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The Martian Sky
DDAVIS
post Oct 16 2009, 11:01 PM
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So would I, but the mirror should be easier to arrange in the competition for room etc.
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djellison
post Oct 17 2009, 08:41 AM
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It would also be a lot lighter. Beagle 2 used a wide angle mirror that would pop up infront of its camera to get a complete 360 very quickly and dirtily.
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Ant103
post Oct 17 2009, 11:32 AM
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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).
Attached thumbnail(s)
Attached Image
Attached Image
Attached Image
 


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Bjorn Jonsson
post Oct 19 2009, 12:58 AM
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QUOTE (Ant103 @ Oct 17 2009, 11:32 AM) *
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:
Attached Image


Similar viewing geometry but looking in roughly the opposite direction:
Attached Image


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).
Attached Image

Attached Image


Flying above the surface, altitude 7 km (upper one) and 607 km (lower one). FOV 40 degrees:
Attached Image

Attached Image


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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vikingmars
post Oct 20 2009, 07:47 AM
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Bjorn, your test images and simulations are really interesting smile.gif
Here are 2 color samples to show the color of the Martian sky (close to the horizon + at zenith) that is considered to be the closest to the "moderate yellowish brown" (although it's not looking so "yellowish brown" close to the horizon) as seen by the VL landers and confirmed later by MPF and MER. It's the average color, useful for computing, that is NOT taking into consideration the dust opacity variations over the Martian seasons. Enjoy cool.gif
Attached Image Attached Image
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mwolff
post Oct 20 2009, 04:53 PM
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QUOTE (Bjorn Jonsson @ Oct 18 2009, 07:58 PM) *
[...snip...]


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).

[...snip...]


I would be interested in whether realistic forward scattering properties (i.e., derived from CRISM EPF sequences) would help. In the tabulation below, you can directly apply the single scattering albedo (Csca/Cext) and the wavelength dependence of the extinction (normalize Cext to your favorite wavelength, say 0.88 or 0.90 microns to scale MER Pancam optical depths). The model below assumes one is interested in an effective particle size (r_{eff}) of 1.5 microns (v_{eff} = 0.3).

wave = wavelength in micrometers
Cext = extinction cross section (arbitrary units)
Csca = scattering cross section (same units as Cext)
kappa = ignore
g = asymmetry parameter

wave Cext Csca kappa g
0.4400 4.4040e+00 3.4910e+00 1.8460e+16 7.6870e-01
0.4600 4.4230e+00 3.5760e+00 1.8540e+16 7.6200e-01
0.4800 4.4360e+00 3.6780e+00 1.8600e+16 7.4980e-01
0.5000 4.4660e+00 3.7980e+00 1.8720e+16 7.4040e-01
0.5250 4.4680e+00 3.9270e+00 1.8730e+16 7.2790e-01
0.5500 4.5060e+00 4.0800e+00 1.8890e+16 7.1330e-01
0.5750 4.5150e+00 4.2010e+00 1.8930e+16 7.0650e-01
0.6000 4.5420e+00 4.3260e+00 1.9040e+16 6.9290e-01
0.6300 4.5830e+00 4.4250e+00 1.9210e+16 6.8760e-01
0.7000 4.6190e+00 4.5100e+00 1.9360e+16 6.8060e-01
0.7500 4.6630e+00 4.5580e+00 1.9540e+16 6.7820e-01
0.8000 4.7110e+00 4.6030e+00 1.9750e+16 6.7700e-01
0.8500 4.7480e+00 4.6350e+00 1.9900e+16 6.7730e-01
0.9000 4.7700e+00 4.6510e+00 1.9990e+16 6.7870e-01
0.9500 4.7850e+00 4.6640e+00 2.0060e+16 6.7990e-01
1.0000 4.8350e+00 4.6940e+00 2.0270e+16 6.7920e-01
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tolis
post Oct 21 2009, 01:09 PM
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One question that is relevant to the appearance of the Martian sky is whether one would expect to see stars during the daytime.
My impression is that the answer to this question would be heavily dependent on the amount of dust in the atmosphere.
Perhaps the more technically-oriented members of this forum can help..

