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[APML] Re: Vignetting and SGBNR info
Hi Juan:
Thanks for that link, it looks very useful! I tried putting it through
Babelfish (the AltaMira translator) and got an adequate english version as
far as it was willing to go (4.5 pages). So I downloaded the whole thing to
my machine and will try feeding segments to Babelfish and see if that works
for getting the rest of it. Any Spanish-speakers on the list who would like
to take it on?
Bert
Bert Katzung
katzung1@attbi.com
www.astronomy-images.com
----- Original Message -----
From: "Juan Conejero" <skycad@ctv.es>
To: <astro-photo@seds.org>
Sent: Thursday, May 23, 2002 11:15 AM
Subject: Vinetting and SGBNR info (Was: Re: [APML] Pleiades response to
Processing Challenge: LMC)
> Bert, Don,
>
> We have an article on vignetting correction at our website, but,
> unfortunately, this article is available in Spanish only ---we haven't had
> time enough to translate it. However, we included a lot of figures and
used
> the English version of Photoshop 6.0 to make them, so I think you won't
> have too much problem to understand the techniques described. The article
> is here:
>
> http://www.pleiades-astrophoto.com/doc/vignetting/es.html
>
> MS Internet Explorer >= 5.0 / Netscape >= 6 required. We'll be glad to
> answer any question or doubt you may have. In spite of language, I hope
> you'll find that article of some help.
>
> Regarding SGBNR, we apologize for having no documentation specific for
> version 1.0.9 yet. We are finishing our new house now. Whoever has built
> his/her own house knows what a sort of nightmare this means. However, I
> have some free time now and the aim of making things clearer for you and
> the rest of SGBNR users, so I'll try to explain it a little. I realize
that
> the explanation below is going to be a bit long ---sorry, but probably
I'll
> found no better way!
>
> As you can see when you run SGBNR, parameters are grouped into three main
> sections: filter, edges protection, and masks.
>
> * Filter
>
> For the filter's order, you usually want small values, which can preserve
> smaller details. A value between 0.7 to 1.0 for order is appropriate for
> many images. A value of 0.4 generates the largest filter matrix and can
> yield very uniform backgrounds, but preserves very small image features,
> which sometimes isn't convenient. However, the filter's order is usually a
> non-critical adjustment.
>
> The filter's radius is the main control available to govern the filter's
> strength. This parameter must be adjusted according to the filter's order.
> Generally speaking, low order filters require smaller radii.
>
> The matrix size informs us about how much filtering strength we are going
> to apply. For instance, for 2700 dpi film originals, matrices between 15
> and 40 are usually required, depending on the importance, size and
> characteristics of film grain. For example, a filter of order 1.0 and
> radius 3.0 gives a matrix size of 41. The same size is achieved with order
> 0.4 and radius 1.6. But the 0.4 filter is more conservative. The higher
the
> order, the more averaging effect.
>
> The iteration parameter is new to version 1.0.9. For mathematical reasons
> which are out of scope here, and as demonstrated by many test results,
> applying a smaller filter matrix several consecutive times is more
> efficient (in terms of better preserving low-contrast features) than a
> single pass with a stronger filter. For example, instead of a matrix size
> of 41, try two iterations with 19, or even three iterations with 13. Use
> this feature mainly for images with subtle low-contrast features.
>
> * Edges Protection
>
> Protection of edges plays a key role in SGBNR processing. For example,
> suppose we have an image of the Moon. You don't want the darkest features,
> as shadows on the bottom of craters, contaminated by surrounding bright
> areas. Similarly, you don't want extremely small bright features, as
> certain prominent peaks or thin crater borders, dimmed by pollution from
> neighbor dark areas. Edges protection helps protecting small detail and
> preserving contrast.
>
> For both the dark and bright sides of edges, you have threshold and
> strength parameters. The most important parameter is by far threshold. Low
> threshold values are more aggressive when protecting edges from pollution.
