How about playing with Titan's atmosphere parameters!?

[Cham]

I'm using Fridger's parameters for Titan, and I noticed some strange artifacts in the atmosphere, dependant of the observer's position (look at the bottom-right of the moon) :



This looks like some interference patterns. Is it related to the screen resolution and FSAA ? Or is it a bug in the atmosphere rendering ?

Also, the atmosphere cut-off is too abrupt, too strong. It doesn't feel very natural.

[t00fri]

I'm using Fridger's parameters for Titan, and I noticed some strange artifacts in the atmosphere, dependant of the observer's position (look at the bottom-right of the moon) :



This looks like some interference patterns. Is it related to the screen resolution and FSAA ? Or is it a bug in the atmosphere rendering ?

Also, the atmosphere cut-off is too abrupt, too strong. It doesn't feel very natural.

Martin,

I also noted some artefacts when playing with the settings, but by far not as strong as what you seem to notice. Also the atmosphere cut-off in my case of rendering seems to match pretty well the natural color photos:

see here (from higher up):



You should also realize that the present rendering is without any special tuning wrto Titan's very special atmosphere. So I think it is pretty amazing to come that close already.

Bye Fridger

[Cham]

The colors are okay. I'm just talking about the "interference" like patterns (shown above), and the atmosphere cut-off, as shown below :



The blue layer should fade away in a smoother way.

[chris]

I'm using Fridger's parameters for Titan, and I noticed some strange artifacts in the atmosphere, dependant of the observer's position (look at the bottom-right of the moon) :

This looks like some interference patterns. Is it related to the screen resolution and FSAA ? Or is it a bug in the atmosphere rendering ?

The majority of the atmospheric scattering calculation is done in the vertex shader, so if the atmosphere is tesselated to coarsely, you will see artifacts like the ones in your images. The solution is to increase the tesselation of planet spheres when there's an atmosphere. It's a simple adjustment, and shouldn't noticeably affect performance.

--Chris

[Cham]

Chris, is this adjustement at the code level ? I guess I can't do it in the SSC level ?

[chris]

Chris, is this adjustement at the code level ? I guess I can't do it in the SSC level ?

There's nothing that you can do in the ssc file; the code needs to be changed.

--Chris

[Cham]

Why not define a new parameter to be declared in the SSC, about the planet resolution ? Without this parameter, it's just the default value.

And what about the atmosphere cut-off ?

[chris]

Why not define a new parameter to be declared in the SSC, about the planet resolution ? Without this parameter, it's just the default value.

I think it would be best if Celestia automatically did the right thing. The tesselation artifacts you noticed are a bug in Celestia, and I should fix the problem.

And what about the atmosphere cut-off ?

I think Fridger's Titan atmosphere looks quite good. It should be more opaque, but that's a limitation of the atmosphere code. I'm working to fix it.

--Chris

[Greewi]

I tried your parameter for titan and if the effect is impressive "behind" the saturnian moon, it seem strange "front".

Some screen shots :
"Behind" :


"Side" :


"Front" :


I guess, some thing is missing : perhaps a smooth brown daze for daytime ? (Sun set is really impressive, just a too luminous)

[t00fri]

Well, generally the blue color above the orange haze has been documented by true color imaging. It has also been clarified in discussions between the lead scientist of the Cyclops imaging group and myself.

There are a number of issues, however, of which Chris and myself are fully aware. Ideally, for example, the haze should be generated by means of the new atmosphere code. Also it's upper boundary should be washed out rather than sharply limited.

All that means pretty nontrivial coding and will take it's time before completion. Nevertheless the striking /qualitative/ agreement of critical atmosphere views from Cassini seems pretty encouraging to us.


Bye Fridger

[Greewi]

It is not the blue that make problem in my opinion, but the missing lower part of the atmosphere. Perhaps we could add a layer (or a fake layer by using a clone of titan) ?

[t00fri]

It is not the blue that make problem in my opinion, but the missing lower part of the atmosphere. Perhaps we could add a layer (or a fake layer by using a clone of titan) ?

The lower part of the atmosphere is there, but the haze is lacking. The Mie atmosphere code just does not yet render it sufficiently opaque.