Regards to All,

Tolis.
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vikingmars
post Oct 21 2009, 01:37 PM
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QUOTE (tolis @ Oct 21 2009, 03:09 PM) *
One question that is relevant to the appearance of the Martian sky is whether one would expect to see stars during the daytime. (...) Tolis.


Unfortunately, NO stars are visible during daytime : this fact was confirmed after the VL1 landing in 1976, because the dust makes the sky too bright for stars to be visible even at zenith.
Maybe the only place where the brightest stars (and planets) could be visible from the ground on Mars AND only when dust opacity is at its lowest, is at the summit of the highest volcanoes... But to see them you would have to concentrate your vision a bit, avoiding ambiant sunlight, staying in the shadow of a big rock for example, as the Apollo astronauts did on the Moon by looking at the brightest stars from the shadow of the LM... cool.gif
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Bjorn Jonsson
post Oct 21 2009, 05:39 PM
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QUOTE (mwolff @ Oct 20 2009, 04:53 PM) *
I would be interested in whether realistic forward scattering properties (i.e., derived from CRISM EPF sequences) would help. In the tabulation below, you can directly apply the single scattering albedo (Csca/Cext) and the wavelength dependence of the extinction (normalize Cext to your favorite wavelength, say 0.88 or 0.90 microns to scale MER Pancam optical depths). The model below assumes one is interested in an effective particle size (r_{eff}) of 1.5 microns (v_{eff} = 0.3).

[Snip]

Thanks - I'll try this and see what happens. Might take some time though.

Meanwhile, I've been having a look at some more images. This one is particularly interesting because of the caption:

http://photojournal.jpl.nasa.gov/catalog/PIA00917

The image caption says:
QUOTE
"The dust in the atmosphere absorbs blue light, giving the sky its red color, but it also scatters some of the blue light into the area just around the Sun because of its size. The blue color only becomes apparent near sunrise and sunset, when the light has to pass through the largest amount of dust."


This is interesting because I was not sure the bluish color near the sun when it is high in the sky in my images is correct. However, according to the Tomasko et al. phase function much more blue light than red gets scattered at low scattering angles and I have been having problems 'suppressing' the bluish color with the sun high in the sky without losing that color at sunset too (I might be able to fix this using data from mwolff's table though). Does anyone know of true color images showing the Martian sky relatively near the sun and with the sun high in the sky? I haven't found any that I'm happy with.

QUOTE (vikingmars @ Oct 21 2009, 01:37 PM) *
[Snip]
Maybe the only place where the brightest stars (and planets) could be visible from the ground on Mars AND only when dust opacity is at its lowest, is at the summit of the highest volcanoes... But to see them you would have to concentrate your vision a bit, avoiding ambiant sunlight, staying in the shadow of a big rock for example, as the Apollo astronauts did on the Moon by looking at the brightest stars from the shadow of the LM... cool.gif

And the 'star' easiest to see might be the Earth...
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Bjorn Jonsson
post Dec 1 2009, 12:33 AM
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I have now made significant improvements to my rendering software. It's about 6 times faster now, meaning I can do nice animations without keeping my computer running for something like 100 hours (it's an old one BTW). More importantly, I'm getting results that I consider much better. I'm using the parameters posted by mwolff in an earlier message and cannot completely rule out bugs but using these parameters definitely resulted in a big improvement. I had to modify the phase function to reduce the dynamic range though - I'm using somewhat lower values for g because of this. Also I'm using an improved Henye-Greenstein function devised by Cornette but the resulting difference is in this case negligible compared to a normal HG function.

