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High order bit reconstruction
Malmer
post Feb 24 2015, 12:16 PM
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I have noticed that Cassini sometimes sends back images as 8bit by dropping the high order values in the 16 bit data. (edit: correction, the 12bit original data)

I wrote some code that follows the gradients in the images and tries to guess the most likely high order bits. Then i use manual methods to fix the areas where my code got it wrong...

Am I reinventing the wheel here? Is there any official method for dealing with these kinds of images?

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Ian R
post Feb 24 2015, 02:35 PM
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I *think* this problem may have been addressed recently:

http://photojournal.jpl.nasa.gov/catalog/PIA18411


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ugordan
post Feb 24 2015, 05:35 PM
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That's a very nice reconstruction. Back when I was playing around with Phoebe encounter data, I contemplated doing the same thing, but never got around to actually implementing it for what appeared to be a remarkably grey body anyway. I'm not aware of any official method for reconstructing the dynamic range, but who knows what kind of plugins exist for Isis 3 nowadays...

I will say that I'm glad they employ this encoding very rarely.


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4throck
post Feb 24 2015, 06:51 PM
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I think that type of reconstruction can be used on other images.
For example, here's a CIVA image just after Philae's release from Rosetta:



http://blogs.esa.int/rosetta/2014/11/12/farewell-rosetta/


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Malmer
post Feb 24 2015, 09:19 PM
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QUOTE (Ian R @ Feb 24 2015, 03:35 PM) *
I *think* this problem may have been addressed recently:

http://photojournal.jpl.nasa.gov/catalog/PIA18411



I will continue chugging along. I have done most of the high resolution images from the phoebe encounter anyway.

My code works reasonably well but when there are lots of highfrequency components in the image it becomes hard to know if it is a "step" or just a big change in brightness. That's where human input comes in to play... click "up" click "down"...

I will upload a zip file with the images when I'm done.
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Malmer
post Feb 25 2015, 05:31 PM
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work in progress on the trickiest to get right... lots of highfrequency stepping...

The first image is the source. Second my work in progress. Third another raw image of the same area shot with overexposed highs.

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Malmer
post Feb 25 2015, 05:35 PM
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QUOTE (4throck @ Feb 24 2015, 07:51 PM) *
I think that type of reconstruction can be used on other images.
For example, here's a CIVA image just after Philae's release from Rosetta:


Since it is a .jpg image it might not work as intended. A .png would have been nice.
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Bjorn Jonsson
post Feb 25 2015, 11:37 PM
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This is great, somehow I always regarded these images as (nearly) useless but I'm very happy to see that I was wrong about that. Luckily these images are very rare, the Phoebe images are the only interesting images like these that I can remember and I have looked at most of the Cassini images (very quickly for the vast majority of them of course). There are probably some images I have forgotten about though.
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stevesliva
post Feb 26 2015, 07:50 PM
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Why are images sometimes mangled this way? What factors lead to it?
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Malmer
post Feb 27 2015, 12:19 PM
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I would guess that it is a storage space issue. When saving 8 instead of 12bit images one can save 4 bits per pixel to do other things. You end up being able to shoot extra frames before the memory fills up. For a once in a mission encounter I guess they try to eek out as many observations as possible. Since it is an orbiting mission we only see this happening on a few occasions.

And if someone as crappy a programmer as me can get a reasonable reconstruction of the data it seems like a good trade off... smile.gif
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