Don`t know how many will remember my spaceship challenge - where I had a problem with rendering in layers and it was producing a black line around each part that I had to remove in Photoshop. AKA "pain in the ass"

Well I just found out 1) Why and 2) How to stop it. I thought it would be useful to share it here.

(Info from "The Area")

This issue is most evident when rendering an image with an alpha channel and then trying to use it in a compositing application.

The dark edging is due to the “Premultiply” option in mental ray found in the render settings window Quality tab > Framebuffer:

To remove this outer edge you can either:

A. In your compositing application, use the option to remove the outer edge. For example, in Photoshop you can do the following:
Layer > Matting > Remove Black Matte

B. or disable the Premultiply option before rendering with mental ray

Both methods will remove the undesired outer edge.