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Re: [APML] Pleiades response to Processing Challenge: LMC
Bert,
Thank you. It's always a pleasure when one works with such a good raw material.
We use two methods for vignetting correction. Both methods are based on a
quite simple principle: we try to isolate all the background pixels of the
image to obtain a vignetting correction mask. Background pixels are those
corresponding exclusively to the sky background. An ideal, vignetting-free
image would have a perfectly uniform distribution (exception made of local
variations due to random distribution) of background pixels over the whole
image. Vignetting correction is difficult (and can be terribly difficult)
because stellar and nonstellar objects hunt us background pixels. The
correction techniques try to figure out what the sky background should look
like in the absence of objects.
Our first method, which is also our preferred one, is what we call "the
object suppression method". A fundamental property of background
pixels is that they are locally (i.e., at small scale) randomly
distributed. If this is true, then we can safely copy neighbor, nearly
adjacent background pixels over stellar and nonstellar images without
altering the background pixel distribution's properties, including both its
small-scale (random) as well as global (due to vignetting) properties. In
Photoshop, we basically create a selection comprising all the stellar
images (through Select>Color_Range channel by channel) and then use the
clone stamp tool to copy adjacent background pixels over stars. Then we use
the vignetting simmetric properties to suppress nonstellar objects, again
by copying adjacent background pixels. We use a special circular concentric
grid to help doing all that. This method is extremely accurate, but it
requires a considerable amount of work and can be very difficult to
implement when nonstellar objects are covering large areas of the image.
The second method is similar to Jerry Lodriguss' vignetting correction
method. We take a number of samples, for background pixels, to reconstruct
the vignetting profile over each diagonal of the image. Then we set up four
gradients (one for each diagonal), usually with 10 to 20 sampled values,
and apply them. This method is simpler (also more elegant from an
algorithmic point of view) but less acurate than object suppression. We use
it when object suppression is causing us a headache.
We have in mind, since some time ago, the possibility of automating these
procedures (or at least most of them) with a vignetting correction
software. Time permitting...
For the Matt's LMC image, we have used object suppression for the entire
image except its central area, which is too covered by the LMC. For these
central regions we have used a gradient, and for the very central area we
have had to guess a constant value, since no background pixels are
available. As we have used both methods we talk of an "hybrid" technique.
For chromatic balance, we make a number of careful selections of background
pixels with the lasso tool, then save this selection, and create a curves
layer. With the Image>Histogram command, having the selection loaded, we
can see the median values for each color channel. We adjust curves until
the three median (*not* average!) values for the R, G and B channels are
identical. This way we achieve a neutral sky background. This doesn't
guarantee prefect color balance over the whole image, indeed, but is a
necessary starting point for vignetting correction and image integration
techniques.
Hope this helps. Regards,
______________________________________
Juan Conejero, Pleiades Astrophoto Lab
skycad@ctv.es
http://www.pleiades-astrophoto.com/en.html
At 10:04 22/05/2002 -0700, you wrote:
>A very impressive job of processing, Juan! Can you give us more of a
>tutorial on the first set of Photoshop techniques (sample background zones,
>hybrid object suppression, etc)? Those are new terms for me. And
>congratulations to Matt for a fantastic raw image!
>Bert
>
>Bert Katzung
>katzung1@attbi.com
>www.astronomy-images.com
>
>----- Original Message -----
>From: "Juan Conejero" <skycad@ctv.es>
>To: <astro-photo@seds.org>
>Sent: Wednesday, May 22, 2002 9:21 AM
>Subject: Re: [APML] Processing Challenge: LMC
>
>
> > Matt,
> >
> > Here is our attempt on your LMC:
> >
> > For 1024x768 resolution:
> > http://www.pleiades-astrophoto.com/APML/MattBenDaniel-LMC/en.html
> > For 1280x1024 or higher:
> > http://www.pleiades-astrophoto.com/APML/MattBenDaniel-LMC/en.1024.html
> >
> > Congratulations for such a magnificient shot. A really funny and teaching
> > challenge as well. Let's see how do we do with Monte's challenge!
> > Best regards,
> > ______________________________________
> > Juan Conejero, Pleiades Astrophoto Lab
> > skycad@ctv.es
> > http://www.pleiades-astrophoto.com/en.html
> >
> > At 21:39 14/05/2002 -0400, you wrote:
> > >Below is a URL to an unprocessed 8.5MB TIFF of the Large Magellanic
>Cloud.
> > >Also below is a link to a processed version of the image. I chose this
> > >image because I could not make it look as nice as I want. I am hoping
>that
> > >y'all will show me a thing or two with it. That said, this is a difficult
> > >image. It has major vignetting and red fogging (probably from film age
>and
> > >or humidity). If you're new to image processing, this is a tough image to
> > >start with.
> > >
> > >Because there is some nice stuff on the edges, I'm hoping that you won't
> > >crop your version.
> > >
> > >One thing I don't have yet is SGBNR or Grain Surgery. I think that would
> > >help this one a lot.
> > >
> > >You know that if you do a good job with it, the folks on the APML
> > >(especially me) will want to know what you did in gory detail.
> > >
> > >I found that WinZip did not compress the file very much, so I didn't
> > >bother with it. If you're on a 56K modem, it will take at least half an
> > >hour to download. I downloaded the image and opened it in Photoshop to
> > >test its validity.
> > >
> > >http://world.std.com/~mattb/lmc.tif
> > >
> > >http://world.std.com/~mattb/gallery/astro/lmc.html
> > >--
> > >Matt BenDaniel
> > >matt@starmatt.com
> > >http://starmatt.com
> > >
> > >
> > >-- APML Archives at <http://astro.umsystem.edu/apml/> ---
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> >
> >
> >
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> >
>
>
>
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