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Re: Sorting and image rescaling [message #25311 is a reply to message #25176]

Mon, 28 May 2001 14:50

Wayne Landsman
Messages: 117
Registered: January 1997

Senior Member

Bill wrote:

>
> Some of our band images are on the order of 2500 by 10000. For such band
> images this line can take over 30 seconds per band. This is a moderate
> nuisance at the moment, but we are planning to update our calibration ,
> and reprocess 1000s of multiband images with a new calibration.
> Naturally we want to update the jpegs to reflect this new calibration.
> It appears that this single line will extend reprocessing by a couple of
> days. I don't like this. This yields the following questions:
>
> 1. Does anyone know a better general approach to such a rescaling that
> avoids the need to sort the data, or sort more than a fraction of the
> data?

Provided your images are well-behaved, I would think it would suffice to
sample a few thousand pixels distributed uniformly over your image to
establish the scaling. For all the but the most bizarre histograms of
pixel intensities, I would think a few thousand samples is a good
approximation.

An astronomical application can be found in the program sky.pro in
http://idlastro.gsfc.nasa.gov/ftp/pro/idlphot/. This program establishes
a grid of about 4000 pixel values uniformly distributed across the
image. It then runs a fairly complicated program to throw out outliers
(assumed in astronomy to be overwhelmingly positive) to establish a mean
sky and sigma. For display purposes one can then choose to display
between say sky - 2*sigma to sky + 20*sigma.

--Wayne Landsman landsman@mpb.gsfc.nasa.gov

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[Message index]

		Sorting and image rescaling By: William Clodius on Fri, 25 May 2001 15:34
		Re: Sorting and image rescaling By: John-David T. Smith on Wed, 30 May 2001 09:44
		Re: Sorting and image rescaling By: Wayne Landsman on Mon, 28 May 2001 14:50
		Re: Sorting and image rescaling By: Craig Markwardt on Mon, 28 May 2001 10:25
		Re: Sorting and image rescaling By: Martin Schultz on Mon, 28 May 2001 09:54

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