Saturn's atmosphere needs updating

[granthutchison]

At the level of radial info only, what's wrong with the Cassini albedo

Well, it's a mix of albedo and transparency, isn't it? In part due to reflected light, and in part due to the darkness of space beyond the rings. So if I add that to my transparency map, I'll darken the transparent areas excessively.
I originally thought I might need to process a photograph like yours by brightening each area according to the degree of transparency associated with that region of the rings in the dataset - effectively removing the space-darkness from the image. But I find that a flat albedo map pretty much reproduces the photographic appearances once transparency is added. It would appear that most of the brightness variation is due to transparency, not reflectivity (with the exception of the well-known difference between the bright A & B rings and the darker, bluer material of the C ring and the Cassini ringlets).

Grant

(Sorry for the late reply ... I seem to have missed recent posts on this thread.)

[Evil Dr Ganymede]

The subtle purple "atmosphere light-scattering" effect is also interesting.

If you're talking about the vaguely magenta outline around the disk below the rings, that might actually be due to a slightly misaligned image in one of the filters used to make the colour image.

[Toti]

Well, the effect is also visible in the december 5 image. Take a look at:
http://saturn.jpl.nasa.gov/news/press-releases-03/20031205-pr-a.cfm

It is also featured -with this exact color- in StarStrider. See http://www.starstrider.com/

Still, I am not really sure about this effect's nature (Note the quotation marks in my original post).

Bye

[JackHiggins]

Could it be the same type of thing as the blue "haze" in HST images of Mars from last august...?

[Guest]

Well, the effect is also visible in the december 5 image. Take a look at:
http://saturn.jpl.nasa.gov/news/press-releases-03/20031205-pr-a.cfm

It's also visible in Celestia, though right now the haze effects only work on GeForce cards.

--Chris

[Toti]

Thank you, Chris.
Well, I have a Riva TNT2 card and therefore can't see it, but, it is very apparent in the screen captures of this thread: http://www.shatters.net/forum/viewtopic.php?t=4539&sid=badb61d89fdac2842c6184295a841650

Bye

[jim]

Hi all,

Since Grant send me a very nice alpha channel texture for the Saturn rings I've played a bit with it and the Saturn texture. For the shots I change further following settings for Saturn in solarsys.ssc.

HazeColor [ 0.6 0.6 0.9 ]
HazeDensity 0.4
...
# Color [ 1.0 0.88 0.82 ] (ring color removed)

Here is the result in Celestia 1.3.2pre5:



The image in the centre is the Cassini shot from 9. February but a little bit color corrected by me .

Now this little test shall show what can be further improved at the texture or in Celestia.
Here is what I found:
1. The ring shadow on Saturn is not dark enough
2. The A ring is to faint.
3. Despite a magnitude limit of 0.93 most of Saturn's moons are still visible from a distance of 70 million km.
4. Carefully looked you can see that Saturn's shadow isn't correct. I hope Chris new shadow code does a better job.

Nevertheless I hope you like this shot.

Bye Jens

[granthutchison]

I think we have to bear in mind that the Cassini image is contrast- and colour-enhanced, so we can't make any final judgement on things like the faintness of the A-ring. Me, I'd rather stick with the measured transparency from stellar occultation than try to tweak the model right now to match a manipulated photo.
The shadows are definitely too pale, but there was no way to make sensible adjustments until we had a good transparency channel - that's now in place for the next prerelease, so I'm hoping that Chris will also make a final tweak to the ring shadows, too. But even in its densest region the B ring will forward-scatter some light on to the planet, so the absolute blackness of the shadows in the Cassini image is again misleading.

For those who can't wait until the next prerelease, my own version of the rings is available at 1k from the CVS tree:
http://cvs.sourceforge.net/viewcvs.py/celestia/celestia/textures/lores/saturn-rings.png
and at 4k from Selden's site:
http://www.lns.cornell.edu/~seb/celestia/hutchison/saturn-rings.html
Both of them require the following Rings definition for Saturn in solarsys.ssc:

Code: Select all

   Rings {
      Inner   74500  # includes some ringlets inside edge of C ring at 74660
      Outer  140220
      Texture "saturn-rings.png"
   }

(Notice that the Color statement has been deleted, so that the colour of the ring texture itself can be displayed.)

Grant

[jim]

I think we have to bear in mind that the Cassini image is contrast- and colour-enhanced, so we can't make any final judgement on things like the faintness of the A-ring. Me, I'd rather stick with the measured transparency from stellar occultation than try to tweak the model right now to match a manipulated photo.

Hi Grant, this was only a fist test by me. My textures are only optimized for this Cassini picture. We need more and of course unmanipulated pictures from Cassini before we can say more about Saturn and it's rings. That's the reson while I have not up uploaded my textures.

Bye Jens

[granthutchison]

Oh, and a link to an image of my 4k rings, courtesy of Selden:
http://www.lns.cornell.edu/~seb/celestia/hutchison/saturn-4k-rings.jpg

Grant

[granthutchison]

Hi Grant, this was only a fist test by me. My textures are only optimized for this Cassini picture. We need more and of course unmanipulated pictures from Cassini before we can say more about Saturn and it's rings.

Sure, Jens. But as the person responsible for the new Celestia ring texture, I was just chiming in to explain why the Celestia rings haven't been tuned to reproduce this one Cassini image. What I've done at present is use very solid transparency data, coupled with good (but rather general) albedo information, and a mild set of tints to suggest the rather stronger shades that appear in the processed Cassini image. The link above lets folk see what they're likely to get in the next prerelease (the image uses my 4k texture, but the 1k won't look much difference at that sort of distance).

