Craig Markwardt wrote:
> David Oesch <oesch@giub.unibe.ch> writes:
>
>> Hello outthere,
>>
>> I know this topic was up before, but all I could find in the list was to
>> go for CALL_EXTERNAL and use a FORTRAN etc. program. Here's my problem:
>> 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.
>> Any progs so far in IDL for this problem?..or a decent fortran or C program?
>>
>
>
> 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
>
> It's a loop, but unless you have a bazillion images, it will be fast.
>
> Craig
>
Yes, I got that one with TOTAL, the problem is, it only works with
imagesets where all datalayers consists of 100% valid pixels...but this
is not always the case...
to eliminate those values, I came as far as
mean = total((image NE -9999),3)/nimages
but the problem is, nimages needs to be changed to...
up to now, I use the following simple function to get to my data...but
it is time consuming ..
function mean3d,file,nvalid=nvalid
;*********************************************************** ****************
; finding the mean at each pixel position for a set of equal size 2-D images
;*********************************************************** ****************
;file= 3d array
;nvalid= novalid values
;calling sequence: a=mean3d(array,nvalid=-9999)
s=SIZE(file)
outarray=FLTARR(s(1),s(2))
i=0l
FOR i=0,fix(s(1))-1 DO BEGIN
j=0l
FOR j=0,fix(s(2))-1 DO BEGIN
check=WHERE(file(i,j,*) NE nvalid,count_check)
IF count_check GT 0 THEN BEGIN
outarray(i,j)=MEAN(file(i,j,check))
ENDIF
ENDFOR
ENDFOR
RETURN,outarray
END
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