Now some test renders. In these two the sun is high in the sky. The difference is that in the left one I reduced the dynamic range as described above but in the right one I'm using the g values posted by mwolff (and a regular HG function and not Cornette's version). I'm posting the right one solely to show the problems with dynamic range; all of the other renders I'm posting are rendered using the same parameters as in the left one. These renders have a field of view (FOV) of 90.
Attached Image
Attached Image


In the renders above the ground is brighter than the sky. As the sun gets lower in the sky the sky/ground brightness ratio increases when looking towards the sun. Actually I still haven't made any attempts to get the sky/ground brightness ratio correct - it's clear I need to take a careful look at the MER images to do this. The FOV is 90.
Attached Image


The sky is much darker when looking away from the sun. In this one the sun is equally high in the sky as in the first two renders but the sky is much darker since we are looking away from the sun. The FOV is 90.
Attached Image


I think I'm getting more realistic sunsets now, compared to the MER and MPF images they not perfect but I think they are fairly realistic. In the following one the sun is visible through the dust (actually it should be brighter). The FOV is 90.

Attached Image


Here's another version of the sunset image, this time with a FOV of 45, making it roughly comparable to some of the MER sunset mosaics:
Attached Image


I don't know how realistic this one is, the altitude is 10 km and the FOV 45. I enhanced the brightness a bit so unlike all of the other renders here it has been post-processed:
Attached Image


The limb is darker now as seen from space than it used to be but probably still too bright, also it's too uniform (there should probably be visible haze layers). The altitude is 607 km and the FOV 35:
Attached Image


And Mars as seen from space. The texture map I'm using isn't of high quality and the color is a bit strange but this is intended to show the atmosphere near the limb. As in the previous render I think it's too bright:
Attached Image


An interesting result is that the sky is bluish near the sun even when the sun is high in the sky (the color is less pronounced than near sunset though). Looking at the parameters posted by mwolff it seems obvious why: The g parameter is higher at short wavelengths. This actually may be consistent with the image caption I mentioned in my above message ("The blue color only becomes apparent near sunrise and sunset") - the color is less obvious with the sun high in the sky but it still is there. So I'm now leaning towards thinking the sky really has a subtle bluish color near the sun when it is high in the sky. However, I still haven't searched the MER images to see if there are any images that might show this effect.

Finally I'll end this big message with an experimental Martian sunset animation. It has a FOV of 70. In this animation I darkened the sky a bit compared to the renders above.
Attached File  martian_sunset_test.avi ( 380K ) Number of downloads: 282


Any feedback appreciated. These test renders have improved greatly since I started this thread, largely thanks to suggestions and feedback received here.
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vikingmars
post Dec 1 2009, 07:11 PM
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Really interesting Bjorn ! My 1st impression is that it looks like a good a render close to the horizon with the Sun having its bluish halo, but close to zenith there should be no blue halo visible around the Sun and the sky should be darker and less gray (i.e. a little more dark pink). Anyway, what a great & nice work you did. Congrats ! smile.gif
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Bjorn Jonsson
post Dec 1 2009, 11:49 PM
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Which reminds me - there really should be a test render showing the sun at the zenith. Here it is:
Attached Image


It has a FOV of 90.

Needless to say I now have become really interested in seeing some kind of a 'sky camera' (as discussed briefly earlier in the thread) on a future lander mission smile.gif.
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djellison
post Dec 2 2009, 08:15 AM
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Don't encourage him Bjorn wink.gif
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DDAVIS
post Dec 2 2009, 08:04 PM
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I would like to encourage a small hemispheric mirror be placed on the deck of a future lander/rover to allow a modest fisheye view of the sky to be had by a camera looking down on it.
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James Sorenson
post Dec 2 2009, 09:17 PM
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I have imagined a modified MER hazcam camera, but with 180 deg FOV optics, and with a color CCD chip which is mounted on the top of the camera mast of a lander/rover. That way, it is not limited to just pointing up smile.gif.
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