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Re: Finding the mean of a set of images [message #32582 is a reply to message #32578]

Tue, 22 October 2002 09:33

Jaco van Gorkom
Messages: 97
Registered: November 2000

Member

"Craig Markwardt" <craigmnet@cow.physics.wisc.edu> wrote in message
news:onznt6k38x.fsf@cow.physics.wisc.edu...
> David Oesch <oesch@giub.unibe.ch> writes:
>> ...
>> Does anyone have an algorithm for finding the mean/standardeviation etc
>> at each pixel position for a set of equal size 2-D images? Currently the
>> only way I have to do this is to extract all the values for a given
>> pixel position into a 1-D array and find the mean/standardeviation etc
>> on that. Doing it pixel by pixel like this is inefficient in IDL so I am
>> looking for an *array* based algorithm that would find all
>> the mean/standardeviation etc in parallel. ...
>
> Sure, if you stack your image into a 3D image cube, then you would
> have something like IMAGE = FLTARR(NX, NY, NIMAGES)
>
> Then the mean image is:
>
> mean = total(image,3)/nimages
>
> The standard deviation is:
>
> meancube = rebin(reform(mean,nx,ny,1),nx,ny,nimages)
> std = sqrt(total((image - meancube)^2,3)/(nimages-1))
>
> Now, what you meant by "etc" can get a little hairier. If you want to
> do median you are probably in trouble, but min and max are easy too:
>
> minimage = image(*,*,0)
> maximage = minimage
> for i = 1, nimages-1 do begin
> minimage = minimage < image(*,*,i)
> maximage = maximage > image(*,*,i)
> endfor

I believe that IDL 5.5 offers the luxury of
maximage = MAX(image, MIN=minimage, DIMENSION=3)

As for median, well, there was a thread on "Finding the median of a set
of images" back in '96. Should be valid still, I guess. Improvising with
transpose(), reform(), median(image, Nimages) and rebin() should be fun, but
not be very fast.

Jaco

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[Message index]

		Re: Finding the mean of a set of images By: Jaco van Gorkom on Tue, 22 October 2002 10:48
		Re: Finding the mean of a set of images By: Jaco van Gorkom on Tue, 22 October 2002 09:33
		Re: Finding the mean of a set of images By: David Oesch on Tue, 22 October 2002 08:23
		Re: Finding the mean of a set of images By: Craig Markwardt on Tue, 22 October 2002 06:33
		Re: Finding the mean of a set of images By: David Oesch on Wed, 23 October 2002 07:28
		Re: Finding the mean of a set of images By: David Oesch on Tue, 22 October 2002 22:18

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