Rendering thick glass and light question

Rendering thick glass and light question

I'm just getting over the learning curve in Cheetah - much thanks to Luke's sunset tutorial for helping me "get it".

I'm trying to model the attached lighting fixture - sphere.pdf It's a cast glass fixture, very thick with a small filament bulb inside.

I've attached a small rendering of three lights. They all have in common point sources with 0.1 intensity, ray trace with transparency, and clamp attenuation off, the glass has a refraction index of 1.5:
The differences are as follow:
-The Left one has no geometry, no attenuation but you can't see the glowing filament.
-The middle one has geometry, attenuation 1/(r^2), problem: a black sphere suddenly appears
-The right one has no geometry, attenuation 1/(r^2), problem: the fixture disappears

I have a number of questions, all leading to: How do you rendering this thick glass lighting fixture? The attached close-up shows the left light with glass that looks as thin as a wine glass. And what's wrong with my lighting settings to show the glowing filament (not intending to model the filament, just a small spot of light)

The model was made in Sketchup with the follow-me tool. I'm just quicker with SU and haven't used C3d for making the shapes - maybe that's the problem.

Any help would be appreciated.
 

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  • Sphere light.jpg
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To do a really good job you're going to have to either fake the heck out of the light thrown out by this thing or use caustics maxed out to an astonishing degree since the refractive properties of the glass are going to be fairly spectacular.

That aside, the "thin glass" problem is probably the geometry of your lamp. For refraction to work properly you need to create objects with well-defined interiors, and I'm guessing you've either built something with thin walls or wrong normals or something.

All that said, I tried to set up a test scene in C3D, and I'm seeing all kinds of problems with transparency and lighting.

This is 4.3.1.

Note that I got very different results based on where I set the transparency via the color/opacity/use alpha channel versus setting reflection intensity, but neither rendered as expected.
 

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To get glass to look right in 3D you usually need to include an air layer which has reversed polygon normals and a different index of refraction. I've never managed to get this right in Cheetah as there seems to be something weird happening when you place the polygons on top of each other. I'd be interested to hear how this can be resolved too.
 
Dear spwong

I'm not sure if this the sort of thing you are after ? if it is then file herewith to pull apart

Regards

Luke
 

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re Meshman post

I did some more testing and indeed got quite different results when I simply reversed the the interior polygons of the light.

The reversed polygon render to me a least does create a more realistic effect..

Regards

Luke
 

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... when I simply reversed the the interior polygons of the light, .....the reversed polygon render to me a least does create a more realistic effect..

Because your camera is looking on the right side ;)
You should have now an eye at the index of refraction not to run into weird distortions.

Cheers
Frank
 
hi,

I tried to make it. as podperson wrote, it seems to make wrong shadow when closed object with transparent material covers light source... so I attached Render tag for turning off 'cast shadow'. please check attached file, and I hope it helped.

glass object's 'cast shadow' off, without Caustics tag.
ch_20071129_l_a1.jpg


glass object's 'cast shadow' off, with Caustics tag.
ch_20071129_l_a2.jpg


ch_20071129_l_b.jpg


regards.

tg_jp, Hiroto.
 

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Last edited:
Thanks Frank, Luke and Hiroto.

It looks great and will suit my purposes fine. I can't tell you how extremely helpful it is for me to dissect your files because "inverting polygons" goes over my head, but once I study your files, I'm sure i'll get it.
 
I did some more testing and indeed got quite different results when I simply reversed the the interior polygons of the light.

The reversed polygon render to me a least does create a more realistic effect..

Regards

Luke

Thanks Luke. That's kind of what I've been after. I always used to invert the whole object in Lightwave and set the refractive index to 1 for the air layer but haven't had a lot of joy in C3D with that method. This looks like what I've been after. Will mess around with your file a bit. Muchos gracias. :)
 
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