Grant

[Darkmiss]

Jim will you be releasing your ring texture too ?
and maybe your updated saturn as well ?

[TERRIER]

Hello Grant and Jim
I really appreciate your efforts concerning Saturns rings and planet texture.
Can I be 'a bit of a pain', and ask you to post some screenshots showing a comparison of your latest rings, as opposed to the Celestia default and Jens current rings ?
Just to help highlight the differences.

regards
TERRIER

[jim]

Jim will you be releasing your ring texture too ?
and maybe your updated saturn as well ?

Ok I will upload my Saturn texture. It's only a modified one of Bj?rn J?nsson great texture. But my ring texture... I think Grant's rings are very nice and it makes no sence to upload my current rings too. .

TERRIER, here is the wanted comparison.



Grant, personally I think your F ring is really to thin.

Bye Jens

[granthutchison]

Grant, personally I think your F ring is really to thin.

I think you're judging from long-duration exposures that show up the faint detail of the ring, like this:

But the stellar occultation data I used to build my alpha channel show that much of the F-ring is highly transparent, with the exception of that dense "core", ~10km wide, more or less at the outer edge of the ring in the right-hand image.
Try sailing up close to the F-ring and Pandora in my 4k texture and comparing it with this view:

(For comparison, Pandora's long axis is 114km.) You'll see that Pandora is quite overexposed, but there's still really nothing of the F-ring visible except for the very narrow core. It actually looks about the right width in my 4k texture, whereas just one pixel in the 1k makes it too broad.
But if you zoom in on the 4k texture you'll also see that all the structure present in the upper right photo is visible in the texture - it's just appropriately faint.

(You'll also notice the F-ring is a little closer to Pandora in the image than in my texture - that's because the ring is eccentric in reality, but in Celestia I've had to portray it as a circle placed at its mean distance from Saturn.)

Grant

[chris]

I've made a few adjustments which improve the appearance of Saturn:

- Accounted for the oblateness when rendering shadows on rings--for Saturn, this has a notable impact on the positioning of the shadow.

- Reduced the brightness of moons. The limiting magnitude for planets was being automatically adjusted based on the field of view regardless of whether or not automag was enabled. Now Enceladus is much dimmer (remember that in the image from Cassini, Enceladus was brightened so that it would show up more clearly.)

- Made the shadows cast by rings much dark.

In this image, I'm using Grant's new 1K ring texture.



Things that could be improved:
- The blue/purple haze should be more blue, less purple
- In the Cassini image, the dark hemisphere of Saturn is lit faintly by ringshine. You can get approximately the same effect by adding a bit of ambient light, but I think that it should be simulated automatically.
- There should be a slight bluing near the terminator.
- The placement of the blue haze isn't quite right--in the Cassini image, it seems to be concentrated in the northern latitudes.

--Chris

[t00fri]

I've made a few adjustments which improve the appearance of Saturn:

- Accounted for the oblateness when rendering shadows on rings--for Saturn, this has a notable impact on the positioning of the shadow.

- Reduced the brightness of moons. The limiting magnitude for planets was being automatically adjusted based on the field of view regardless of whether or not automag was enabled. Now Enceladus is much dimmer (remember that in the image from Cassini, Enceladus was brightened so that it would show up more clearly.)

- Made the shadows cast by rings much dark.

In this image, I'm using Grant's new 1K ring texture.

Things that could be improved:
- The blue/purple haze should be more blue, less purple
- In the Cassini image, the dark hemisphere of Saturn is lit faintly by ringshine. You can get approximately the same effect by adding a bit of ambient light, but I think that it should be simulated automatically.
- There should be a slight bluing near the terminator.
- The placement of the blue haze isn't quite right--in the Cassini image, it seems to be concentrated in the northern latitudes.

--Chris

looks like real progress!

--Some days ago I have scanned specialized photos (from the 80's), concerning the location of the blue haze. There is indeed relatively little to be seen in the southern latitudes unlike your image above. Most of it is in the northern hemisphere.

-- Another area for improvement concerns the placement of the ring designations from the ring_locs.ssc file. There are many configurations where there is almost total overlap of the labels!


Bye Fridger

[Commander David]

Hi Chris,

the screenshot looks fantastically, the only thing which tunes me sadly is, that I would get not to see this probably so soon, because I have an ATI 9500 and the Haze effect is not represented But, please further, the other visual effects are working.

Thanks for this great programm.

[TERRIER]

In this image, I'm using Grant's new 1K ring texture.

That's a screenshot ?

Things that could be improved:
- The blue/purple haze should be more blue, less purple
- In the Cassini image, the dark hemisphere of Saturn is lit faintly by ringshine. You can get approximately the same effect by adding a bit of ambient light, but I think that it should be simulated automatically.
- There should be a slight bluing near the terminator.
- The placement of the blue haze isn't quite right--in the Cassini image, it seems to be concentrated in the northern latitudes.

--Chris

OK then, I believe you.

Gotta say, Saturn is starting to look much better, I'm looking forward to seeing all the changes in Celestia....
....and just to think there is soon going to be Cassini's close-up data, which should help answer some more questions about the planet.

Cheers
TERRIER

PS
Is the colour in the rings a result of the reflection of the planets 'surface' ?

[granthutchison]

Is the colour in the rings a result of the reflection of the planets 'surface' ?

More likely intrinsic - the brightness of reflected sunlight will drown out any illumination from the planet itself. But I just don't buy the varied pastel shades in the colour-enhanced Cassini image ... it looks like a paint sampler from an interior decorator!
We should, of course, have more images soon, and I've no doubt there's a lot of further tuning to do with regard to the ring colours.

Grant