Processing VIMS cubes, An attempt at "true" color Options

[ugordan]

Right, a suggestion I made here in another topic made me wonder why not try that myself. A bunch of data was sitting on the PDS, after all. After a hassle figuring out just how the image cubes are organized and trying to read them, finally I was able to produce some results. This is all very rough work, can be considered first-iteration only and not particularly accurate.
Basically, I used the cubes to extract the visible spectrum in the 380-780 nanometer range which was then input to color matching code I found here by Andrew T. Young.
The code integrates over 40 10-nm steps to produce CIE XYZ color components. I then converted these to RGB values.

I'm aware of at least three inaccuracies in my code as of yet: one is the above sampled code apparently uses Illuminant C as the light source, not true solar spectra so the color turns out bluish (has a temp. of 9300 K instead of 6500 K, AFAIK). I tried to compensate at the moment by changing the final RGB white balance, but this is probably an inaccurate way to go. Another inaccuracy is I don't do bias removal from the cubes. This likely affects the outcome. Also, I don't use the precise wavelengths the code requires, but use the closest one in the cube. I intend to fix this by interpolating between nearest wavelengths.

All images are enlarged 4x.


The leftmost image is a 4-cube mosaic. The colors in all four frames turned out identical which gives me at least some confidence. The image in the middle shows Dione's disc creeping in front of Saturn. Dione's disc appears elongated probably because as the lines were readout, it moved considerably in its orbit. The rightmost image shows a very overexposed Saturn image, the part below the ring shadows got overexposed. From what I've seen browsing through the PDS, a lot of the cubes are badly overexposed at some wavelengths.

Here's a couple of Jupiter images. I'm not very satisfied with them as they seem to look somewhat greenish, but overall the color looks believeable:



Lastly, two Titan composites. They turned out way more reddish than I thought they would.


It'll be interesting to see how much the results will change once I do a more proper processing pipeline working.

[Guest_DonPMitchell_*]

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.

[Malmer]

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