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Processing VIMS cubes, An attempt at "true" color
ugordan
post Sep 14 2006, 10:47 AM
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I am very well aware of the ISS data specifics. While it is mostly the case a LUT was used to convert linear (more or less, barring uneven bit weighting) 12 bit DNs to 8 bit, approximately square root encoded numbers, it is not always the case as you suggest. Sometimes full 12 bits were returned. The third case being the 12->8 bit conversion by returning only the lower 8 bits. This is also linear. In any case, my ISS code takes care of that.

However, I found absolutely no mention anywhere of lookup tables used in the VIMS instrument, in fact only mentions of lossless 12 bit encoders. The calibration code I checked doesn't use such conversions anywhere. It would be logical to save it via a square root LUT if they were downsampling the data, but all points to that not being the case.
The raw data is linear which can be seen from the calibration steps used in the provided code:

1. subtract dark background
2. divide by flatfield
3. divide by solar spectrum
4. multiply by detector performance as function of wavelength
5. convert into radiometrically correct units (optional)

Steps 1-4 should already yield accurate colors since I'm not interested in exact units.


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Guest_DonPMitchell_*
post Sep 14 2006, 03:39 PM
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QUOTE (Malmer @ Sep 14 2006, 02:30 AM) *
correct me if im wrong but: sRGB does not use a 2.2 gamma. it uses its own custom crve that is reasonably close to gamma 2.2...

[attachment=7432:attachment]

the red curve is gamma 2.2
the green is sRGB:s "gamma" curve
the blue is just linear.

The difference is mostly visible in the dark regions.

/M


I asked Gary Starkweather about this a few years ago, because the documentation talks about gamma of 2.2, and then also describes a piece-wise function. He told me the piecewise function was just introduced because there is some way to calculate it that is much faster than pow(x, 2.2). He claimed that gamma of 2.2 is what is intended. Just now, I found this page which seems to say the same thing.
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Malmer
post Sep 15 2006, 12:33 PM
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I think the strange piecewise funktion has its benefits. it gives slightly better coverage in the black regions... when handling 8bit images...
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Malmer
post Sep 15 2006, 12:50 PM
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QUOTE (ugordan @ Sep 14 2006, 12:47 PM) *
I am very well aware of the ISS data specifics. While it is mostly the case a LUT was used to convert linear (more or less, barring uneven bit weighting) 12 bit DNs to 8 bit, approximately square root encoded numbers, it is not always the case as you suggest. Sometimes full 12 bits were returned. The third case being the 12->8 bit conversion by returning only the lower 8 bits. This is also linear. In any case, my ISS code takes care of that.

However, I found absolutely no mention anywhere of lookup tables used in the VIMS instrument, in fact only mentions of lossless 12 bit encoders. The calibration code I checked doesn't use such conversions anywhere. It would be logical to save it via a square root LUT if they were downsampling the data, but all points to that not being the case.
The raw data is linear which can be seen from the calibration steps used in the provided code:

1. subtract dark background
2. divide by flatfield
3. divide by solar spectrum
4. multiply by detector performance as function of wavelength
5. convert into radiometrically correct units (optional)

Steps 1-4 should already yield accurate colors since I'm not interested in exact units.



I stand corrected.

I guess i should have been a bit more clear with the fact that I was speculating...

While im at it, here is another speculation that might or might not have scientific grounds;

One reason that could make the images look washed out in gamma 2.2 is that the solar intensity at saturn is only 1% of the intensity here on earth. So maybe the images on your screen are actually brighter than the real thing. And since the eye has a logatitmic response the colors wash out.

So maybe you should either calibrate the images to the actual intensity of your screen or just use an arbitrary gamma that looks good. I would go for the "looks good option"



Speculativly yours
/Mattias
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ugordan
post Sep 15 2006, 06:10 PM
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QUOTE (Malmer @ Sep 15 2006, 01:33 PM) *
I think the strange piecewise funktion has its benefits. it gives slightly better coverage in the black regions... when handling 8bit images...

If you're referring to the linear portion close to 0, it's supposedly for suppressing low DN noise that would otherwise be apparent. As for different curves for different channels, I don't know anything about that. I assume that would totally trash the colors on all but those displays that were precisely calibrated - not many of those surely.


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ugordan
post Sep 15 2006, 06:26 PM
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QUOTE (Malmer @ Sep 15 2006, 01:50 PM) *
One reason that could make the images look washed out in gamma 2.2 is that the solar intensity at saturn is only 1% of the intensity here on earth. So maybe the images on your screen are actually brighter than the real thing. And since the eye has a logatitmic response the colors wash out.

