Fast 9-point mean matrix smoothing in R

smooth9.R

s9<-function(M,iters=1){
 M*0+9->n;
 n[,1]<-n[1,]<-n[,ncol(n)]<-n[nrow(n),]<-6;
 n[1,1]<-n[1,ncol(n)]<-n[nrow(n),1]<-n[nrow(n),ncol(n)]<-4;
 for(e in 1:iters){
  s<-M+
     cbind(M[,-1],0)+
     cbind(0,M[,-ncol(M)])+
     rbind(M[-1,],0)+
     rbind(0,M[-nrow(M),])+
     rbind(cbind(M[,-1],0)[-1,],0)+
     rbind(0,cbind(M[,-1],0)[-nrow(M),])+
     rbind(cbind(0,M[,-ncol(M)])[-1,],0)+
     rbind(0,cbind(0,M[,-ncol(M)])[-nrow(M),]);
  s/n->M;
 }
 return(M);
}