Planar Tracker, soften edges

I can't for the life of me figure out how to soften the edges of a planar tracker.

Is this at all possible?

I should clarify, I have a corner pin from a planar tracker that is like the edges to be softer to help with blending into the composite.

You can't do it in the planar tracker itself. The way I'd do it is by softening the edges before feeding it to the planar tracker, by adding a matte control with a rectangular garbage mask with a soft edge.
Or if you need more control over your edges, create a double edged polygon to create point by point softness.

Thanks Sander,

Your approach worked, but, If you place anything to adjust the image such as a color balance or curves to help blend the image into the scene requires that you also create a soft mask on the adjustment. Not sure if this sounds be classed a bug but that adjustment affects the BG plate even though it's placed on the FG object.

If your fg element color changes change the bg you need to unpremultiply the image before grade and premult after again. There should be a predivide/postmultiply checkbox in color nodes that does this automatically.

Reason why it is needed (besides proper color handling in semitransparet areas) is that if you change rgb values in areas where alpha is zero (for example lift the blacks), it will have an additive effect when merged over bg. With unpremult and premult you force those pixels back to their proper values in relation to alpha.