RE: [APML] Gradient and Vignette removal software

["R. Scott Ireland" <ireland@gate.net>]

This is essentially the same method Sean described in his article.  Blur and
subtract.

This method works pretty well, but not for all images. I find it can handle
about 60% of my images, the rest need a more complex A/V layer mask.

Scott

website:  http://www.rsiphotos.com/
email:  ireland@gate.net

> -----Original Message-----
> From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
> Behalf Of Richard Klappal
> Sent: Thursday, June 27, 2002 11:48 AM
> To: astro-photo@seds.org
> Subject: RE: [APML] Gradient and Vignette removal software
>
>
> 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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