How to set up a Mask Render Pass?

[Jess_Bradmon]

How does one configure the mask render pass?

For example if I want to mask out (black out) everything in the render output but the steering wheel?

How do you set the steering wheel to be visible and everything else to be black in the mask render pass?

[michael_nikelsky]

You should use the RenderLayer for this: Just add the objects want to mask out to a renderlayer, set the visible in Alpha to false and apply the renderlayer. 

However, when using this to combine objects to a final rendering don´t even try to do it in photoshop, it will not work. Use a compositing tool like Nuke, which can handle these kind of composites.