#' Mexican hat brush
#' to use with filter2
#' @param n size of brush
#' @param sigma2 standard deviation
#'
#' @return brush
#' @export
mexican.hat.brush<-function(n=7,sigma2=1)
{
matrix<-array(0,c(n,n))
m<-(n+1)/2
for (x in (1:n)-m)
for (y in (1:n)-m)
{
matrix[x+m,y+m]<- -exp(-(x*x+y*y)/2/sigma2)*(1-(x*x+y*y/2/sigma2))/sigma2/sigma2/pi
}
matrix<-matrix/sum(matrix)
}
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