Luminosity vs Lights

Started by paulcurtis, December 17, 2021, 06:54:31 AM

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I am testing approaches for bringing a whole series of hot air balloons into clouds. Whether i do this externally or in terragen itself, which is a nice idea if possible, i don't yet know.

I need the fires to light up the inside of the balloons and the objects are external. I didn't think it was possible to import a model with a light so i was wondering whether a luminous object would do the trick.

So i have test rendered a light in the clouds (looks good) with sphere and a luminous texture. Came to the conclusion they worked very differently. I don't think luminosity is lighting clouds at all but some surfaces. I haven't really gotten to the bottom of it but its different enough i thought i'd ask before spending hours troubleshooting.

Are luminous objects treated differently to lights internally?

Is there a subsurface scattering shader, think lighting the inside of the balloon?

If i comp externally i cannot see any render elements to create deep images? There is a cloud position pass though, i am rendering spherical and i don't know how that will work but that's another area...


Kevin Kipper

Hi Paul,

You can import a light as part of an FBX file, but by default only its heading and elevation data are imported, not its colour or strength.

In your external 3D application, if you can match the light's translation and rotation with another object, like a null, you can bake the motions to the null and import the null as part of the FBX.  You can then add a Spotlight or Lightsource type light in Terragen and copy/paste the motion channels from the null object to the lights position and rotation channels.  The process of copy/pasting motion channels is covered in Parts 3 - 5 of the "Terragen for VFX" videos.

Luminous surfaces do affect clouds, see attached image.

See the wiki documentation on the Glass shader for subsurface scattering.

Terragen's Render Layers can save render elements in EXR format, but not deep EXR.


If you don't need to see the lights themselves, but just some glow of the balloons, you could use the objects' luminosity and some mask to fade it towards the top of the balloon.


It's also good to note, with luminous surfaces, you may need to have a duplicate object set to invisible, and use it as your emission, by that, I mean your actual light level above Lum=1. So your visible object will have luminosity at 1, or whatever level, masked by whatever, and then your hidden object will have luminosity at say 1000 to create a very strong glow, while your render shows and normal glowing object. Anything above 1 in lum tends to just be a solid, un-detailed thing.


Thanks all,

I've been troubleshooting this and i'm pretty convinced there are some bugs in this approach. I'm just wrapping my head around what they actually are.

For example on a spherical projection i can get illumination but it's not where the emission object is. On a rectilinear i am seeing issues with luminosity values in terms of their scale/units. So the scene is blown out 10,000 - take that down and then increase and now i am not seeing any illumination - it's like there's some internal data not updating. So when i get back to 10,000 units i still see nothing.. The scene and render settings themselves have not changed - just the lum value.

I'll post something in the support thread when i'm closer to having steps to repro.