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Re: Image analysis and ring identification [message #30272 is a reply to message #30271]

Thu, 18 April 2002 06:44

Paul Van Delst[1]
Messages: 1157
Registered: April 2002

Senior Member

David Fanning wrote:
>
> Rachel Pepper (Rachel_Pepper@brown.edu) writes:
>
>> Actually, the rings often have a bright spot that is not in the center of
>> the ring, so I don't think the centroiding technique will work. Do you have
>> any other suggestions?
>
> I think Paul's suggestions are right on the money,
> but I wouldn't give up on the centroid theory too
> quickly. The wonderful thing about centroids is that
> they are really center of mass calculations. So if
> the values you total are actual image values,
> then the centroid will naturally zero in on
> bright spots, since they are "heavier" in the
> calculations.

Personally I prefer the centroid method. I think its more accurate than simple peak-finding. If
the bright spot is a nice gaussian shape, then determing the maximum will probably do the job.
In the real world nice smooth shapes are rare, so the centroid method is the way to go.

My experience in this area is with finding the centroid of a satellite instrument channel
spectral response function (SRF). An SRF is determined by the detector response, properties of
optical components directing the radiation onto the detector, the instrument field of view etc.
etc. The resultant SRF is replete with bumps, wiggles, and shimmies. Calculating the first
moment of the SRF (the frequency centroid) is really the only way to do it accurately. If it's
not done accurately, then any calculated Planck function radiances will be slightly incorrect
and this will introduce a frequency dependent bias in any comparison of satellite observations
with calculations. That is not a Good Thing (tm).

If the OP's particles are "macroscopic" (i.e. she's not searching for Higg's bosons amidst a
slurry of those pesky W and Z particles :o) and have a shape to them, then the centroid method
is recommended.

paulv

--
Paul van Delst Religious and cultural
CIMSS @ NOAA/NCEP purity is a fundamentalist
Ph: (301)763-8000 x7274 fantasy
Fax:(301)763-8545 V.S.Naipaul

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[Message index]

		Re: Image analysis and ring identification By: James Kuyper on Thu, 18 April 2002 08:07
		Re: Image analysis and ring identification By: Paul Van Delst[1] on Thu, 18 April 2002 06:44
		Re: Image analysis and ring identification By: David Fanning on Thu, 18 April 2002 06:05
		Re: Image analysis and ring identification By: Paul Van Delst[1] on Thu, 18 April 2002 05:37
		Re: Image analysis and ring identification By: Rachel Pepper on Wed, 17 April 2002 21:21
		Re: Image analysis and ring identification By: David Fanning on Wed, 17 April 2002 07:29
		Re: Image analysis and ring identification By: Dan Larson on Thu, 18 April 2002 08:37
		Re: Image analysis and ring identification By: David Fanning on Thu, 18 April 2002 08:33

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