Alpha Premultiplication
You may have experienced this at some point during your post production work when working with render passes. A dark line is surrounding your models and you have no idea why.
You're simply experiencing an alpha premultiplication issue, where the black background is factored into the pixel color before the alpha is applied, causing a dark pixel along the alpha edge. You can look at it as the background color is blending into the image where the alpha value is semi-transparent (between 0 and 100%). Alpha premultiplication applies to all passes with an anti-aliased alpha channel and can cause problems when compositing.
1.) Render the entire beauty pass as a whole. Don't break it up into elements, but rather render out masks for your various objects instead.
2.) Scale the render up 2-4x, disable anti-aliasing and scale it back down when compositing/editing. Because the alpha is no longer semi-transparent without anti-aliasing, the premultiplication issue will be omitted entirely.
You're simply experiencing an alpha premultiplication issue, where the black background is factored into the pixel color before the alpha is applied, causing a dark pixel along the alpha edge. You can look at it as the background color is blending into the image where the alpha value is semi-transparent (between 0 and 100%). Alpha premultiplication applies to all passes with an anti-aliased alpha channel and can cause problems when compositing.
1.) Render the entire beauty pass as a whole. Don't break it up into elements, but rather render out masks for your various objects instead.
2.) Scale the render up 2-4x, disable anti-aliasing and scale it back down when compositing/editing. Because the alpha is no longer semi-transparent without anti-aliasing, the premultiplication issue will be omitted entirely.