I want to as accurately as possible simulate lights shining through not-quite-transparent coloured plastic. The manual says the wax shader is the way to go, and it looks like with a bunch of fiddling I should get something superficially plausible-looking, in combination with photons, but I'm trying to actually figure out how this might look before prototyping it, what are the shortcomings of the wax shader and photons going to be? Can I expect to actually be able to get this in the ballpark or is there some major effect Brazil will miss?

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I think the wax shader method will work for you Jim but the sample rate as you know will effect render time. Keep it as low as possible. The depth for light to penetrate should also be set in relation to the actual size of the model. For a translucent plastic that has an even transmission of light through it's thickness, I'd use a BAM with glossy refraction at say 98% and then keep the sample rate as low as you can. 

Photons don't have any issues with wax shaders that I am aware of but if you see something odd please let me know. 

Hi Brian,

Yeah I added SSS to a BAM and it seems to look okay, though yes I need to crank down the sampling or it takes forever. My concern about SSS and photons is that from what I can tell so far the photons seem to ignore the SSS, the objects look more realistic but the light they're throwing off looks identical(minus the attenuation from the translucency.) If that is the case, is that physically incorrect? Think about something like a taillight reflector, that's one of the things I'm looking to simulate.

Well to try to figure this out I'm rendering a test of a part that I do have in reality and while I can't compare a crystal-clear plastic to translucent I can compare translucent plastic (a cloudy "clear" plastic) to no housing, and at least in this case with small LEDs and fairly thin, clear plastic I can tell that the overall contribution of any sub-surface scattering to the overall illumination pattern is trivial compared to the caustics of the overall housing shape and dust and scratches.

Next question with this...I've got a translucent material that looks not bad, but I've got a lot of disco light effects in the renders from something being off in the photon settings...but darned if I know if it's the GI or caustics causing it. What exactly should the workflow be for sorting this out? It doesn't help that when I do an isolated test on part of the model that can render in less than an hour and get it looking smooth, once I add every else back in the solution no longer works.

Well it turns out that enabling caustic photons has no apparent effect on the rendered result, in this case anyway, except to greatly increase the render time. In trying to get rid of the splotchy GI I've cranked up the photon settings to the point where a non-regathered render of just the photons looks almost smooth enough on its own--and I'm talking a 235MB photon map here--but with regathering it's still garbage. Good grief...

I only use GI photons for interior renderings. Mainly because of the issues you've encountered with varied material types in combination scattering light splotches. This seems connected in my experiments to HDR environments as well as some shader types like Car Paint BAM and sample amounts in those shaders. Personally, my go to technique is almost always AO render cache and an HDRI in a GI environment. It's the most dependable. It's easy to burn time with so many controls trying to figure out what setting is causing what. Not gathering the GI photon map is great for speed if you can get away with it... the lighting however determines if this can work even with high GI sample rates in the luma server in my experience. 

Well I had a chance to look at this again and learned a couple things. I guess the help says this but could make it more clear that you don't want to mix caustic photons with QMC caustics the way you do GI. But, the only way to get illumination passing through materials in QMC AT ALL is to enable caustics--but that's not the case with photons. So armed with that I got the smoothest looking and probably closest to correct result using photons without regathering, but I'd like to figure out why regathering still looks like mud no matter how high I crank  the options.

After fiddling around trying to "simulate" this stuff, now I need a super-fake fast-enough-for-animation trick.

The Brightness material might be a quick approach. 

Does the brightness material differ in the effect it produces from the 'illumination' channel in an advanced material?

I'm not sure about that one Jim and would have to test it out. Please let me know if you come to a conclusion first. 

Well on my Neon test using the brightness shader turns the BAM pure white no matter what I set the level to...

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