in regard to antoine clappier's gemstone render (see attached). my question is: how to light the scene to get the same brilliance - the little spots and lines of caustics. I know where to get cut stones, what general settings to use in refraction and transparency. all those are covered in previous threads here. however, I wasn't able to find a thread that describes the lighting. there are at least two refs to antoine's gem tutorial, but I believe that's about how to model the gem. when I try the render, I get pools of caustics under the gems, but not the lines and spots as in the attached. p.s. I tried it in poser first, as somebody said it could handle caustics thru some means, but I found it was hopelessly slow in any case.

Look in the Browser>Scenes>Global Illumination.

Either replace your objects into those caustics scenes or analyse them to study the settings.

Hi Miss Nancy, Lighting gemstones is always a tough one - whether it's 3D or with an actual camera. I've got some hastily modeled ones in my example that aren't "real" cuts. That means that they don't actually reflect the way true cuts are supposed to. I believe Antoine's version is modeled after a true, reflective cut To start with, the scale of the stone needs to be larger rather than smaller as compared to the light. Notice even Antoine's image has only pools of caustics under the small stones. As you can see, I have three lights in my scene. I'm not quite sure how Antoine did his but you can tell that there is multiple lights based on the color of the caustics in the shadow areas (blue). Only one light is allowed to cast shadows in my scene - the main white light. The blue fill light and light under the scene have shadows turned off. Caustics need to be pretty much maxed out. Additionally, experiement with various Refraction settings - this will make a huge difference. Mine uses, duh, diamond. However, other settings may create something you'll like better. I'd play some more but I don't have the time today... :-/

I've found that part of the process is getting the right gemstone cut, and importing it into carrara using the correct settings. carrara is surprisingly sensitive to small variations in the cut of the stone. I started with a new poser diamond file that was just posted here, and it gave a pool of caustics under the gem, but no brilliance nor fire. the latter (splitting white light into a spectrum) is likely beyond carrara's capabilities. however, using a brilliant-cut poser model from the lapidary site gave reasonable results for reflected caustics, but not for refracted caustics that are displayed in the above image. hence, I daresay, unless antoine pops into this forum on occasion, I'll hafta ask him directly. did he remain with the firm after daz bought eovia?

I don't know if Antoine stops by. He is involved with a new effort in France revolving around digital imaging but I don't know the details.

o.k., thanks, mark. i was using one spot and one bulb, which gave bad shadows, so I killed the bulb, which wasn't present in the default carrara caustics scene. scene background/backdrop also made it look wacky, so I killed that. it worked better with the default ambient than with no ambient. had caustics at 100, but found varying photon count and filter size (whatever that is) didn't affect the caustics much. the gems in my case were all pretty big, ranging from 2 inches to 5 inches, which of course is absurd, but all of 'em did reflected caustics, actually better than in yer example, but it seemed as if only the emerald refr. index did refracted caustics, which were poor compared to antoine's example above. I also jacked up reflection/refraction raytracing to 8 or 16, but it didn't help much, except to slow down rendering.

Quote - ... and filter size (whatever that is)...

controls the 'blurriness' of the edges of the caustic emmissions. Higher numbers make blurrier caustic patterns.
I can't render caustics, because C5 crashes every time I try... so I couldn't give any advice about what might be a good place to start for filter size.

yes, dv, just to concur with ya. I found that a filter setting of 0 gives a blotchy-looking caustic, and a setting of 100 gives a caustic that is blurred to the point of roundness. so the default setting (around 30%) works well. in my case it doesn't crash when trying to do 'em. the "filter" setting appears analogous to something like "photon exposure" (the inverse of ambient occlusion), and increasing photon count increases the sharpness of the caustic edge, as noted by mark in the msg above. I also tried an rgb spotlite, but there was no birefringence. although lowering the filter setting caused colour separation in the caustics, it was in blotches, and not in accord with the laws of optics. antoine is easy enuff to locate, but I daresay he would consider intrusive any questions about a render he did years ago, using software with which he is no longer involved. e.g. a google search using his name, and limited to domain www.daz3d.com, gives no results.

