| Re: Median filter the hard way [message #36750 is a reply to message #36748] |
Fri, 17 October 2003 14:34   |
Dick Jackson
Messages: 347
Registered: August 1998
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Senior Member |
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Hi Peter,
"Peter Payzant" <pce@accesswave.ca.nospam> wrote in message
news:WvGjb.92982$PD3.4887868@nnrp1.uunet.ca...
> Hello, all-
>
> This is my first posting to the group.
Welcome aboard!
> He is applying a median filter to a 2-dimensional image.
> [but...]
> If there less than 7 good values in the 3 x 3 array, he discards
> the original pixel.
["loopy" :-) code snipped]
> Obviously, the nested loops are the source of the problem. Is there
any
> other way to accomplish this, in a more IDL-esque way?
As Mike mentions, the Median filter is part of it, but your "7 or
better" requirement makes it a bit more interesting.
Here's an array, a:
IDL> a=Float(Byte(RandomU(seed,7,7)*10))
IDL> a[2:4,2:4]=!values.F_nan
IDL> print,a,Format='(7F4.1)'
8.0 1.0 7.0 2.0 2.0 5.0 4.0
5.0 7.0 8.0 8.0 3.0 3.0 4.0
3.0 6.0 NaN NaN NaN 9.0 0.0
5.0 0.0 NaN NaN NaN 4.0 9.0
6.0 5.0 NaN NaN NaN 0.0 0.0
9.0 5.0 3.0 4.0 5.0 3.0 4.0
7.0 5.0 9.0 7.0 4.0 5.0 3.0
Median will give a result with even one actual number in the 3x3
neighborhood:
IDL> print,Median(a,3),Format='(7F4.1)'
8.0 1.0 7.0 2.0 2.0 5.0 4.0
5.0 7.0 7.0 7.0 3.0 4.0 4.0
3.0 5.0 7.0 8.0 4.0 4.0 0.0
5.0 5.0 5.0 NaN 4.0 4.0 9.0
6.0 5.0 4.0 4.0 4.0 4.0 0.0
9.0 6.0 5.0 5.0 4.0 4.0 4.0
7.0 5.0 9.0 7.0 4.0 5.0 3.0
Let's use the Finite function to figure which other ones to knock out to
NaN:
IDL> print,Finite(a)
1 1 1 1 1 1 1
1 1 1 1 1 1 1
1 1 0 0 0 1 1
1 1 0 0 0 1 1
1 1 0 0 0 1 1
1 1 1 1 1 1 1
1 1 1 1 1 1 1
The Convol function can be used to count up neighborhoods. If you need
better counting around the edge, you could pad the array before calling
Convol.
IDL> print,Convol(Finite(a),Replicate(1B,3,3))
0 0 0 0 0 0 0
0 8 7 6 7 8 0
0 7 5 3 5 7 0
0 6 3 0 3 6 0
0 7 5 3 5 7 0
0 8 7 6 7 8 0
0 0 0 0 0 0 0
OK, here goes:
IDL> m3=Median(a,3)
IDL> nGood = Convol(Finite(a), Replicate(1B,3,3))
IDL> m3[Where(nGood LT 7)] = !Values.F_NaN
IDL> print,m3,Format='(7F4.1)'
NaN NaN NaN NaN NaN NaN NaN
NaN 7.0 7.0 NaN 3.0 4.0 NaN
NaN 5.0 NaN NaN NaN 4.0 NaN
NaN NaN NaN NaN NaN NaN NaN
NaN 5.0 NaN NaN NaN 4.0 NaN
NaN 6.0 5.0 NaN 4.0 4.0 NaN
NaN NaN NaN NaN NaN NaN NaN
Hope this helps!
Cheers,
--
-Dick
Dick Jackson / dick@d-jackson.com
D-Jackson Software Consulting / http://www.d-jackson.com
Calgary, Alberta, Canada / +1-403-242-7398 / Fax: 241-7392
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