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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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 21 2006, 07:03 PM
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There's a brown Moon for you, taken by Galileo. I doubt if the image was created with any fancy colorimetric processing, but it is amusing. Color perception is effected by viewing conditions. Among other things, your brain does a sort of automatic white-point compensation -- adaptation. Everyone has noticed how bad color photographs look when they are taken with indoor lighting, but to our eye, the lighting never seemed as bad. For looking at dyes and fabrics or doing graphics arts, people buy expensive precision lighting, typically D50 lamps that meet ISO standards. Monitors generate their own light, so its not as critical to have D50 lamps, except for the secondary effect of your visual system's color adaptation. Here is a nice article I found by an artist, about how he set up his workspace: Kevin Mills Photography. However, sRGB is well defined. If you are synthesizing an image or if you have spectral sensor data, then there is one unique standard thing to do, calculate 24-bit sRGB. All the messy stuff happens when people view it, but that is someone else's problem. |
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