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_*]

It is true that Judd and Vos made improvements to the color matching functions. But I don't believe those changes are part of the XYZ color value talked about in televsion and sRGB standards. So I think you want to integrate against the CIE 1931 matching functions, not the Judd or Vos versions.

Why is there a white-point correction? Are you calculating an image of the reflectance function, or an image of what would be seen by an observer in space?

[ugordan]

Why is there a white-point correction? Are you calculating an image of the reflectance function, or an image of what would be seen by an observer in space?

As I was saying, using Young's matching code, uniform reflectance gives me converted RGB values of let's say 0.9,0.8,1.0 - somewhat bluish. I want that to be 1.0,1.0,1.0 so the surface would turn out white (this of course gamma-corrected and scaled to 255). Hence that XYZ scaling. I'm trying to calculate what would be seen by an observer in space. What's the difference to the other approach anyway?

The caveat with Young's code is I think in that it takes reflectance spectra of a material illuminated by Illuminant C. Hence, I have to divide the observed VIMS brightness spectra by solar spectra. This code then gives me the appearance of the object by this illumination. As I say, it's an approximation of daylight on Earth so an all-white material (such as Enceladus, effectively) will turn out bluish. That's where the Illuminant C -> D65 scaling comes in. It's not a magic fudge factor I made up, but found in the table slinted also referenced. This correction gives me 1.0,1.0,1.0 for white materials.
This worked well for the time being. I've been more focussed lately on trying to remove those pesky lines than do the color matching thing from scratch. In any case, a more "neat" matching code is on my todo list.