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Re: efficient kernel or masking algorithm ? UPDATE [message #23919 is a reply to message #23918]

Sun, 25 February 2001 19:00

John-David T. Smith is currently offline

John-David T. Smith
Messages: 384
Registered: January 2000

Senior Member

Martin Downing wrote:
>
> From thread: http://cow.physics.wisc.edu/~craigm/idl/archive/msg03957.htm l
> a.. Date: Wed, 29 Nov 2000 16:30:54 -0500
> Richard Tyc wrote:
>>
>> I need to apply a smoothing type kernel across an image, and calculate the
>> standard deviation of the pixels masked by this kernel.
>>
>> ie. lets say I have a 128x128 image. I apply a 3x3 kernel (or simply a
>> mask) starting at [0:2,0:2] and use these pixels to find the standard
>> deviation for the center pixel [1,1] based on its surrounding pixels, then
>> advance the kernel etc deriving a std deviation image essentially.
>> I can see myself doing this 'C' like with for loops but does something
> exist
>> for IDL to do it better or more efficiently ?
>>
>> Rich
>
> I was wandering through new Craig's IDL archive site (which is brilliant by
> the way) and came across this question asking for an efficient way of
> calculating the loacal standard deviation in an array. It seemed to me that
> the thread had not reached a full solution so perhaps some of you might be
> interested in this method which is very fast. It is based on the crafty
> formula for variance:
> variance = (sum of the squares)/n + (square of the sums)/n*n

Righto. I knew I was fishing for something like this. Except I think you mean:

(population) variance = (sum of the squares)/n - (square of the sums)/n*n

Luckily, that's how you've coded it too. Sample variance (=population
variance*n/(n-1)) is of course the more common case in science (as opposed to
gambling).

JD

P.S. I think I originally got the idea from sigma_filter.pro, a NASA library
routine, dating back to 1991. It's chock-full of other good tidbits too.
Thanks Frank and Wayne!

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[Message index]

		Re: efficient kernel or masking algorithm ? UPDATE By: Craig Markwardt on Mon, 26 February 2001 10:51
		Re: efficient kernel or masking algorithm ? UPDATE By: Martin Downing on Mon, 26 February 2001 07:32
		Re: efficient kernel or masking algorithm ? UPDATE By: Martin Downing on Mon, 26 February 2001 02:21
		Re: efficient kernel or masking algorithm ? UPDATE By: Martin Downing on Mon, 26 February 2001 02:21
		Re: efficient kernel or masking algorithm ? UPDATE By: Martin Downing on Mon, 26 February 2001 02:00
		Re: efficient kernel or masking algorithm ? UPDATE By: Martin Downing on Mon, 26 February 2001 01:52
		Re: efficient kernel or masking algorithm ? UPDATE By: John-David T. Smith on Sun, 25 February 2001 19:00
		Re: efficient kernel or masking algorithm ? UPDATE By: Craig Markwardt on Sun, 25 February 2001 12:39
		Re: efficient kernel or masking algorithm ? UPDATE By: Martin Downing on Sun, 25 February 2001 10:44
		Re: efficient kernel or masking algorithm ? UPDATE By: PMan on Tue, 23 July 2013 13:35

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