[R] circular filter

[Christof Bigler]

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