Calibrating And Processing Pds Cassini Images Options

[mike]

I would say the second, I suppose because the left one appears to have a uniform yellowish-brown tint. Having never seen Saturn that close myself, however, I have absolutely no idea how it should look.

[ugordan]

The right one also looks more natural to me. Well, I better put it this way: It looks more like the ones released by the imaging team. The problem is, as I said that with that scheme all moons turn out looking blue. In fact, when looking at CICLOPS images of the planet and the moons being merely specks of light, they are in fact looking bluish as well. So I'm wondering if the imaging team is embellishing those Saturn images just a bit, to make them look more "oooh!".
For example, these are a few shots of Titan and Iapetus I put together using the normal method, which makes Saturn all yellowish.

The two shots of Titan were acquired on Dec 13, 2004 with the wide angle camera at a distance of about 220 000 km and were corrected for the changing distance between respective exposures. The one on the left is a RGB composite, the right one uses violet for the blue channel, enhancing the haze above the moon's limb.
The Iapetus shot is similar to the one the imaging team released, but is a RGB composite (the imaging team used IR/G/UV, IIRC). Color fringing is pretty apparent in this pic because the spacecraft changed the viewing geometry noticeably, especially the green channel. Had I corrected for the blue channel issue here, the fresh snow visible would have turned up blue...

[Bjorn Jonsson]

I've attached two sample images (they weren't taken at the same time, but were both taken through CB1/GRN/BL1 filters)...

Which of the two would you say looks more like true color?

The right one definitely looks more realistic but based on what I've seen through telescopes I'd say the color is too saturated in both cases.

BTW now is a good moment to 'announce' a new version of my IMG2PNG utility:

http://www.mmedia.is/bjj/utils/img2png/

Previous versions were announced/discussed in this thread:

http://www.unmannedspaceflight.com/index.php?showtopic=291

The major change in this new version is that it can calibrate Cassini images. It should be noted that this is not a user friendly utility, neither the 'installation process' nor the user interface. For example it runs in a command prompt window so if you are not comfortable with that don't use it ;-). I plan to release a version with an improved user interface later but it's not a high priority.

Also you will need to download lots of data from the COISS_0011 Cassini DVD volume to be able to calibrate Cassini images.

Any feedback is welcome.

PS I'm attaching an image showing the importance of flatfielding when working with low contrast images. The two images are processed in a similar way (including sharpening with an unsharp mask) with the main difference being that the left one was not flatfielded. This is a section of a BL1 image of Saturn.

Attached thumbnail(s)

 

[ugordan]

The right one definitely looks more realistic but based on what I've seen through telescopes I'd say the color is too saturated in both cases.

BTW now is a good moment to 'announce' a new version of my IMG2PNG utility:

http://www.mmedia.is/bjj/utils/img2png/

The saturation issue is probably due to the fact the image is too dark. I've kicked up the gamma in Photoshop to 1.33 and the result for the left image is looking better, IMO:


Regarding IMG2PNG, I'm curious, why do you need the detached LBL files as well? Isn't all information needed for calibration and regarding instrument states included in the IMG file as is?

[Bjorn Jonsson]

Regarding IMG2PNG, I'm curious, why do you need the detached LBL files as well? Isn't all information needed for calibration and regarding instrument states included in the IMG file as is?

I do not remember exactly what's in which file but the way IMG2PNG is currently written it needs the LBL files. It will look for an LBL file with the same name as the IMG file (except for LBL instead of IMG in the name) so this shouldn't make it more difficult to use, also the LBL files are very small.

[Bjorn Jonsson]

I've been experimenting with "deterministic" ways to produce true color images. I've also been playing around with procedures to calibrate the images, adding flatfielding to my rudimentary converter.
My problem is, using NAC RED/GRN/BL1 filters...

I have also been experimenting with calibrated color images using CB1/GRN/BL1 - I assume that by RED you mean the red filter and not CB1?

The color I get looks somewhat different from yours, possibly because I'm using CB1 and not RED. Some time ago when I was processing images of Jupiter I had to add 'fudge factors' to get realistic color (the main problem was the dark BL1 image). Interestingly, after applying these same fudge factors to a color image of Saturn the color becomes very similar to your left image.

[ugordan]

I have also been experimenting with calibrated color images using CB1/GRN/BL1 - I assume that by RED you mean the red filter and not CB1?

The color I get looks somewhat different from yours, possibly because I'm using CB1 and not RED. Some time ago when I was processing images of Jupiter I had to add 'fudge factors' to get realistic color (the main problem was the dark BL1 image). Interestingly, after applying these same fudge factors to a color image of Saturn the color becomes very similar to your left image.

You're right, the above image was taken through the CB1 filter. There aren't that many RED images taken by NAC as yo've noticed a while ago. I've found a case where both a RED and CB1 filtered images of Saturn (the same view) exist and compared them, they indeed turn out very similar, the CB1 being only slightly brighter and the "blue cranium" is the only place noticeably brighter in CB1 than in the RED.
As I said before, I have a feeling the imaging team enhances their true color images of Saturn to make them look more bluish. If you take a look at the small moons (Mimas and Enceladus) often visible in those shots, they're colored distinctly blue, while the shots of them alone are obviously predominantly whitish or grayish in Mimas' case.
Perhaps volcanopele could shed some light on the "true" color issue of Saturn?

