matte elements and refraction/reflection

[toby™]

Hi there...

I got a problem and don't know what I've might done wrong.

Scene
Several cars are standing in front of a glass facade in a parking lot.

What I want
I rendered the scene separately without trees and cars to have a better control of them in postproduction.

What I experienced
A render of the cars for example revealed my problem, I'm talking about. The cars and their shadows are rendered well and occur in my resulting Alpha Channel as expected when doing a matte rendering, but the cars' reflections aren't displayed in the Alpha Channel.

Does anyone around already encountered such a problem and does have a workaround?

I attached 3 files to furthermore illustrate my problem.

[stef.thomas]

The reflections are there in your alpha, just very faint. Try adjusting the exposure of the image and you'll see what I'm talking about.

[toby™]

Hum. Seems as if it's not working for me.

[stef.thomas]

There's definitely something there, look what happens when you adjust the levels in the alpha. It's there just very faint, not sure why.

[toby™]

That is what I'm not sure about, why this occurs. It's quite annoying.

Is there maybe another way of doing this?

[stef.thomas]

You could render out your reflections as a seperate render element then composite it.

[toby™]

I thought of this, but it isn't a very nice solution, if I've got the main image already rendered with all passes included.

I guess there won't be an easy workaround on this.

Thank you very much for your help so far, stef.thomas

[RErender]

What are you using to make the composition? what you want is to add the area of inverse alpha on top of your glass. Rendering the cars against black would be best too.

[dlparisi]

The reflections are there in your alpha, just very faint. Try adjusting the exposure of the image and you'll see what I'm talking about.

No they're not. Reflection alpha has never been supported AFAIK. You'll need to render out a matte pass (self illuminated white objects over black with black glass for your regular glass) if you want to get an alpha for the refelction. It's not really necessary though...

Rendering the cars against black would be best too.

You'll get better edges with this and you can simply add (linear dodge in PS) the reflections over the glass that's already there. This really won't be correct though since you already have the reflection of the pavement on the glass. Should be pretty minor though.

[toby™]

I went the hard way on this

Rendered the whole image with all trees and cars but with an additional VRayReflection Element and the image with no trees 'n' cars, to have a better control of the reflection and trees/cars.

Thank you for all your help and suggestions.