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Eroding Alpha
CG inserts can stick out when the boundary between the CG effect and the background is too sharp. This is different than Matching Defocus, which applies to the interior of the effects.
Instead, you want to blur the alpha channel but only inside the existing area.
You don't want any non-zero alpha creeping outside the portion which has been rendered.
In Fusion, you can configure the Blur to blur only alpha. Then a Custom tool with i1=1-2*(1-a1), r=r1*i1, g=g1*i1, b=b1*i1, a=i1 will pull the alpha in while maintaining the premultiplying of Fusion's pipeline. (There's probably a better way; this is a sledgehammer solution.) Just a pixel or two of alpha blur will do the trick.
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