[Bjorn Jonsson]

If you haven't done so you should take a look at this HST image of Saturn:

http://heritage.stsci.edu/1998/29/index.html

This is by far the most realistic spacecraft image of Saturn that I have seen, based on what Saturn looks like when seen visually through a telescope but even so this is not a simple problem. The problem is that even though I have seen Saturn through a fairly big telescope I'm not sure exactly what it would look like from a distance of a few million km. The color saturation increases with increasing telescope size with Saturn becoming more yellowish (instead of whitish) when using a big scope.

[tedstryk]

If you haven't done so you should take a look at this HST image of Saturn:

http://heritage.stsci.edu/1998/29/index.html

This is by far the most realistic spacecraft image of Saturn that I have seen, based on what Saturn looks like when seen visually through a telescope but even so this is not a simple problem. The problem is that even though I have seen Saturn through a fairly big telescope I'm not sure exactly what it would look like from a distance of a few million km. The color saturation increases with increasing telescope size with Saturn becoming more yellowish (instead of whitish) when using a big scope.

Not to mention that the eye can play tricks when looking at something like Saturn in the darm, surrounded by black sky. For example, there is the bright red Mars issue - it sure looks that way in a telescope, but it's color isn't nearly so red.

[ugordan]

I have a question somewhat related to this issue. I do my color compositing in Photoshop CS and, naturally, the inputs are 3 grayscale images. I usually switch the red filtered image into RGB mode and paste the other grayscale images into their respective channels. However, when changing image mode from grayscale to RGB, I notice that while the composite image stays the same, when I switch to individual channels the dark areas of the image get even darker, while the bright ones don't noticeably change. What I'm saying is that the color components of a grayscale image in RGB mode don't look exactly like the original grayscale.
It's like PS is clamping the darks or applying a nonlinear ramp. This produces weird results, especially with Saturn where the rings are inherently fainter than the planet itself. Note that I have color management switched off, though there is an app in my windows startup called Adobe gamma loader or something like that.
Has anyone else come across this?

[tfisher]

BTW now is a good moment to 'announce' a new version of my IMG2PNG utility:

I am trying to use IMG2PNG to process Titan images, with mixed success. Maybe you can give me some help.

First of all, I have been trying to get away without downloading the entire calibration DVD. Instead, I've only downloaded the files named when img2png pops up a dialog complaining it didn't find them. Maybe you can tell me already that I'm doomed with this strategy.

For some images, it doesn't seem to try to do calibration at all. For instance, I download this IMG file and the corresponding LBL file. But img2png -r applied to this produces a file with only 5 distinct shades of gray, and no complaints about missing calibration files. Adding -s10 or somesuch doesn't increase the number of shades.

On the other hand, for an image like IMG/LBL the program requests several calibration files, and after they are present produces an output with 71 distinct shades of gray and partial flatfield correction. (From a 12bpp imager, I would have hoped for more than 71 shades, but at least this result resembles the correct thing!)

Am I doing something wrong?

[Bjorn Jonsson]

Am I doing something wrong?

Probably not - I discovered that in a few cases IMG2PNG didn't complain if it didn't find a calibration file so you probably need to download a few additional calibration files.

There is a new IMG2PNG version available that should always complain when calibration files are missing:

http://www.mmedia.is/bjj/utils/img2png/img2png_exe.zip

This file only contains a new version of the executable so it is smaller than the 'full' download you already have.

Another problem is the fact that there really are very few shades of gray in the images you are using although not as few as you are getting. Here are calibrated PNG versions of these files:

http://www.mmedia.is/bjj/utils/img2png/N1477488603_2.png
http://www.mmedia.is/bjj/utils/img2png/N1477490884_2.png

Also it might be a good idea to try this IMG file and this corresponding LBL file. This is a BL1 image of Saturn. It will be obvious whether it gets flatfielded or not if you sharpen it using an unsharp mask after calibrating and compare it to the uncalibrated version.

Let me know if this helps.

[tfisher]

There is a new IMG2PNG version available that should always complain when calibration files are missing:

Thanks. Actually, my major problem turns out I was opening the images as 8-bit instead of 16-bit png files. (Using ImageJ, which supports 16-bit per chanel, but requires a plugin to load 16-bit png files...).

Here's a sample of what I'm playing with; here is the Huygens landing site with a FFT bandpass filter applied and contrast adjustment:

[djellison]

I find it a bit dissapointing that the flatfield/darkfield work etc hasnt already been applied to Cassini imagery as it has with MER imagery that gets on the PDS as RAD's

Doug

[Bjorn Jonsson]

This is probably because the calibration files may get updated, not only for data that hasn't been released but also for data that has already been released. Still it would be nice if the PDS had calibrated data that would get updated when the calibration files change. IMG2PNG doesn't completely accomplish what CISSCAL can do, for example there's no darkframe subtraction. This is not a big deal - the reason is that it is extremely complicated to do becuase it isn't done using actual darkframe files but by using synthetic darkframes constructed based on a model of the camera's behavior as a function of exposure, gain, compression etc. Also IMG2PNG doesn't remove the 2 Hz noise.