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Re: Convolution, IDL & Numerical Recipes [message #32761 is a reply to message #32678]

Fri, 01 November 2002 10:44

R.G. Stockwell
Messages: 363
Registered: July 1999

Senior Member

David Fanning wrote:
> R.G. Stockwell (sorry@noemail.now) writes:
>
>
>> Perhaps you want to use the following keywords:
>> Check out the help file to see the effects the keywords
>> have on how the arrays line up to be convolved.
>> (Note: you must explicitly set center=0, or else it defaults
>> to 1)
>
>
> Alright, now, can you give me the layman's definition
> of the difference between spacial filtering (CENTER=1)
> and convolution "in the strict mathematical sense"
> (CENTER=0).
...

> Cheers,
>
> David

If I may answer quickly off the top of my head without thinking about
it or looking at the help files, then I'd say, .. uh.... hmmm.... oh I
better look it up.

Ok, convol is just about the most messed up piece of code that IDL has.
(Don't get me started about people using the letter "l" as a variable,
which to me is indistinguishable from the number "1").

The difference is quite profound between the two.

IF center = 0 EXPLICITLY, then
you have the sum of A[t-m/2+i] K[i]
NOTE that the index of A is a constant +i, this is a correlation.
The kernel shifts along, and the time series shifts along in the same direction.

IF center = 1 OR is ommitted, then
you have the sum of A[t-i] K[i]
NOTE that the index of A is now a constant - i, this is a convolution.
The kernel shifts along, BUT the time series is shifting backwards (in the opposite direction).

Also, the offsets move around too.

> Which would I use if I'm trying to make a pretty image? :-)

I suggest running all possible permutations of the keywords, and
selecting the one that matches the textbook examples :)

; test code

a = indgen(10)
k = [1,2,3]

print,'_________________'
print,strcompress(string(convol(a,k,center=1)))
print
print,strcompress(string(convol(a,k,center=0)))

<results>

0 8 14 20 26 32 38 44 50 0

0 0 4 10 16 22 28 34 40 46

Note the way the results are not even similar! YAY!

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[Message index]

		Convolution, IDL & Numerical Recipes By: aceves on Thu, 31 October 2002 12:50
		Re: Convolution, IDL & Numerical Recipes By: aceves on Wed, 06 November 2002 17:02
		Re: Convolution, IDL & Numerical Recipes By: JD Smith on Tue, 05 November 2002 14:42
		Re: Convolution, IDL & Numerical Recipes By: R.G. Stockwell on Tue, 05 November 2002 05:34
		Re: Convolution, IDL & Numerical Recipes By: David Fanning on Fri, 01 November 2002 12:14
		Re: Convolution, IDL & Numerical Recipes By: David Fanning on Fri, 01 November 2002 11:12
		Re: Convolution, IDL & Numerical Recipes By: muzic on Tue, 12 November 2002 06:10
		Re: Convolution, IDL & Numerical Recipes By: muzic on Tue, 12 November 2002 06:10
		Re: Convolution, IDL & Numerical Recipes By: R.G. Stockwell on Fri, 01 November 2002 10:44

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