| Re: ratio imaging [message #29799 is a reply to message #29710] |
Wed, 13 March 2002 04:39   |
gerhard.holst
Messages: 3
Registered: March 2002
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Junior Member |
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Dan Larson <drl16@cornell.edu> wrote in message news:<MPG.16f7fce19edad53f989684@newsstand.cit.cornell.edu>...
> In article <9fe17a3c.0203120028.1a7caf67@posting.google.com>,
> gerhard.holst@pco.de says...
>> Craig Markwardt <craigmnet@cow.physics.wisc.edu> wrote in message news:<on1yev3r9q.fsf@cow.physics.wisc.edu>...
>>> Dan Larson <drl16@cornell.edu> writes:
>>>> I am trying to do simple ratiometric
>>>> imaging with IDL. Of course, since
>>>> there is some pixelation noise, the
>>>> ratio is not very robust. I have
>>>> experimented with a number of
>>>> different filters (median, Gaussian
>>>> deconvolution, smooth) to try and
>>>> remove some of this instability.
>>>> Is there a filtering technique which
>>>> is minimally perturbative that will
>>>> remove some numerical artificats
>>>> without changing the boundaries of
>>>> objects?
>>>
>>> Dan, you should be filtering the two images, *before* computing the
>>> ratio, right? I would have said that goes without saying, but now I
>>> am saying it. [ The reason of course is that the ratio does not have
>>> a nice compact statistical distribution, so averaging is less
>>> robust. ]
>>>
>>> Craig
>>
>> Dan,
>> if your are looking for more edge preserving filters you might
>> search for topics like "Savitzky-Golay" and "LOESS", both are
>> filters that might consume a little more time in calculation
>> (especially the LOESS I have found on the web, if you are interested
>> I can look for the link), but they do a good job in smoothing
>> while edge keeping, much better than boxcar, median etc.
>>
>> Gerhard
>>
> Gerhard,
>
> I am familiar with the Savitzky-Golay filter, but I have never used
> the LOESS filter. If you have an implementation that you like, I
> would like to hear about it. Do you know what the acronym stands
> for?
>
> Dan
Hi Dan,
to my knowledge it stands for an old German geological term,
it is no acronym, there is also a filter called Lowess very similar to that.
I have found the origin of the LOESS routines in IDL that I use,
they were developed by H. Freudenreich.
http://www.astro.washington.edu/deutsch/idl/htmlhelp/slibrar y14.html
There is also the source for his routines based on a library from
AT&T Labs. I have attached 2 files (pdf&ps) that give some background
on the filter. If you use the Loess routine of Freudenreich, keep
the pixelwidth odd, otherwise you'll get error messages, if I remember
correct.
Gerhard
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