Processing VIMS cubes, An attempt at "true" color |
Processing VIMS cubes, An attempt at "true" color |
Sep 10 2006, 07:51 PM
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#1
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Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
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. -------------------- |
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Guest_DonPMitchell_* |
Sep 14 2006, 04:07 AM
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#2
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Guests |
Very good. Gamma 2.2 is technically what should be done, to get the image we see on the monitor to be proportional to actual scene radiance. What is the camera's radometric response function like? Are the pixel values in the VIMS file logarithmic?
As for dark current, making the background be 0 black should be right. But check the camera response. Subtracting background before or after linearization of the pixel values gives a totally different result if the camera response is nonlinear (and it probably is). I'm not sure about the Illuminant C business. I'm not sure I understand what you did. The sRGB matrix assumes a white point of D65, which all new monitors are supposed to do. Don't think about correcting the color on the camera end, just concentrate on linearizing its response. Just plug those spectral radiance values into the XYZ integrals, convert to RGB by matrix multiple, and do gamma of 2.2. And you should see what the camera saw then! |
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Sep 14 2006, 07:09 AM
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#3
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Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
The pixels are linear as far as I know, a 12 bit A/D converter is used. I think a precommanded bias value might be set to get the values out of the converter's low nonlinearity portion, or it might simply be a matter of subtracting dark current. Either way, I used the background algorithm found in the calibration package and then subtracted it.
As for Illuminant C, as I said the integration code I used produces a bluish white color if I pass it a uniform input reflectance spectra. When I used the regular CIE XYZ color matching functions, the colors turned weird. That'll need looking into. At the moment, using the supplied 40-step integration tables and using scaling factors found here, for Illum. C -> D65 got me an exact white for the test input. This can be double-checked with Mimas, Dione and Enceladus which are all very gray and sort of provide "ground truth" for the white-point. Regarding gamma, if I set it to 2.2, the colors are all washed up and all contrast is lost. Somehow that doesn't seem right? Even with a 1.25 gamma Hyperion's reddish tint is pretty much indescernible. -------------------- |
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