Volume lighting depending on the number of lights and the complexity of the scene, can be very compute intensive and require longer render times. To help you avoid some frustration when working with volume effects, I am writing this short blog on how to setup up a render using many lights with volume effects. The first step, is to get an idea of how the volume effects will appear in your scene, as quickly as possible. What you should avoid doing, is trying to render the complete scene until you have the volume effects looking decent, or at least in the ballpark.
Turn off all the geometry and render the camera view that you intend to use for the final render, you can quickly (in a matter of seconds) see how the volume effects will look. It is okay to have some geometry, for instance in my example, I am using a bridge with 43 lights, and I can render the scene with all the light fixtures in just 7 seconds and without the fixtures only 2 seconds. I might as well confess up front, that I am rendering on HP's new Z600, dual quad core worksation that just screams with Luxology's multi-threaded render engine.
Start with a lower number of samples to speed up the render, you can get a good idea with 10 samples and adjust upward for the final render.
3 Meter Radius on Volume Effect above using 10 Samples (2 seconds to render).
With 30 Meter Radius 10 Samples above (2 seconds to render) I am using IES lights, so the spread of light is determined by the IES.... how cool!!
The distance from the light fixture to the highway below is about 30 Meters, I think this looks much better than 1 meter, so now I'll turn on the light fixtures to see how that looks.
7 Seconds to render using a 30 Meter Radius for the effect and with the light fixtures visible.
The noise is a little distracting in the volume effects closest to the camera, so I'll make the samples higher on the lights those lights. The distance from the light fixture to the highway below is about 30 Meters, and I think this looks much better than 1 meter so I will use this for my final render.
You can see in the Light Manager the Settings being tried above.
For best rendering performance the Lights in the distance can use lower samples, you can make the lights close to camera have higher samples and then progressively lower samples as lights become farther away from the camera.
The above images shows how I adjusted the samples with the nearest light set to 60, the next two are 40, then 20 for the 5th light back. After the 5th light back 10 samples seems adequate, so all the lights beyond the 5th light use 10 samples.
I also changes the Scatter Color to a yellowish color as you can see in the above capture.
Turn off volume effects on the lights you can't see, so in this scene all the lights behind the camera. You can see 5 lights behind the camera in the below capture.
Now that I have my light's volume effect the right size, I prefer on more subtle effect, so I made the Density and Scattering both 5.0 instead of 50 for the final render.
Don't use fine or very fine shadows when you have a lot of lights, you can. but it could just be a waste of time.
The above image used Soft Fine and had a render time 12 Minutes 42 Seconds (HP Z600 dual quad core Nehalem processor).
In the above image with shadows set to Coarse, the render time was only 3 Minutes 10 Seconds.
Are the soft fine shadows worth the extra render time (In this case I don't think so)?
I hope you find this information useful and imformative.