This is a multi-prong question that may end up being a bit of a feature request.
Quite frequently in my work, and teaching, I want to use brightness materials to show the actual light bulbs in a scene, and/or to serve as a kind of material ID render pass (I give various materials different brightness materials with a variety of colors). The brightness material is useful this way because generally it renders a matte color that is easy to select and work with later in PHotoshop.
In both these cases (though they are very different), I actually *don't want any light* to come out of the brightness material... I just want it to be pure white (or whatever color). Then, in the case where I'm representing real lighting, I put in a Rectangular Light or whatever is appropriate to simulate the actual lighting. This gives me a significantly faster and smoother rendering than if I were relying on the brightness material to actually light the scene.
This works in Brazil, but with apparent flaws: I turn the 'amount' on the brightness material to zero, but unfortunately, *light still leaks out*. I don't seem to have a performance overhead for calculating that (see my following example), but I get some grain or blotches in the image that I wouldn't have with just the rectangular light working. Here are my questions:
1. Shouldn't an 'amount' of zero mean that NO light be projected from the object? Could this be fixed?
2. Is there any way to get a brightness material to generate photons?
3. Is there another good way to do the 'material ID' pass I described above? E.g. could I make a matte material with a solid color in Brazil that would not actually slow down the rendering (and not bleed color all over the rest of the scene)?
Thanks for any help. I've attached four renderings:
One is with just a brightness material, at 'amount' = 15. Rendering time: 2 mins. 24 secs.
Second is with just a rectangular light. Rendering time = 1 min 38 s.
Third is with a rectangular light and the brightness material at 'amount' = 0: time = 1 min 38 s again, but now the image is grainy...
I also tried one with just the b. material, at 'amount' = 0. It took 59 s and there was a tiny bit of grainy light in the scene.
Thanks for any recommendations!
1- If you use an amount of zero and a color of black no light will be cast from the brightness material.
2- No... unfortunately not.
3- Yes, try the "Brazil Tint" material and then disable all but the "Primary camera rays" and "Renderable" options in the Brazil section of object properties for the selections assigned this Tint material. I think it's what you're looking for.
The grain artifacts of rendering brightness materials can be smoothed by raising AA and GI sample rates but this will add time of course. Using the Render Cache is the quickest way to smooth things out quickly but I suggest a decent AA and sample rate for the GI in unison with the cache for best results.
Brian, thank-you for the helpful information! The Tint material is GREAT - though a proper material ID channel in the rendering would be much easier to use, of course (since this way I have to re-assign all materials...).
Your answer to #1 is a little surprising - I guess because the Tint material exists, the issue I'm raising does not really matter, but on principle I still think a Brightness material with amount 0 should not emit any light! Anyway, I realize now I could use a white Tint material for those bulbs, also, and get an even faster rendering, so if this is a bug, it's a very minor one..