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Re: coherence test implementation [message #15424 is a reply to message #15336]

Thu, 13 May 1999 00:00

Dick Jackson
Messages: 347
Registered: August 1998

Senior Member

Hi Mark,

Mark Rehbein wrote:
>
> Hi,
>
> I'm doing some cloud detection on some rather large images using what
> some people call a coherence test. To do a coherence test on a pixel
> you take a 3x3 matrix around that pixel and assign that pixel the
> standard deviation of the 3x3 matrix.

You piqued my curiosity, since you're right, there had to be a more
efficient way! My attached coherence.pro should do the trick, and I
clock it at about 60 times faster with a 300x300 test.

From your code, just call:

sddevimage = Coherence(ch4)

I return an image of the same size and leave the outer ring of pixels as
0.0, is that reasonable?

I used the formula for the SD of a set of 9 values:

SD = Sqrt((Sum(X^2) - (Sum(X)^2 / 9)) / 8)

Generalizing coherence.pro to allow variable 'width' (not fixed at 3) is
left as an exercise to the reader. :-) I guess it wouldn't be hard,
changing all 'magic numbers' (3, 2, 9 and 8) to width, width-1, width^2
and width^2-1.

Cheers,
--
-Dick

Dick Jackson Fanning Software Consulting, Canadian Office
djackson@dfanning.com Calgary, Alberta Voice/Fax: (403) 242-7398
Coyote's Guide to IDL Programming: http://www.dfanning.com/

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[Message index]

		coherence test implementation By: Mark Rehbein on Thu, 13 May 1999 00:00
		Re: coherence test implementation By: Struan Gray on Thu, 13 May 1999 00:00
		Re: coherence test implementation By: eddie haskell on Thu, 13 May 1999 00:00
		Re: coherence test implementation By: Dick Jackson on Thu, 13 May 1999 00:00
		Re: coherence test implementation By: Craig Markwardt on Thu, 13 May 1999 00:00
		Re: coherence test implementation By: Dick Jackson on Thu, 13 May 1999 00:00

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