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Re: image subtraction [message #29781 is a reply to message #11675]

Wed, 13 March 2002 18:48

Dick Jackson
Messages: 347
Registered: August 1998

Senior Member

"Craig Markwardt" <craigmnet@cow.physics.wisc.edu> wrote in message
news:onbsdsxalv.fsf@cow.physics.wisc.edu...
>
> aburden@mpl.ucsd.edu (Art) writes:
>> Hi,
>>
>> Can anyone come up with an efficient way to subtract a single image
>> from an array of images without using a loop, as in
>>
>> imgArr = imgArr - img1
>>
>> where,
>>
>> imgArr = lonarr(12,512,512)
>>
>> and,
>>
>> img1 = lonarr(512,512)
>>
>> as opposed to
>>
>> For ii=0,11 do imgArr[ii,*,*] = imgArr[ii,*,*] - img1
>
> (a) You can do this:
> imgarr = imgarr - rebin(reform(img1,1,512,512),12,512,512)
>
> This makes IMG1 into a 1x512x512 array, and then uses REBIN to
> expand it to the desired size.
>
> (b) I don't think a FOR loop will be that much slower than the REBIN
> approach, at least for 512x512 arrays.
>
> Good luck,
> Craig

I second Craig's suggestion, and add a small addendum:

The assignment in the FOR loop can be sped up a bit by writing it as:

FOR ii=0,11 DO imgArr[ii,0,0] = imgArr[ii,*,*] - img1
^ ^
The values will be laid in as expected, but this dropped my timing from
1.402 seconds to 1.232 seconds.

Now, having said that, the REBIN method (useful if you have enough memory to
make the second big array) comes in at 0.210 seconds!

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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[Message index]

		Image subtraction By: FRANCOIS LAROSE on Mon, 18 May 1998 00:00
		Re: Image subtraction By: noname on Tue, 26 May 1998 00:00
		Re: image subtraction By: Dick Jackson on Mon, 18 March 2002 21:45
		Re: image subtraction By: K. Bowman on Sun, 17 March 2002 08:11
		Re: image subtraction By: Dick Jackson on Thu, 14 March 2002 08:00
		Re: image subtraction By: Kenneth P. Bowman on Thu, 14 March 2002 04:55
		Re: image subtraction By: Dick Jackson on Wed, 13 March 2002 18:48
		Re: image subtraction By: Craig Markwardt on Wed, 13 March 2002 16:21

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