RE: [APML] Gradient and Vignette removal software

[Matt BenDaniel <matt@starmatt.com>]

How can the method below account for the difference between background and nebulosity?

Matt

At 11:48 AM 6/27/2002, Richard Klappal wrote:
>>From "The Image Processing Handbook", 3rd Edition, CRC Press/IEEE Press,
>Editor: John C Russ, one of the standard ways to correct for uneven
>illumination (should work for both vignetting and sky gradients) is to make
>a copy; repeatedly replace each pixel with its darkest neighbor from a 5x5
>octagonal neighborhood, until only backgrounds remins; subtract from
>original and stretch/relevel the contrast.  Another uses brightest neighbor
>from the 5x5 (probably depends on the predominant background).
>
>I suspect that a darkening blur sequence would work similarly, with with
>radius dependent on detail size.
>
>Rich
>
>
>> -----Original Message-----
>> From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
>> Behalf Of Jerry Lodriguss
>> Sent: Thursday, June 27, 2002 02:30
>> To: astro-photo@seds.org
>> Subject: Re: [APML] Gradient and Vignette removal software
>>
>>
>>
>> Hi John,
>>
>> I NEVER remove stars by hand! <G>
>>
>> That's what a 100 pixel median filter is for... <G>
>>
>> Jerry
>>
>>
>> >You are right - this isn't the perfect answer due to film's non-linear
>> >response over time, and it won't touch gradients. It might be
>> possible that
>> >adjusting curves or contrast to the control layer might help offset the
>> >effect of reciprocity. Or not, but I do plan to try this. This has to be
>> >more accurate and easier that manually removing stars from an image,
>> >removing any DSO's, applying a massive blur, inverting and applying as a
>> >control layer which is how I do it now. With unfiltered Schmidt shots -
>> >which Chris does - exposures are short, generally under 10
>> minutes. If all
>> >goes well, I will try this out on the weekend.
>>
>>
>> Astronomical photography: http://www.astropix.com
>>
>>
>>
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>
>
>
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--
Matt BenDaniel
matt@starmatt.com
http://starmatt.com


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