Fiddling with an additional layer may be fun for an add-on creator at this point in time. But it's certainly not what we are after eventually in the Celestia dev team.

Bye Fridger

[Greewi]

It is not the blue that make problem in my opinion, but the missing lower part of the atmosphere. Perhaps we could add a layer (or a fake layer by using a clone of titan) ?

The lower part of the atmosphere is there, but the haze is lacking. The Mie atmosphere code just does not yet render it sufficiently opaque.

Fiddling with an additional layer may be fun for an add-on creator at this point in time. But it's certainly not what we are after eventually in the Celestia dev team.

Bye Fridger

Ok. I was thinking that it was wrongs parameters.

But I have one question (almost in the subject) : does all atmosphere are mono-layered ? Is it physically true in the reality ? I was thinking that Titan has really two layers (like an blue atmosphere layer and a brown cloudy layer). May be, I'm searching to do a cloud layer with atmosphere layer. It may be here where I'm wrong.

[t00fri]

Right now, any "atmospheric layer" we might want to add has ZERO thickness. Cloud layers, for example, have zero thickness. So that's no real improvement in case of Titan.

As I said. It will become just right, but it's difficult stuff and takes it's time. So patience is required.

Bye Fridger

[fsgregs]

Sorry to reactivate this thread, but I have been testing out 1.5.0 final, and I noticed that when I went to Titan, its atmosphere is quite different now in appearance from the images seen here. I realize that the pictures posted earlier on this thread are well over a year old, but as Fridger and Chris pointed out then, they looked quite realistic when compared to photos taken by Cassini.

The Titan atmosphere in 1.5.0 looks very different (much pinker, less orange). I've tried to go to about the same place on Titan. Here are the two images compared:

Fridger's 2006 image:


Titan as seen in 1.5.0 final


I am not complaining. Both atmospheres look great. I was just wondering which one is the more accurate with respect to what Cassini has photographed recently?

Frank

[chris]

The current atmosphere code has some deficiencies that make it not well suited for optically dense atmospheres like Titan's. We're working on better approaches, however:

http://forum.celestialmatters.org/viewt ... 16&start=0

(In particular, see the Titan screenshots on page 2 and later . . .)

--Chris

[t00fri]

Sorry to reactivate this thread, but I have been testing out 1.5.0 final, and I noticed that when I went to Titan, its atmosphere is quite different now in appearance from the images seen here. I realize that the pictures posted earlier on this thread are well over a year old, but as Fridger and Chris pointed out then, they looked quite realistic when compared to photos taken by Cassini.

The Titan atmosphere in 1.5.0 looks very different (much pinker, less orange). I've tried to go to about the same place on Titan. Here are the two images compared:

Fridger's 2006 image:


Titan as seen in 1.5.0 final


I am not complaining. Both atmospheres look great. I was just wondering which one is the more accurate with respect to what Cassini has photographed recently?

Frank

Frank,

both images that you contrasted in your post can be generated with the 1.5.0 atmosphere parameters, if you chose the right configuration of the sun and the observer. Mie atmospheres are capable of describing VERY rich optical phenomena. But you must understand qualitatively what's going on.

I am sure you have been teaching your students regularly why the sky on Earth is blue and why the sky is turning red in the evening, if viewed in a particular direction! Which one ?

Mie scattering is nothing but a generalization of Raleigh scattering of light on atmospheric droplets, so it also includes Raleigh scattering as a limiting case.

Surely Titan has a much more dense atmosphere and correspondingly there are many new phenomena arising compared to Earth. Backscattering halos is one of many.

If you want to see particular atmospheric colors you must be aware of the direction dependent absorption of light that occurs.

Here is a simple way to retrieve the more yellow-orange atmospheric light at lower altitudes that you incorrectly claimed missing above:

type titan and hit G
Then hit H and type SHIFT+C.

That command places the sun behind Titan as seen from the observer's position and you can see the striking backscattering halo. Without changing anything, zoom in and you will see again the yellow orange atmosphere merging into blue at high altitudes. By turning Titan with your right mouse key, you can certainly also retrieve the purple-blue views you were displaying above.

How about contemplating a bit why things happen this way (and telling your students about it) ? It's beautiful and pretty simple physics that 13-15 year olds should easily be able to grasp qualitatively!


F.