Customizing Nuke with Gizmos - Nuke 101: Professional Compositing and Visual Effects - page 494

You'llusethisnodetodrawtheActionSafebox.Rightnowitisjustawhitebox,
and you only need its outline edge. For that you can insert an EdgeDetect node
afterRectangle1.NoticethatafteryoucreateanEdgeDetectnodeitwillbecalled
EdgeDetectWrapper in the DAG.
2. Add an EdgeDetect node from the Filter toolbox.
Not much different—or is it?
3. In the Viewer, look at the alpha ( FIGURE 12.2 ) .
Figure 12.2. What happened to the alpha here?
You can see that at the top and bottom you have lines, but the sides are all funny.
This is an odd thing that happens due to the bounding box. With the bounding
boxontheedgeoftherectangularshape,theEdgeDetectoperationdoesnotwork
properly.Youneedtomaketheboundingboxlargersoit'snotexactly attheedge
of the rectangle. You can use a Crop node for that as it will, by default, make the
bounding box the same size as the resolution of the image: