Edge allows you to extend the proper color out over transparent edges or areas with a lot of motionblur without eroding away the nice detail. This edge extend has some more advanced features like splitting the core matte and final key which allows you to work on motion blur without effecting small details like hair or a swinging rope for example. There is also more control over how you blend the core detail to the extended edge color. This was shown to me years ago by an old comp sup I had and is the same as the edge extend by Frank Reuter. This is my version of it that has evolved a little bit over the years.
There are 4 inputs in the Edge Gizmo
BG – This is where your Background goes if you are using the node as Final Result
FG – Foreground plate with Spill suppression applied
Core – This is like a core matte, it helps identify the leading edge and where the edge tool should start from. It is important to add this core so that small details are not lost like a thin rope or hair.
Final Key – This is your Key with fine hair details and transparent motion blurred edges. For Roto you want to have motion blur turned on.
The first setting you will see in the properties panel on the right is the Operation selector. Here you can choose if you want to use the Edge Tool to merge together your foreground and background giving you the Final Result or if you would like the Intermediate Result which is unpremultiplied and not merged over the background. The Intermediate Result can be helpful when tweaking parameters like the edgegrow start and edgegrow size.
edge start – determines where you would like to start bleeding out the color from the inside of your key. It does this with a series of unpremult, blur, premult operations instead of a standard erode.
edge size – sets how large the edge with the color bleeding will be from the edge grow start or core matte. This should be just big enough to reach the edge of the largest areas of motionblur.
edge blend – also helps soften the transition bringing back original detail. This can sometimes bring back some lighting information instead of having it completely replaced.
edge blur – helps to soften the transition from the original to the new edge. It tends to erode the bleeding color back inwards if you use high values.
edge smooth – helps with extremely large motionblur and completely takes away detail. This should not be used in most cases but if you do use it sparingly.
The color panel is used for bleeding color from the background through the semi transparent edges, like a color wrap instead of a light wrap. You can alter the edge you created in the controls panel and the color that is input from the BG.
In practice you should only apply Edge to the areas that you need it. With edges that come close enough to each other you can get issues with different areas bleeding together. By using a keymix you can isolate the areas that need it most. After you are finished tweaking the settings you can output a matte generated inside the tool called edge.matte to regrain the edge.