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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-hyakutake.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
>
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