Spotlight volumetrics

[Grolicus]

Hi, I'm trying to create a blue halo around an Earth globe that would only be visible on the side of the Earth that is lit by the Sun. Therefore, I created a spotlight of 180° in the middle of my Earth globe that automatically points at the Sun.
To make this light visible, I activated the Visible Light Cone option, but this only seems to render the edges of the light cone : the interior of the light cone seems not filled by by light. This means you can't see the blue atmosphere when looking straight at the Earth 'and thus into the cone). You can only see the cone when lokking at the 180° edges, thus when looking from the side.
Two questions :
1. Am I correct in assuming that Carrara doesn't render the volume, but only the edges of the light cone ?
2. Is there a way to solve this problem ? By the way, activating the Sphere visibility doesn't help, because then the dark backside of the Earth is lit too.

Any help appreciated.

[bwtr]

Grolicus may be because it's early in the morning here but I dont' seem able to visualise your problem. A drawing may help.

I am trying to understand how one could see a light pointing away from you if it was not by reflection off something? Why, ever, would you point a light at the sun?

[LoneGunman]

Here is the example, see image attached (i did it in Bryce 5 couple of years ago)...

Principle should be the same in Carrara, Earth's athmosphere itself is the halo, therefore it can be simulated with sphere which is transparent.

In order to make it work as a volume, I believe (somebody correct me here if I'm wrong) you have to fiddle with one of the shaders which account for this (translucency and sub-surface scattering), in Bryce every texture can be rendered as volume, so, that was not a problem

Since I never did this in Carrara, all this was assumption, but approach with spotlight IMHO is just not a way to go (cuz halo is actual result of athmosphere beeing illuminated by the sun).

In any case, good luck to you, keep us posted if you make it work !

[Grolicus]

bwtr,

In short, when you order Carrara to render the Light Cone (under Effects tab) of a spotlight, it seems to litteraly render only the edges of the light cone, not the interior light volume within the cone. At least, that's what it looks like to me.
I'd like the spot to point at the sun with an 180° opening, so that only the half of the sphere illuminated by the sun emits a blue halo light. The other (dark) part pointing away from the sun would then not be illuminated by the spotlight.
In C4D this worked like a charm, and the emitted volumetric light nicely interacted with the sunlight.

LoneGunman,

Thanks for the advice, I'll certainly try that approach. I think you're absolutely right that this would more closely mimic a real atmosphere.

[bwtr]

I am still a bit mystified by your method. However I did the attached which is just the earth duplicated, sized up and the Transparency Shader applied to it. (Refraction was reduced from the default setting of the shader.)

[Joppe]

Therefore, I created a spotlight of 180° in the middle of my Earth globe that automatically points at the Sun.

There you go wrong. Try a bulb als light. Give the light a glow and a lightsphere. Chooose blue in the General settings an experiment with size and brightness and you can make any Halo you want.

I try to attach an image but this is my first time so I don't know if I'am doing this right.

Joppehttp://www.eovia3d.net/images/smilies/shiny.gif

[Grolicus]

bwtr,

You're right. This was the method also proposed by LoneGunman. But as you can see from Joppe's image, his fall-off of a light is much more smooth than yours with transparent geometry. But still, I think I prefer your method instead because it solves the problem with the dark half of the Earth. It should be possible to create a smooth fall-off with some tinkering.

Joppe,

Nice halo with a nice fall-off, but the point of the spotlight was to confine the halo to the bright part of the Earth (directed towards the sun). In your image, the right side of the Earth is dark, but the atmosphere is still lit. In real life, the halo is visible because light from the sun illuminates it. So on the dark side of the Earth, the halo shouldn't be visible. That's how this whole thing with the light cone got me started

Thanks for the input, guys !

[bwtr]

Been trying hard to soften the edge of the halo but not there yet. It's good learning to try and solve these problems. Diagrams and images work better than a thousand words--as the saying goes.

[HowieFarkes]

I used a volumetric cloud set to a sphere and twiddled the settings until I came up with this.

Or you might use BWTR's version and use "blur" on the outer sphere.

[Grolicus]

The volumetric cloud looks good too, and by its nature approximates real life closely.

Thanks for all the help. I'll have to wait a while before I can try out both solutions, but your support is much appreciated
I think you guys just bought Carrara another customer (i'm on the demo for now).

Hey Carrara, how about a monthly vote for most supportive forum member ?
(followed of course by the inevitable small goody bag to be sent to the proud winner)

[Grendel]

You could use Joppe's method and move the bulb to the sunside to not illuminate the darkside.

[Joppe]

You could use Joppe's method and move the bulb to the sunside to not illuminate the darkside.

I thought about that too, but I was so busy I hadn't time to try.

[Joppe]

But the cloud method is begging for research on my side ;-)
Love it very much.

[HowieFarkes]

Originally Posted by sailor_ed

Or you might use BWTR's version and use "blur" on the outer sphere.

I tried that and the blur effect is applied by Carrara post-render so the planet underneath the atmosphere gets blurred too.

[bwtr]

Howie you beat me to the punch.

I really should have thought of that, my apologies. However we both learned something. :-)

Maybe this will do better. This uses a color gradient and fake fresnel (from ShaderOps by DCG) to give good control over the fall off of the atmosphere; both outer and inner portions. Attached is the atmosphere shader and a quick render.

Of course this shader is applied to a sphere a bit larger than your earth. Note that because it uses Fresnel the lit edge will always be in the correct position around the earth regardless of how you move the sun. Fall off is adjusted with the color gradient. Transparency is set to 100%. Render with "Light through Transparency" checked.

[bwtr]

Made my day. Thanks.