the speed advantages though. It does
> simplify code.
>
>  Unionall <- function(x) Reduce('union', x)
> leaveout <- Unionall(leaves)
>
>
> On Tue, Jun 1, 2010 at 9:53 PM, 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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>
>