Animating Car Headlights

[efx2004]

We are starting a new animation of a car inside a tunnel. I plan to render the car and it's headlight as a second pass. My question is how to composite only headlight's effect to pre-rendered backgroud.
Thanks for help in advance.

[joconnell]

If you make everything in the scene a white material and have only the headlights turned on, you'll end up with a matte of the headlights effect on the scene. You can then use this in any comping package to add over the background or use it as a mask to colour correct the background to brighten it up and add a bit of yellow in the areas affected by the headlights.

[efx2004]

Thanks for help.
In this case, we are going to have more control over the lighting but it doubles the render time.
Is it a good idea to comp BG and FG or do you recommend single pass?
Actually the background is not so complicated.

[joconnell]

Are you putting a cg car into footage of a real tunnel? I'd definitely do everything as seperate passes then - it does make things more comlicated in terms of scene setup but it gives you more options later on.

[efx2004]

Tunnel is CG.
To have more control is good but render speed has #1 priority for me. Adding lights as a second pass would take more time I guess.

[joconnell]

Well, admittedly it wouldnt be as correct but you could render the tunnel as one pass with a cached irmap and light cache file and then do the car render as a seperate pass - might go quicker and have less flicker issues.

[efx2004]

You are right. I didn't think flicker issues. I'll try separate passes way.
Thank you for all your help.
Ismail