In Cycles, if I use a spotlight object to illuminate a matte object, the matte surface is well behaved with little noise. If I explicitly model a spotlight, the cycles renderer has some terrible caustic noise, owing to the obvious fact that a spotlight is a very small, very bright source of light. If cycles raytracing uses a universal light path following solver, then a spotlight object should have all of the same noise issues present in a discrete model. It does not, how does it achieve this?
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$\begingroup$ The lamp object set to Spotlight sends out light rays in limited defined directions, whereas a modeled spotlight - although you didn't exactly explain how you modeled it, but I would guess some kind of point light in a cone or simliar shaped object to reflect the flight in a certain direction - has a lot of scattered light in different directions to calculate. That's how I would suppose it's working. Also a lamp set to Sun sends a directional light in a defined direction, it's not a giant emissive sphere very far away from the center which would surely be harder to render as well. $\endgroup$– Gordon BrinkmannCommented Nov 30, 2022 at 19:26
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$\begingroup$ So the curious thing about that is that cycles 'traces light backwards' from the camera, not forward from the light source. All I can think is that light sources are a special case where the first bounce is sent directly to the receiving material, which itself becomes the source for back-traced light...? $\endgroup$– J CollinsCommented Nov 30, 2022 at 20:23
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$\begingroup$ Well, more or less, I think there are more sophisticated videos on Youtube explaining raytracing engines then what I could tell you here in 600 characters per comment and actually this site is to help with problems using Blender, not educating about render engines. $\endgroup$– Gordon BrinkmannCommented Nov 30, 2022 at 22:34
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1 Answer
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Modeling and using lighting components such as lenses and reflectors adds complexity to the scene and will induce probabilistic effects on all of the traced rays.
Additionally, there are a couple of default render settings that decrease the amount of light added to the scene by transmission and glossy rays. (Indirect Light and Filter Glossy in this image)
In many cases, if you are willing to dedicate the machine-time, the image will eventually resolve to something that looks better and more accurate than a quick workaround, but this is one of those times where you have to decide if it's worth it to you.
Some suggestions:
- Just use a "spotlight" light object and forget the rest.
- If you need glass on the model, use the old transparency workaround.
- Consider using an IES lighting texture, which should be able to give you some refraction and feathering effects that look more natural.
answered Nov 30, 2022 at 20:04
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$\begingroup$ The reason I'm curious about how spotlights work is that they seem so much more efficient. The problem is that a real headlight has a very complex projection pattern from a field of effective emission sites. What I'd like is something like setting up a headlight in isolation, and kind of 'record' the projection pattern, and bring the 'recorded' light into a full model, where the projected light might be made up of say 1000 spotlights. Also I haven't seen the 'is shadow ray', will have to look up what that and the IES light textures do. $\endgroup$ Commented Nov 30, 2022 at 20:27
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$\begingroup$ As I said in my other comment above, the reason a Spotlight lamp object is much more efficient is that it's a simplified, for a certain use specialized light source. And as you say yourself, a real headlight is much more complex. No light source in real life is a simplified version of a light. Real light sources coming from light bulbs always emit light in all directions, only lenses, reflectors and casings create specific light patterns, and not only for 200 or 1000 samples but billions of photons. Your computer will not be able to calculate this in a reasonable amount of time, if ever. $\endgroup$ Commented Nov 30, 2022 at 22:42
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$\begingroup$ @GordonBrinkmann Could you point me to something about how the raytracer is actually working for spotlights and what shortcuts it is using? I can't seem to find anything. I'm thinking even a handful of spotlights in a line each projecting a canned image might be a good answer. $\endgroup$ Commented Nov 30, 2022 at 23:43
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$\begingroup$ @JCollins No I can't, I'm not a developer and have no deeper knowledge of how it works, and I don't think knowing the technical implementation of a raytracing engine will provide a solution on how to make a good headlight. Read the article about raytracing in Wikipedia if you think you need to know that, but that's like asking a physicist how spectral absorption of visible light works in order to find out how to paint a wall blue. Knowing how raytracing works is not explaining the different implementations of light sources in 3D softwares. $\endgroup$ Commented Nov 30, 2022 at 23:58