[R] circular filter
Peter Wolf
s-plus at wiwi.uni-bielefeld.de
Wed Feb 25 09:22:24 CET 2004
filter.matrix.center implements Manhattan or L 1 distance.
If you want to define neighbor points
by Euklidean distances (L 2) use filter.matrix.center(p=2):
filter.matrix.center.p <- function(n=9,size=5,p=2){
x<-matrix(1,n,n)
center<-(n+1)/2
(abs(row(x)-center)^p+abs(col(x)-center)^p)^(1/p) < size
}
example:
> 0+filter.matrix.center.p(9,4.1)
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,] 0 0 0 0 1 0 0 0 0
[2,] 0 0 1 1 1 1 1 0 0
[3,] 0 1 1 1 1 1 1 1 0
[4,] 0 1 1 1 1 1 1 1 0
[5,] 1 1 1 1 1 1 1 1 1
[6,] 0 1 1 1 1 1 1 1 0
[7,] 0 1 1 1 1 1 1 1 0
[8,] 0 0 1 1 1 1 1 0 0
[9,] 0 0 0 0 1 0 0 0 0
Peter Wolf
--------------------------------------------------
Peter Wolf wrote:
>
> try:
>
> filter.matrix.center<-function(n=9,size=5){
> x<-matrix(1,n,n)
> center<-(n+1)/2
> (abs(row(x)-center)+abs(col(x)-center)) < size
> }
> filter.matrix.center()
>
> some tests:
>
> > 0+filter.matrix.center(5,2)
> [,1] [,2] [,3] [,4] [,5]
> [1,] 0 0 0 0 0
> [2,] 0 0 1 0 0
> [3,] 0 1 1 1 0
> [4,] 0 0 1 0 0
> [5,] 0 0 0 0 0
>
> > 0+filter.matrix.center()
> [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
> [1,] 0 0 0 0 1 0 0 0 0
> [2,] 0 0 0 1 1 1 0 0 0
> [3,] 0 0 1 1 1 1 1 0 0
> [4,] 0 1 1 1 1 1 1 1 0
> [5,] 1 1 1 1 1 1 1 1 1
> [6,] 0 1 1 1 1 1 1 1 0
> [7,] 0 0 1 1 1 1 1 0 0
> [8,] 0 0 0 1 1 1 0 0 0
> [9,] 0 0 0 0 1 0 0 0 0
>
> Peter Wolf
>
> --------------------------------------------------------------------------------------------------------
>
>
> Christof Bigler wrote:
> I try to find a circular filter that I can export to be used in a
> spatial software.
> Assuming, we have a matrix, representing 9x9 regularly spaced points
> with the center point 'filter[5, 5]'. In this example, I want to find
> a function that weighs all neighbor points within a distance of d=4
> units with 1:
>
> > filter <- matrix(0, 9, 9)
> > filter <- function() ...
> > filter
> [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
> [1,] 0 0 0 0 1 0 0 0 0
> [2,] 0 0 1 1 1 1 1 0 0
> [3,] 0 1 1 1 1 1 1 1 0
> [4,] 0 1 1 1 1 1 1 1 0
> [5,] 1 1 1 1 1 1 1 1 1
> [6,] 0 1 1 1 1 1 1 1 0
> [7,] 0 1 1 1 1 1 1 1 0
> [8,] 0 0 1 1 1 1 1 0 0
> [9,] 0 0 0 0 1 0 0 0 0
>
> Finally, I want to use a larger matrix, e.g. with 61x61 points. Is
> there a simple function around that I could use to this end?
>
> Thanks!
> Christof
>
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