This could very well be true. But, since using absolute brightness here on Earth, it wouldn't be very useful as everything would turn out pretty dim on a computer screen. It would be interesting, though, to know how the apparent brightness would change once the eye got accustomed to low light levels at Saturn (if you were there, for example).
I guess we can settle the point on this whole matter being too subjective to quantify in a scientifically meaningful way. My primary goal was to get the "correct" hue of objects, principally Saturn's disc (more importantly the rings as I was wondering whether they're as colorful as ISS would lead to believe) and Titan, Iapetus and Hyperion. Pretty much anything there is gray anyway. Brightness was of lesser importance as that sort of thing is a hard beast to accurately "tame".


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Malmer
post Sep 15 2006, 11:40 PM
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it would be fun to paint spheres in the colors you have derived and put them in a dark room with a dim light and just wait and see... smile.gif

/M
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Guest_DonPMitchell_*
post Sep 16 2006, 01:56 AM
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One concept is to make an image that resembles what would be seen by a human being, there in space, looking through a window. As long as the colors are inside the gamut of your display, it is theoretically possible to achieve this:

1. Convert 12-bit VIMS values to linear radiance.
2. Correct for dark current
3. Apply gain flatfield correction
4. Apply spectral-sensitivity gain correction
5. Integrate against CIE observer functions to get XYZ
6. Convert XYZ to RGB
7. Apply global gain adjustment for desired image contrast.
8. Encode pixel channels, for example, iRed = int(pow(R, 1.0/2.2) * 255 + 0.5)

Gamma has a big effect on the color, so if 2.2 looks drastically wrong, I would double check the process. You don't want to do everything carefully and then fudge gamma in photoshop. I bet there is just a small problem somewhere, and if you find it, the image will come out beautifully.
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Malmer
post Sep 16 2006, 09:42 AM
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QUOTE (DonPMitchell @ Sep 16 2006, 03:56 AM) *
Gamma has a big effect on the color, so if 2.2 looks drastically wrong, I would double check the process. You don't want to do everything carefully and then fudge gamma in photoshop. I bet there is just a small problem somewhere, and if you find it, the image will come out beautifully.


sometimes when i do gamma correction i convert to HLS, apply gamma to the L channel and convert back to RGB. that way the colors dond desaturate as much... maybe its not the most colorimetrically correct process but it looks better sometimes...

/M
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ugordan
post Sep 16 2006, 03:39 PM
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A mosaic of Saturn's northern latitudes:


Appearance of Jupiter and Saturn at similar phase angles:


Two shots of the lit side of the rings:

The rings in the left image are clipped off, it's not Saturn's shadow. The narrow, white ring is the F ring. The dark area in the middle image is Saturn's shadow.

Several narrow slices of the unlit side mosaicked together:


The unlit side appears less dull-brownish and more bluish, possibly due to ice particles forward-scattering blue light stronger.


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john_s
post Sep 16 2006, 04:54 PM
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Just checked out your gallery, ugordan- your processed Cassini images are truly glorious. Great work!
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ugordan
post Sep 16 2006, 05:56 PM
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QUOTE (john_s @ Sep 16 2006, 05:54 PM) *
Just checked out your gallery, ugordan- your processed Cassini images are truly glorious. Great work!

Thanks! After comparing the results with the VIMS cubes, which ought to be more accurate, I may have to change the way I process Saturn (Titan too) composites a bit. The results I got so far seem consistently a bit on the greener side than the stuff that turns out here. A channel mix here and there should do the trick. wink.gif


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Guest_DonPMitchell_*
post Sep 16 2006, 06:47 PM
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QUOTE (Malmer @ Sep 16 2006, 02:42 AM) *
sometimes when i do gamma correction i convert to HLS, apply gamma to the L channel and convert back to RGB. that way the colors dond desaturate as much... maybe its not the most colorimetrically correct process but it looks better sometimes...

/M


That's a good idea. I'd use Lab coordinates, which I think separate "color" from "luminance" even better.
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Guest_DonPMitchell_*
post Sep 16 2006, 07:01 PM
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I also really like what you are doing. I hope you don't feel I am being discouraging. I am poking at the gamma issue, because I think you might have a bug someplace. Track it down, and then you will have rigorous "true color".
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Malmer
post Sep 16 2006, 09:02 PM
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QUOTE (DonPMitchell @ Sep 16 2006, 08:47 PM) *
That's a good idea. I'd use Lab coordinates, which I think separate "color" from "luminance" even better.



Yes thats obviously much better but im often a bit too lazy... Guess I have to shape up. In the light of what you and ugordan have done with cassini and venera i feel that i really have to start from scratch with the stuff i have done.

I think that these pictures deserve the very best in processing that is humanly possible. I believe that it is important to make pictures that are true to reality. If its looks "dull" it should stay that way. These pictures are humanitys only way of experiencing these places and i dont think they should be enhanced or distorted to make them look more exotic.

keep it real!

/M
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