> Each threshold must be adjusted by trial and error (don't worry: practice
> helps :-)). Start with the default values and preview. Try to reduce dark
> threshold. Too low a dark threshold yields undesirable dark artifacts,
> since it exaggerates the importance of extremely small dark features. The
> same is true for bright threshold: a too low value gives many small bright
> spots. It's unlikely that the default values (0.250) be too low, but
> sometimes that occurs. It's a good idea to preview some portions of the
> image with edges protection enabled/disabled; that helps evaluating the
> real effect of this mechanism. Once you've found a nearly correct
threshold
> value, you can play with strength. Try previewing many different areas of
> the image. Areas with small dark features, with small bright features,
with
> both of them, with none of them, and so on. E.P. is a beatiful creature.
>
> * Masks
>
> The masks protect certain areas of the image from excessive blurring
during
> the low-pass filtering process. Masks in SGBNR work exactly like Photoshop
> masks: where a mask is black no change occurs; where a mask is white
> there's no protection at all. You can use two kinds of masks: uniform and
> image masks. Uniform masks are useful when we have to remove noise on the
> whole image, including bright areas. See, for example, a recent test
> processing by Jay Potts with an image of Hyakutake. We have placed our own
> version of the Jay's test on our website:
>
>
http://www.pleiades-astrophoto.com/software/SGBNR-test-potts/sgbnr-test-hyak
utake.en.html
>
> In this test you can see how uniform masks are the only way to get rid of
> noise on the comet's head. With version 1.0.9 we've got even better
results
> with iterative filters:
>
> http://www.pleiades-astrophoto.com/software/SGBNR-test-potts/108_51-4.tif
> http://www.pleiades-astrophoto.com/software/SGBNR-test-potts/108_51-5.tif
>
> The above images are, respectively, the result of two and three
iterations:
>
> 108_51-4.tif
> Two iterations with the following parameters set:
> Uniform masks = 0.75
> Radius = 0.85 (matrix size = 21)
> Order = 0.40
> Dark sides threshold = 0.270
> Dark sides strength = 0.50
> Bright sides threshold = 0.170
> Bright sides strength = 0.15
>
> 108_51-5.tif
> Three iterations with the same parameters as above, except:
> Radius = 0.60 (matrix size = 15)
>
> Image masks are much more restrictive than uniform masks: they protect
> bright areas and allow more noise removal on dark background regions.
> Starting with version 1.0.9, the usage of image masks is much easier
> because only three parameters are required to define the masks for the
> three R,G,B channels: shadows and highlight clipping values, and midtones
> balance. The clipping values work like Photoshop's auto levels. See the
> documentation for SGBNR version 1.0.7, under the "Auto" button. The
> midtones balance works just like a histogram's midtones balance. Again,
the
> 1.0.7 documentation will be of great help.
>
> For the Matt's LMC image we used image masks. We found that uniform masks
> are not appropriate for this case, the main reason being that there is no
> noise to remove in the brightest areas of the image, especially the bright
> nebulas: uniform masks "wash" them too much.
>
> And that's all folks! Sorry if I've bored you too much, but hope it helps!
>
> Sincerely,
> ______________________________________
> Juan Conejero, Pleiades Astrophoto Lab
> skycad@ctv.es
> http://www.pleiades-astrophoto.com/en.html
>
> At 18:46 22/05/2002 -0700, you wrote:
> >Juan,
> >I too am very impressed with your LMC processing. Thanks for the
detailed
> >explaination of your control of vignetting. Now if I read it four or
five
> >times and try some of it, I may begin to understand it. I do hope you
can
> >automate the anti-vignetting process.
> >
> >I downloaded your SGBNR settings used for LMC and notice they are widely
> >different from the default settings. At the risk of asking Rembrandt how
to
> >paint masterpiece art, how do you know what settings to adjust in SGBNR,
and
> >in which direction? I'm at the trial and error stage.
> >
> >Don
> >
> >-- APML Archives at <http://astro.umsystem.edu/apml/> ---
> > Unsubscribe at <majordomo@seds.org>
>
>
>
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