Use the Matte Edge effect to create an image based on the detected edges, or use it to modify the edges of a matte to help in creating a key.
To access the Matte Edge menu, use:
Timeline, then use ConnectFX (See Creating ConnectFX).
Tools, then select from the menu.
The Modular Keyer, then select a node from the Node bin.
This node accepts a front and matte clip as inputs, and outputs a result. You can add the Matte Edge node to any part of the pipeline except the Front pipe of the CBLend node. The Matte Edge node is not appropriate for this pipe because its result is a matte.