Rendering in Layers?

[execpres]

I have a night time road scene that I have rendered as a background.

Now I have two cars that I want to render separately so I can composite them.

My first question is how to handle the headlights that are illuminating the road. I want to render the lights to be on the same layer as the cars (or at least another layer separate from the background).

Number two question, is how can I do this and maintain the reflections in the car materials?

Is it possible?

Is it worth it?

[dbuchhofer]

Gonna toss out some brain droppings

Well.. you can break it up a bunch of ways.. one long way that gives a lot of leeway in the comp stage for someone..

You can render your background in the 'night' scene..

Render a version of the background that is much brighter.. almost daytime..

Create a .. Volume? so to speak that will be your light cone.. that you can use as a mask to break through to the bright scene.. I guess one way to do it would be to use a Volume light as the headlights with a white volume and everything else in the scene set to matte -1 alpha.. (Atmospheric pass i guess would be the term)

Another way to work it since i think the above method wouldn't work too well on second thinking of it.. make a Standard material.. in the Diffuse color add a Falloff map, Set the falloff type to 'shadow/light' have a white material, and a matte shadow vraywrapper in the black slot.. (Can you use that in a map slot? i forget?) Render a pass will this on everything.. use this as a mask between the Bright version/dark version in your compositing app of choice..

Also dont forget shadows for the cars if you plan to split off to a seperate pass.. You can get these a couple ways.. the one Richard talked about a few weeks ago in the Animation thread whereby you render a diffuse white pass, and a diffuse white pass with shadows.. then you use a difference map between the two to get your shadows seperate.. (Long way, but works, and no worries about self shadowing..). Or you could go the Matte route if you dont have much that requires self shadows.. you can just have the shadows checkbox in the render 'object settings' dialog box on, set your shadow color, and set it to affect alpha.. (This is on the objects that will recieve shadows.. you may have to turn shadow casting OFF on these objects on a case by case basis in the properties right click menu..)

Couple of the tricks for layered renderings.. theres so many more its not even funny

[dbuchhofer]

Also.. is it worth it question...

If the camera movements aren't going to change, yes !

if you're working with things that aren't planned out beforehand well then you should probably stay away from rendering out in large numbers of layers..

[execpres]

Thanks... those sound like good tricks -- but in my case definitely not worth the trouble.

However, I don't see how either method would solve the reflections issue?

[dbuchhofer]

There is no reflections issue.. reflections work fine with matte objects..

(Hell, its hard to turn them OFF)