The Martian Sky |
The Martian Sky |
Oct 11 2009, 06:41 PM
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IMG to PNG GOD Group: Moderator Posts: 2254 Joined: 19-February 04 From: Near fire and ice Member No.: 38 |
I have been attempting to make computer generated images of the Martian atmosphere, both as seen from the surface and from space. To check the results I have been looking for spacecraft images to use as ground truth. I have found lots of images - by far the best ones I have found are from UMSF in this thread: http://www.unmannedspaceflight.com/index.php?showtopic=3324
However, I'm always looking for more ;-). So if anyone knows of more and/or better images I'm interested in them. What would be best are mosaics showing the sky from the horizon (with the horizon/surface visible) and towards the zenith. The sky varies a lot because of variable amount of dust but the general impression I get is that the sky is bright near the horizon (usually brighter than the surface) but gets much darker higher in the sky. There is probably a fairly large, bright area in the sky near the sun, possibly less reddish (lower R/B ratio) than parts of the sky farther from the sun. I'm already getting fairly interesting results, this one has a field of view of 90 degrees: (needless to say this one is 'overexposed' near the horizon; dynamic range is sometimes a problem) The problem is that even though this may not be bad the limb currently appears far too bright as seen from space : This shows that my atmospheric model is erroneous in some way - I suspect that as seen from the surface the Martian sky is darker high above the horizon than I have been assuming. |
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Oct 17 2009, 11:32 AM
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Senior Member Group: Members Posts: 1619 Joined: 12-February 06 From: Bergerac - FR Member No.: 678 |
Here is the settings of the atmosphere in TG2, sets by default to have a classical blue sky. Maybe you can give me some advise to have a martian sky.
And the settings to hava "martian sky", with the result rendering (notice that the fov is 104° with a sun at 45° high in the sky). -------------------- |
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Oct 19 2009, 12:58 AM
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#3
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IMG to PNG GOD Group: Moderator Posts: 2254 Joined: 19-February 04 From: Near fire and ice Member No.: 38 |
Here is the settings of the atmosphere in TG2, sets by default to have a classical blue sky. Maybe you can give me some advise to have a martian sky I assume "exp height" is the scale height. If that is the case it is incorrect - it should be ~11 km for both haze (dust) and bluesky. Thanks to everyone for useful information - mwolff's message was especially helpful. As I was starting to suspect, using a Henyey-Greenstein (or similar) phase function didn't work well (before seeing mwolff's message I was even considering using the sum of two or more HG-functions). I was able to get nice sunset images but in that case images with the sun high in the sky looked bad or vice versa. So I replaced it with the Tomasko et al. functions (444 nm and 671 nm) mentioned by mwolff. Actually I simply measured the function values from a graph in the Tomasko et al. paper and construced a lookup table. The function isn't perfectly smooth yet - this shows up in some of the images where the sun is visible. Dynamic range is a big problem so I processed the function in Excel to greatly reduce its dynamic range while preserving the color ratio (R/B) as a function of scattering angle. The results are very promising. I still need to do some tweaks, for example I probably need to increase the optical depth (the normal optical depth is currently ~0.18) and I'm currently assuming it doesn't vary with wavelength which I'm not sure is correct. Some test images: The sun as seen from the surface, the field of view (FOV) is 90 degrees. Interestingly, the sky near the sun has a bright bluish color. I don't know how accurate this is: Similar viewing geometry but looking in roughly the opposite direction: Sunset, the upper one has a FOV of 90 degrees and the lower one a FOV of 30 degrees. From these images I suspect I need to change some parameters because if I slightly increase the solar elevation a large area around the sun gets completely saturated. Because this didn't happen with the sun high in the sky (see above) I suspect the amount of inscattering is too high relative to absorption. From a comparison with actual images from the surface I suspect the sky is also too bright near the sun here which also implies I need to increase the optical depth (i.e. absorption). Flying above the surface, altitude 7 km (upper one) and 607 km (lower one). FOV 40 degrees: As previously, these are rather crude test images without antialiasing. Also to speed things up I'm using a low resolution texture map and no DEM. I might eventually make my renderer available one day - this has been a very helpful discussion. However, a lot of work remains before that's even possible. |
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