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It??s unbeliveable,but Saturn is not transparent in visual wavelenghts,because almost all of Cassini??s images of Saturn atmospheres are in ultraviolet or in methane band.So I think we have to adopt a haze in Saturn??s definition,to be more realistic(at least until Celestia don??t support other wavelenghts).Is there any haze texture available?

danielj wrote:It??s unbeliveable,but Saturn is not transparent in visual wavelenghts,...

Can you read the original text by NASA once more carefully:

NASA wrote:Saturn's atmosphere is essentially transparent at wavelengths visible to the human eye, but when the view through the atmosphere is oblique, as it is along the planet's limb (edge),

http://www.nasa.gov/mission_pages/cassini/multimedia/pia07521.html

Bye Fridger

I have a very silly question: Where does the 'atmosphere' end and the 'planet' start (in case of a gas giant)?

danielj wrote:It??s unbeliveable,but Saturn is not transparent in visual wavelenghts,because almost all of Cassini??s images of Saturn atmospheres are in ultraviolet or in methane band.So I think we have to adopt a haze in Saturn??s definition,to be more realistic(at least until Celestia don??t support other wavelenghts).Is there any haze texture available?

Oi danielj,

prepare to be embarrassed : the Saturn SSC already has a haze definition:



It's what makes Saturn appear to have a blue limb to match Cassini's global colour photos of Saturn. Check your Saturn SSC for this (and ask yourself why you didn't think of doing that before posting).

We don't (yet) have haze textures in Celestia.

julesstoop wrote:I have a very silly question: Where does the 'atmosphere' end and the 'planet' start (in case of a gas giant)?

Not a silly question, cos it's not easy to answer .

My understanding is that below the cloud tops, the atmosphere goes down for 1,000's of km, getting denser (and hotter) until it becomes so dense that at some depth the gas condenses into a liquid, and that would be the 'surface' of a global ocean, itself 10,000's of km deep, until finally you reach a solid core of something rocky/metallic.

It's probably pitch black at this liquid surface.

Spiff.

Spaceman Spiff wrote:

danielj wrote:It??s unbeliveable,but Saturn is not transparent in visual wavelenghts,because almost all of Cassini??s images of Saturn atmospheres are in ultraviolet or in methane band.So I think we have to adopt a haze in Saturn??s definition,to be more realistic(at least until Celestia don??t support other wavelenghts).Is there any haze texture available?

Oi danielj,

prepare to be embarrassed : the Saturn SSC already has a haze definition:

Code: Select all

  HazeColor [ 0.0 0.0 1 ]
  HazeDensity 0.25
...



It's what makes Saturn appear to have a blue limb to match Cassini's global colour photos of Saturn. Check your Saturn SSC for this (and ask yourself why you didn't think of doing that before posting).


Spiff.

Haze is NOT functioning in the more advanced rendering paths like OpenGL 2.0 ....So you can write whatever you like in the SSc file.

Since a long time, Chris has a complete rewrite of atmospheres on his agenda, but ....the climbing season has started....

Bye Fridger

In this same text,it says that in some angles,the haze turn the atmosphere opaque.I think you know why I have this doubt...it??s because the Cassini??s photo of Saturn in visual wavelenghts are no better than Hubble??s,despite being much more closer.In the case of Jupiter,when the probes got closer,the details were much more intricate.But in Saturn,even much nearer,the photos in visual wavelenghts made the atmosphere of planet rather bland.

t00fri wrote:

danielj wrote:It??s unbeliveable,but Saturn is not transparent in visual wavelenghts,...

Can you read the original text by NASA once more carefully:

NASA wrote:Saturn's atmosphere is essentially transparent at wavelengths visible to the human eye, but when the view through the atmosphere is oblique, as it is along the planet's limb (edge),

http://www.nasa.gov/mission_pages/cassini/multimedia/pia07521.html

Bye Fridger

t00fri wrote:Haze is NOT functioning in the more advanced rendering paths like OpenGL 2.0 ....So you can write whatever you like in the SSc file.

That is surprising to me! Not that I doubt it. It's just that going from Celestia 1.4.0pre6 under Win2K to Cel 1.3.2. under Linux was the first time I saw the obvious blue haze of Saturn's north pole. It made Saturn look so much like Cassini pictures. I was impressed.

Note that the haze definition for Saturn is original to the Celestia distributions though, so danielj's request for haze remains a little bit redundant (but see below).

t00fri wrote:Since a long time, Chris has a complete rewrite of atmospheres on his agenda, but ....the climbing season has started....

I hope he'll think about doing it from a basic spec of ground pressure, and a derived scale height after mass/radius specs of planets. For gas giants, the 'ground' pressure should just be that at the 'radius' of the gas giant (which is cloud tops, really). Rayleigh scattering should be worked out from that to get RGB (or HSV?) values to render atmosphere. Oh, and tell him to calibrate sky blueness at altitude while he's up there!

I hope we can start to use 'mass' in SSC's then, I like to see what values non-experts put. It's always interesting what people might guess for their planets.

danielj wrote:In this same text,it says that in some angles,the haze turn the atmosphere opaque. [snip]

danielj, I may not quite follow your English here (but I have to say how much it has improved since you started with the Celestia forum - I am impressed!) but I think you are supposing that there is a haze which makes visual photos of Saturn bland.

I think this is true. But remember, the methane and ultraviolet pictures are to help look at or through the haze or the clouds. Don't mix pictures of haze and clouds up! However, what I think has happened is that the surface texture we have for Saturn already has the blandness of Saturn in it. It doesn't matter if Hubble from far way took the photo, or Cassini close up took the photo - it's the same haze in the way. The haze is not totally opaque though - we can see the cloud bands below. But I suppose the NASA site (do you have a link?) is saying that when anyone looks to Saturns's edge you look through a greater slice of haze, and then it's almost opaque in visible wavelengths.

There is a kind of 'mistake' in Celestia SSCs with gas giants though: gas giant surface textures should actually be cloud textures, which must be opaque in the end, because we can't see the liquid hydrogen/helium surface underneath. Runar noticed this in his Ran add-on. There should be a blue ocean texture underneath. Then there should be this bland-making haze on top.

At the moment, I think the blue haze is modelling where the real haze has condensed out of Saturn's atmosphere due to the ring shadows cooling the northern hemisphere. That's where the blueness from Rayleigh scattering is seen above the clouds.

Do you see the blue haze for Saturn in your Celestia, danielj?

Spiff.

The only rendering path where haze works is

OpenGL Vertex Program/NVIDIA Combiners

It does not work anymore in the more recent paths:

NVIDIA Geforce FX
OpenGL 2.0

Spiff wrote:I hope he'll think about doing it from a basic spec of ground pressure, and a derived scale height after mass/radius specs of planets. For gas giants, the 'ground' pressure should just be that at the 'radius' of the gas giant (which is cloud tops, really). Rayleigh scattering should be worked out from that to get RGB (or HSV?) values to render atmosphere. Oh, and tell him to calibrate sky blueness at altitude while he's up there! Wink

Actually, right NOW he is up there but its rather the top of Mt. Baker (3285 m) in Washington's cascade range.

http://www.mountainmadness.com/namerica/baker.cfm


Bye Fridger

I can see the haze in Celestia 1.3.2,but not in Celestia 1.4.0 pre6.And the strange thing is there is a haze definiton.But changing from Basic render path to Open GL Vertex Program/Nvidia Combiners,in the Celestia 1.4.0 pre 6,there isn??t any difference I can realize.

Do you see the blue haze for Saturn in your Celestia, danielj?

Spiff.[/quote]