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View Full Version : Re: Real or fantasy ??


Chris L Peterson
September 2nd 03, 06:28 AM
On Mon, 01 Sep 2003 17:47:04 +0200, Marco A. > wrote:

>Hello from Marco from Italy.
>I'm a newbye in digital elaboration.
>Often we can see in web pictures with a lot of small detail (0,5'' or
>smaller).
>My question is this: how can I know if a detail is 'real' or it depend
>only from digital elaboration?
>Ok, I can compare the image with an Hst picture.
>But does exist a method that tell me if my elaboration does not
>generate 'unreal details' ??

The kind of processing required to bring out details always introduces
artifacts. The best solution to understanding and detecting them is to take lots
of images as the planet moves. Actual structures will usually be apparent. Also
comparing your images to those made by others is a good test. I don't know of
any good way to know for sure what's an artifact, but any frequency components
higher than the best PSF your optics are capable of are certainly artifactual.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Roger Hamlett
September 2nd 03, 12:10 PM
"Chris L Peterson" > wrote in message
...
> On Mon, 01 Sep 2003 17:47:04 +0200, Marco A. > wrote:
>
> >Hello from Marco from Italy.
> >I'm a newbye in digital elaboration.
> >Often we can see in web pictures with a lot of small detail (0,5'' or
> >smaller).
> >My question is this: how can I know if a detail is 'real' or it depend
> >only from digital elaboration?
> >Ok, I can compare the image with an Hst picture.
> >But does exist a method that tell me if my elaboration does not
> >generate 'unreal details' ??
>
> The kind of processing required to bring out details always introduces
> artifacts. The best solution to understanding and detecting them is to
take lots
> of images as the planet moves. Actual structures will usually be apparent.
Also
> comparing your images to those made by others is a good test. I don't know
of
> any good way to know for sure what's an artifact, but any frequency
components
> higher than the best PSF your optics are capable of are certainly
artifactual.
One thing that is well worth looking for, are artefacts on 'known' edges. On
Mars images for instance, the edge of the planet is about the sharpest
contrast contour present. If you look at some of the posted images, you will
see a second 'ghost' ring, following this edge, sometimes on the dark sky,
and sometimes on the lighter surface of the planet (depending on the
particular processing used). This is in a sense, a 'sure sign' of
everprocessing. A high frequency filter has been applied, to the point where
the sharp edge is creating overshoot, and producing these artefacts. Compare
these images with others, where this type of artefact is not present, and
you will see that the latter tend to have a 'softer' look, with some of the
sharp edges missing, but you can be far more confident that the latter's
details are more likely to be real...

Best Wishes

Chris L Peterson
September 2nd 03, 02:54 PM
On Tue, 2 Sep 2003 12:10:31 +0100, "Roger Hamlett"
> wrote:

>One thing that is well worth looking for, are artefacts on 'known' edges. On
>Mars images for instance, the edge of the planet is about the sharpest
>contrast contour present. If you look at some of the posted images, you will
>see a second 'ghost' ring, following this edge, sometimes on the dark sky,
>and sometimes on the lighter surface of the planet (depending on the
>particular processing used). This is in a sense, a 'sure sign' of
>everprocessing. A high frequency filter has been applied, to the point where
>the sharp edge is creating overshoot, and producing these artefacts...

It is worth emphasizing that there is nothing necessarily wrong with
"overprocessing" and the creation of artifacts. You see it all the time in
professional images. The processing required to bring out certain kinds of
details will often generate artifacts elsewhere in the image. What's important
is to recognize what's real and what's not, lest you fly off thinking you've
discovered a Martian city or something <g>.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com