On Mar 27, 2011, at 10:56 AM, chuan_zl wrote:
Hello,everybody. My name is Chuan Zun Liang. I come from Malaysia. I
am just
a beginner for R. Kindly to ask favor about median filter. The
problem I
facing as below:
x<-matrix(sample(1:30,25),5,5)
x
[,1] [,2] [,3] [,4] [,5]
[1,] 7 8 30 29 13
[2,] 4 6 12 5 9
[3,] 25 3 22 14 24
[4,] 2 15 26 23 19
[5,] 28 18 10 11 20
This is example original matrices of an image. I want apply with
median
filter with window size 3X# to remove salt and pepper noise in my
matric.
Here are the script I attend to writing.The script and output shown as
below:
MedFilter<-function(mat,sz)
+ {out<-matrix(0,nrow(mat),ncol(mat))
+ for(p in 1:(nrow(mat)-(sz-1)))
+ {for(q in 1:(ncol(mat)-(sz-1)))
+ {outrow<-median(as.vector(mat[p:(p+(sz-1)),q:(q+(sz-1))]))
+ out[(p+p+(sz-1))/2,(q+q+(sz-1))/2]<-outrow}}
+ out}
Noting that median is probably the "rate-limiting" factor, I looked
for another way to get the "middle" of 9 items. Using order seem faster:
> system.time( replicate(100000, x2s <- sort(x2)))
user system elapsed
9.829 0.212 10.029
> system.time( replicate(100000, x2m <- median(x2)))
user system elapsed
7.169 0.126 7.272
> system.time( replicate(100000, x2s <-x2[order(x2)[5] ]))
user system elapsed
1.907 0.051 1.960
So see if this is any faster. On my system it's about three times
faster:
x <- matrix(sample(364*364), 364,364)
out <- matrix(0, 364,364)
for(xi in 1:(nrow(x)-2)) {
for(yi in 1:(ncol(x)-2) ) {
xm <- x[xi+0:2, yi+0:2]
d[xi+1, yi+1] <-xm[order(xm)[5] ]}}
#---------tests ------
> system.time(for(xi in 1:(nrow(x)-2)) {
+ for(yi in 1:(ncol(x)-2) ) {
+ xm <- x[xi+0:2, yi+0:2]
+ d[xi+1, yi+1] <-xm[order(xm)[5] ]}} )
user system elapsed
3.806 0.083 3.887
> system.time(MedFilter(x,3) )
user system elapsed
11.242 0.202 11.427
MedFilter(x,3)
[,1] [,2] [,3] [,4] [,5]
[1,] 0 0 0 0 0
[2,] 0 8 12 14 0
[3,] 0 12 14 19 0
[4,] 0 18 15 20 0
[5,] 0 0 0 0 0
Example to getting value 8 and 12 as below:
7 8 30 8 30 29
4 6 12 (median=8) 6 12 5 (median=12)
25 3 22 3 22 14
Even the script can give output. However, it is too slow. My image
size is
364*364. It is time consumption. Is it get other ways to improving it?
David Winsemius, MD
West Hartford, CT
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