Shadow within a comet coma

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Old 01 January 2007   #1
Shadow within a comet coma

I'd like to post a thinly-disguised question in the form of a "mini-challenge".

These two pictures represent the nucleus of a comet (the rock) surrounded by its coma (the foggy glow):

The coma is a radial light with fog enabled. Both pictures were made with EIAS but the shadow in the top image was added (in post-production) using Photoshop. The challenge is that a camera will fly through the coma so the shadow must be generated within EIAS. This means the coma cannot be a fog-enabled radial light.

How would you make the rock cast a shadow within an EIAS project? Thanks in advance for your suggestions.

Joe T
Old 01 January 2007   #2
Here is something to look into... until someone post the right way to do it.

My test comet shadow
Old 01 January 2007   #3
Hi Rick,

Thank you for posting your clip. How did you make your coma? Is it a volumetric cloud from someone like Northern Lights or Konkeptoine?


Joe T
Old 01 January 2007   #4
Volumetric cloud plugin?... No, it's a simple sphere. The shadow is an extrusion from the mid part of the comet ending at the end of the sphere. I used a clipping map for the shadow object so it matches the transparency of the sphere edge. That's it. Of course this is not the perfect solution but it would work for far to mid shots.

Here is the project file (The starfield background is in low rez, size does matter...)
Old 01 January 2007   #5

Hello again Richard,

When we enter the coma in your sample animation, there is a sudden change in the overall appearance as we cross the sphere's boundary. Ideally, the change in brightness would be gradual since the edge of the coma is more transparent than it is near the nucleus. I know your project was a first approximation (thank you for making your project available) but I wonder if anyone can suggest another method for portraying this?


Joe T
Old 01 January 2007   #6
Hey Joe, this is a simple (old school; no volumetric plugins) method of creating shadows in glows

You cut the shadow by enabling glow in your illuminating light (ie: the sun) and enabling ray, then inverting fog and glow and finally setting the glow intensity to -1 (assuming your primary glow, ie: coma glow light, was 1)

There are a couple things to watch out for: the negative shadow (inverted ray) must match the distance (or slightly less than) of your primary glow field. Extend your negative glow past the glow field and you will rip a hole in the universe;-) (a large white "hole" will appear in your rendered image)

A smoker can be used but again the primary glow and the "negative shadow" settings must match exactly (set up a smoker the way you want it and use the smoker settings unmodified in both the lights (coma and sun) any difference will rip a white hole in your
image where the negative glow light (shadow) differs from the glow
Really simple, once you grasp the mechanics, and renders quickly (that entire animation rendered in about a minute). If you want the project let me know and I'll copy it up. (it took literally 5 minutes to do)


Last edited by WmH : 01 January 2007 at 06:32 PM. Reason: typo(s)
Old 01 January 2007   #7
thank you

Hi WmH,

Thank you for posting your recipe and video. I had set up a project based on Richard's method. His recipe worked great until I made my nucleus rotate. My nucleus and its shadow object (a suitably-configured cylinder) could not match perfectly unless I made my nucleus almost perfectly round. My nucleus is irregular in shape, as a true comet nucleus is, so now I'll give your method a try.

I had known about using negative intensities but I've never used them myself. My background is in science and in the real world there is no such thing as light with a negative intensity. I try to remind myself that the digital world is much more flexible. I have to keep telling myself that I'm not really arranging objects in a real-world scene; instead I'm writing lines of code which allows me to do things I could never do in the real world!

See you later,

Joe T
Old 01 January 2007   #8
Glad to help, I played with it a bit (during some idle time I had yesterday) I added a smoker to the glow and pushed the coma back slightly (to account for the push of the solar wind). I am not happy with the (temporal) progression (drift and offset) that I have, it just looks to swirly, but with the smoker in the glow the render time increases substantially and I don't have the time to discover what settings would look right.
It does however look ok in stills.

You will need to point the project to your local ubershape plug and bump shader (I didn't include) Turn off the smoker in the sun AND coma glow light to get reasonable render time when you don't need it .
Project file

PS I forgot to mention, negative light is a surreal concept to begin with, and this is an inverse use of it (the sun's ray shadow in the glow is inverted to positive and then set to negative illumination) so taking a bit of effort to wrap your head around it is to be expected.

Last edited by WmH : 01 January 2007 at 03:32 PM.
Old 01 January 2007   #9
great shadow

Hi WmH,

I threw together a project using your recipe and it works great! For the "sun" light I set the Glow Intensity to -0.5 and the Glow Factor to 2.0. For the "coma" the Glow Intensity is 0.5 and the Glow Factor is 1.4. These values produce a realistic image and they avoid the abrupt disappearance of the shadow at the edge of the coma glow (seen in your original video).

Joe T
Old 01 January 2007   #10
Originally Posted by interstellar: Hi WmH,

I threw together a project using your recipe and it works great! For the "sun" light I set the Glow Intensity to -0.5 and the Glow Factor to 2.0. For the "coma" the Glow Intensity is 0.5 and the Glow Factor is 1.4. These values produce a realistic image and they avoid the abrupt disappearance of the shadow at the edge of the coma glow (seen in your original video).

Joe T

Glad you are getting good results with it, however the shadow edge was likely related to the lights glow extents rather than to intensity or glow factor... I was just quickly scrapping the project together and set the reach of the suns shadow so that is was "for sure" inside the coma glow (as the reverse yields rendering artifacts).

You will find this "trick" sometimes requires fairly precise alignment of the two extents for a believable shadow (primary and negative glow) and that the negative must be "just short" of the primary glow extent or you will get artifacts.
Old 01 January 2007   #11
William you are amazing. Always comming up with the right answer... I remember couple months ago with the Storm challenge, your solution was excellent. How many tricks like that do you have up your sleeve...? I knew this shadow in a glow could be done as I have seen it before. The question as where...!

We should bring back an updated tutorials section on the upcoming New Ei Webside: "Old school trick that works perfectly well with recent EIAS version..."

Thanks for sharing your knowledge.
Old 01 January 2007   #12
I agree

I agree with Rick and I owe you one, William. A couple beers, if you're ever in the area (Space Coast of Florida).

I found those artifacts you mentioned; even when the glow regions are precisely set, minor artifacts appear when the Camera is near the juncture of the shadow and the back edge of the coma. However, careful positioning of Camera avoids that and allows me to get a nice "sunrise" off the edge of the nucleus with a dramatic shadow!

Joe T
Old 01 January 2007   #13
Joe, would you care to post your final image or animation. Would be a nice conclusion to the thread.
Old 01 January 2007   #14
Thanks for the kind words Richard, I was a heavy user of EI 2.X and spent many nights knocking my head against the CRT. (perhaps why it stuck so long;-) Amazing that many of the old "tricks" still work and still hold up by todays (visual) standards (further proof how amazing EI really is)

No problem for the assistance Joe, just pay it forward; if sometime you see another EI user struggling and (think) you have an answer then take the few minutes to share.
I owe Bear for many hours that I would have spent struggling years ago if not for his always generous assistance.

And I agree with Richard it would be great to see some of the final product.

Last edited by WmH : 01 January 2007 at 04:20 PM. Reason: typo
Old 01 January 2007   #15
my nucleus

Although it's just a WIP at this time, here's my cometary nucleus. The gas jets erupting from the nucleus are spotlights shining through smokers. I'll probably add several more of these and they'll be animated.

William & Rick, thanks again for your help.

Joe T
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