Use Background produces harsh edges between shadows and objects

[loganb]

I am Rendering a bunch of furniture to composite into a pre-existing room image.

All objects are placed on a polyplane with a Use background  Shader. 

When I render, it all looks great except the edges of objects that border the shadows are harsh, aliased, and dark.

How can I minimize this?

[joojaa]

Wild guess. its because the image is premutiplied alpha and the alphablending on the shading is acting on straight alpha, wich would kindof cause a exponential antialias on antialias.