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 Post subject: Comet Plumes
PostPosted: Mon, 05-05-14, 16:56 GMT 
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Posts: 9
Well m having a real difficulty in creating a comet tail, i cant seem to figure out how they work, so I tried creating a 3ds model which look pretty but doesn't keep its orientation and for some reason in celestia its model center is mid tail and not in the head (even though its so in blender).

Could a kind soul please help me.


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 Post subject: Re: Comet Plumes
PostPosted: Mon, 05-05-14, 21:51 GMT 
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Have a look at comets.ssc in the Celestia/data directory for examples.

You may find the "MeshCenter" directive useful, and BTW the tail will only be evident when the comet gets close to the Sun.

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 Post subject: Re: Comet Plumes
PostPosted: Tue, 06-05-14, 0:12 GMT 
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and be pointing away from sol so a 2 vector orientation
-- from the wikibook
Code:
BodyFrame {
    TwoVector {
        Center "Sol/Test"
        Primary {
            Axis "z"
            RelativePosition { Target "Sol" }
        }
        Secondary {
            Axis "x"
            RelativeVelocity { Target "Sol" }
        }
    }
}

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 Post subject: Re: Comet Plumes
PostPosted: Tue, 06-05-14, 7:57 GMT 
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This reminds me that it was me years ago, who coded the comet tail illumination and orientation as function of the distance from a star in Celestia. Yes, it was less trivial than it may sound at first: we all know that the comets we see in our solar system stop having a visible tail at a distance beyond 4- 5 AU from the Sun. But then how about the tail illumination if a comet traverses a multiple star system? How will the tail be oriented?

That was what I worked out and coded for Celestia at the time. The comet story needs a thorough polishing in celestia.Sci, ...when time allows that is. After all, comets frequently have TWO tails, a gas tail and a dust tail that are oriented differently....

Fridger

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 Post subject: Re: Comet Plumes
PostPosted: Tue, 06-05-14, 9:51 GMT 
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Well thank you all!, yes that did the trick. and Fridger it seems that my tail forms towards the larger star, but that not an issue, just thought i mention it.

But now im encountering this: why? (or better yet, how can i fix it?)


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 Post subject: Re: Comet Plumes
PostPosted: Wed, 07-05-14, 12:53 GMT 
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zhar2 wrote:
Well thank you all!, yes that did the trick. and Fridger it seems that my tail forms towards the larger star, but that not an issue, just thought i mention it.

But now im encountering this: why? (or better yet, how can i fix it?)


Since you didn't write what you did, the chances for a qualified answer are small indeed...

I suppose the bright star in the center is the Sun?? The squares might be artefacts due to an enforeced alignment with a comet or some other buggy object?? Which one?? Which data in comets.ssc? Any add-on used?? Which Celestia Version?? Which OS??...

Fridger

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 Post subject: Re: Comet Plumes
PostPosted: Wed, 07-05-14, 15:03 GMT 
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Oh my apologies. Its the Ran III addon. I think the squares are supposed to be sun flares. They are being textured but for some reason they are not coming out right and get the bright yellow cubes.


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 Post subject: Re: Comet Plumes
PostPosted: Wed, 07-05-14, 16:51 GMT 
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This is probably due to the way that Runar designed his addons in earlier versions of Celestia.
He sometimes used "transparent" boxes as placeholders to enclose some of his models.
If I recall correctly, earlier versions of Celestia had some issues with 100% transparent objects, so he probably gave them a very slight opacity of 0.001 as a workaround.
In earlier versions of celestia these placeholders were effectively invisible, but due to changes in the rendering engine (sometime around v1.5) these boxes became more visible (slightly) - I think due to the fact that they are not 100% transparent.

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 Post subject: Re: Comet Plumes
PostPosted: Wed, 07-05-14, 23:15 GMT 
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If the case is that as Chuft is saying, instead of editing the 3D cube mesh, you can try to map on it one transparent PNG. "transparent PNG" mean a 0.5k, 1.0k transparent PNG canvas; that is an empty, no background colored canvas saved in PNG format. It will be saved as "alpha channel" being held only the "A" of "RGBA" acronym; every mesh mapped in this manner will be invisible whether its opacity in not zero.

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 Post subject: Re: Comet Plumes
PostPosted: Wed, 07-05-14, 23:45 GMT 
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to fix the cubes use "th_placeholder2.cmod" in the ssc and NOT "th_placeholder.cmod"

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 Post subject: Re: Comet Plumes
PostPosted: Sun, 11-05-14, 12:04 GMT 
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I fixed the issue by creating new spheres with no opacity, now i dont get any artifacts.


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