Emissive question

benniewoodell

So I'm wondering if there's something I don't know about regarding emissives and rendering. Whenever I use an environment that has lights preloaded as emissives, even bringing them down to like 10 and changing the tone mapping of the f stop and ISO, it still takes a long time to render, whereas the same scene if I put spotlights where the emissives lights are it takes a couple of minutes to render. So obviously that works and that's what I've been doing for the past year or so, but it would be great to be able to use the preloaded lights sometimes and I'm wondering if maybe there's something I don't know. And this isn't isolated to one render, it's everything I use. Anyone have any tricks or is this just the way it is and I should stick with reworking all lights as spotlights?

JonnyRay

When an object has an emissive surface, Iray has to calculate light for every face of the surface. For example, if I create a light bulb that has 200 faces on the bulb part, and I make that surface emissive, Iray calculates the light as if there were 200 individual light emitting faces in the scene.

On the other hand, in your spotlight example, there's only one light source to be calculated; so it works much faster.

This is why products like KindredArt's Iray Ghost Light Kit 2 will render faster than similar props that are using emissive surfaces on their geometries rather than "faking" it.

Gogger

Plenty of light equals faster render. Low light and the renderer has to work harder.  As with anything, there is a limit and too many lights can work against you too.  You might be surprised how fast things can render with HDRI light sources.  And, as always things in your scene affect rendertimes too. 

benniewoodell

Oh, thank you both for the information! That makes so much more sense now if it's calculating each face why that's shorter with the spotlight. I'll look into using the ghost lights too now and keep spotlights going. I might be more apt to waiting for the emissives because they always look so great, but I do a lot of animation so I try to keep frames for all the layers of a particular shot under six minutes, three is always my goal. I also didn't know more light made a quicker render, I would have thought the opposite, but now that I'm thinking about it, I could see how that makes it longer. I have my two favorite HDRI's for when my scenes are outdoors that give wonderful glows, but a lot of my scenes for the animation projects I'm doing are inside and at night lol. I do have one moonlight HDRI that I absolutely love from this pack https://www.daz3d.com/ig-iray-dawn-to-dusk-2-atmospheric-lighting it gives a stellar moonlight and you're right, when I place the character directly where the moon hits, these renders do go much quicker. Maybe I'll turn off the roof and try more HDRI and see how that works. I appreciate your time!

nonesuch00

Practically speaking the way emissive lights are being calculated is wrong. While the number of surfaces is relevant for refraction, reflection & such, practically speaking,  there is only one light source there, especially given the speed of light and the close proximity of the emissive surfaces, and it should be calculated as such. I know you can see individual LEDs in LEDs lights but the light is practically speaking arriving everywhere else in the room at the same time. I don't know if those calculations are part of the nVidia iRAY SDK or DAZ Studio is setting those light sources up but internally bordering emissive surfaces should  be subbed out by an appropriate singular light source like a point or spot light.