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Re: Q: Quantil calculation in IDL? [message #17527 is a reply to message #17379]

Fri, 15 October 1999 00:00

davidf
Messages: 2866
Registered: September 1996

Senior Member

Joerg Mosthaf (joerg.mosthaf@urz.uni-heidelberg.de) writes:

> I have been searching the help files and David Fannings great book, but I can't
> find a way to calculate 25%- and 75%-quantils. Unfortunately I don't know the
> english name for this so let me explain: A 75%-quantil is like the median, but
> with 75% instead of 50% i.e. the number in a data spread, that 75% of all
> data points are less or equal to. Is there a way to do this fast on an
> 256x256 array? I need it to cut off noise at a specific level and to get a
> reliable min/max value, not including data spikes. I am probably overlooking
> something very easy, but I just couldn't find it.

Well, I don't know the "IDL way" to do this (Stein
Vidar or someone else can fill us in on the details),
but the "get the thing done" way to do this goes something
like this:

1. Calculate the HISTOGRAM of the pixel distribution.
2. Do a "running sum" of the number of pixels in each
bin of the histogram until you exceed your target
amount. The value of that histogram bin will be
your quantil.

Sorry I can't be more explicit, but aside from not wanting
to embarrass myself, I have a plane to catch in about a half
hour. :-)

See you guys (and Rose, good to hear from *you* again!) in
a couple of weeks.

Cheers,

David
--
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155

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[Message index]

		Q: Quantil calculation in IDL? By: joerg.mosthaf on Fri, 15 October 1999 00:00
		Re: Q: Quantil calculation in IDL? By: m218003 on Mon, 25 October 1999 00:00
		Re: Q: Quantil calculation in IDL? By: James Tappin on Mon, 18 October 1999 00:00
		Re: Q: Quantil calculation in IDL? By: joerg.mosthaf on Mon, 18 October 1999 00:00
		Re: Q: Quantil calculation in IDL? By: J.D. Smith on Fri, 15 October 1999 00:00
		Re: Q: Quantil calculation in IDL? By: J.D. Smith on Fri, 15 October 1999 00:00
		Re: Q: Quantil calculation in IDL? By: Pavel Romashkin on Fri, 15 October 1999 00:00
		Re: Q: Quantil calculation in IDL? By: davidf on Fri, 15 October 1999 00:00

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