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median downsampling [message #29921] Wed, 27 March 2002 11:09
Jonathan Joseph is currently offline  Jonathan Joseph
Messages: 69
Registered: September 1998
Member
This is sort of a follow-up to my previous rebin question. I'm having
trouble writing a routine to *efficiently* downsample an 2D array
(ala rebin) but using the median value instead of the average value
over the box (sub-image) being downsampled into a given pixel.

If my downsampling box-size were guaranteed to be the same dimension
in X and Y, I think this problem could be easily solved using the
median routine with a "width" argument. That's not guaranteed to be the
case though, and I can't see how to do a median filter with different
dimensions in X and Y.

I can use methods like those in the astronomy library's boxave.pro to
"quickly" (looping only over the number of pixels in the downsampling
box) reform the 2d array into a 3d array such that A[*,i,j] gives me
all of the pixels from the original image that get downsampled into the
single pixel [i,j] in the output image. Unlike total though, I can't do
a median or sort across a particular dimension of a multi-dimensional
array, so it seems I would still have to do a median operation for each
pixel in the downsampled image.

Can anyone think of a way to achieve the desired result w/o the
large offensive looping?

Perhaps there's a way to do this using histogram (eh JD?)

-Jonathan
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