I'm going to be compositing smaller videos / png sequences on a larger background image. Any
good effects for blending the edges in?
I tried Roughen Edges which helps.
I thought there was one that let you animate the feathering of the edges ut I can't seem to find it. Any advice on that (or similar simple methods to soften the edges while I bump on the alpha on incoming clips)?
You could just use a mask and animate the feather. Or you could animate the edge sharpness on the Roughen Edges effect.
Michael's right about simply feathering a mask drawn around the boundary of a layer.
Another way to blend the edges of one layer with an underlying layer is to use a light wrap. I don't know if this is what you're after, but it's a good tool to have in the utility chest. There are some links to some light wrap recipes on this page: "Compositing overview and resources"
Thanks to Szalam and Todd Kopriva -
Useful advice. Especially appreciate the links where I can get more info.