Were you using the anisotropic lighting model?
Since Birefringence is often caused by a distortion in an otherwise isotropic material (an extreme bend in a fiberoptic line, for example), The anistropic effect might yield something similar; though I'm not sure that Carrara has the ability to simulate the prismatic dispersion.

Also, I have been able to get birefringent refractions by using: Operaters -> Overlay, in the refraction slot, and using two different indices of refractions. It shows up as a dual refractive image through the transparency in question, but I couldn't sat what effect that would have on the caustic emmissions, if any.

oops! my misteak! I used the term wrong. birefringence is two indices of refraction along certain axes, but (as ya noted) with diamonds the term I shoulda used is called dispersion - the ability to split up white light into its components. I see how your method should work, but i was hoping for a spectrum-type effect. dunno how to do it in carrara. although I also dunno how to apply anisotropic lighting, the term applies to a material that reflects light differently from sources at different angles. I reckon it also applies to materials that refract light in different directions, e.g. birefringent materials. I dunno if diamonds or other gems have that property.

Applying anisotropy:
Top Shader -> Lighting Models -> Anisotropic Model
clcik the little double downward pointing arrow beside the trash can looking icon.

Yes, most singular crystal cut stones should have optical anisotropy, especially in the refraction (more so than the reflections, which are simple surface reflections). Specular anisotropy may or may not be present. Most also have at least some birefringence. Observing anisitropic reactions to light is one of the main ways to distinguish between glass (isotropic) and real crystal (anisotropic). Polycrystals either have no anisotropy or a lot of it, depending on the structure of the crystalline grains.

Still don't have an answer for you about prism effect. Sorry.

I'm pretty sure this gem render was done before Carrara had caustics. This discussion has opened up some interesting info about the caustics renderer, but if I'm right you'll never get the same results as in the image (since they were faked).

holly

so it was faked, like the thing they use to do caustics in poser? it's a good fake, anyway. the attached is what my attempt looked like in carrara. it's not as good as antoine's IMVHO, but it's easy to set up and only takes about 5 minutes to render. I tried doing a diamond caustic render in maxwell render, and it looked promising, even tho the default render settings caused it to be grainy, like a photo taken with bad film in a bad camera. but then I checked the maxwell render forum, and the dispersion setting apparently causes the render engine to crash, or at least to take many hours or days to get a good render. so maxwell render is no-go, at least for me. I also checked the maxwell render forum on why default renders are grainy - no answers, just more questions. keyword "grainy" didn't appear in the maxwell render manual, nor did keyword "diamond", and the "gem" shaders in the maxwell materials were actually shiny metals, not gems.

for the effect of "fire" (the prism effect) carrara has the ability, but not "out of the box" so to speak. if someone programs a plugin to do this in 1 step, and change refraction values and calculate more beams of light, then it'll be easier, but until then, to get the effect that is most realistically what happens, is make multiple renders. 

remember the lighting you used, and refraction values, then turn off all light values except for red, (ie you have 3 lights, RGB 255/200/200, change to 255/0/0 ) and render.

then switch to green channel of your lightiing, and change refraction just a bit (forgot exact values, you can play with a straight 3 sided prism and 1 light to see these effects) and render. then do this step for blue channel as well.

put the 3 color channel renders together in some paint program, and you're done.

if you want more exact pictures, split the color changes a bit and use red, yellow, green, cyan, blue, violet, or even more if you want to put the time into it, and understand how to add layers together.
 
this is one of the main reasons i moved to carrara in the first place, you can do it in poser, but no direct way for getting caustics. poser can fake caustics i'm sure if someone spends months to find how, but carrara is more direct. and be aware that if your material is a perfect red, or perfect green, that it get rid of the rainbow, but there are multiple shades of red in the world, so the idea still applies.