EdgeMix mixes the edge of an FG element with a BG Element, allowing you to control the expansion of the edge area, the 'general' mix and the Light Mix(mixing only the parts of the BG that are brighter than the FG element).
You can also Mask the area of interest and make a quick EdgeExtend. (As for example, if you want to put over a CG element a blurry car that was inside the original plate, you can mask the car, premultiply it and use EdgeMix to mix the blurry edges of the car with the CG element. By doing that, you can also bring back over the CG some other undesired element of the plate, such as the road. By using the EdgeExtend tab inside the gizmo you can avoid bringing up those elements. Anyway, the EdgeExtend tab inside the gizmo it's a bit rough, I would recommend you to use the one inside Nuke 12.0 or any other gizmo for that purpose, but It can give a quick result to test.)
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