Color Correction - Nuke 101: Professional Compositing and Visual Effects - page 172

The composite is almost ready. You just need to punch a hole in the foreground
car so it appears to be behind the snow that's piling on the windshield. For that,
you'll bring another image in (you will learn how to creates mattes yourself in
Chapter 6 ) .
6. From your chapter04 folder bring in Windshield.png and display it in the Viewer.
Here you can see this is a matte of the snow. It is a four-channel image with the
same image in the R, G, B, and alpha. You need to use this image to punch a hole
in your foreground branch. To do that you will need another Merge node.
7. Select Read3 and insert a Merge node after it.
8. Drag Merge2 on the pipe between Read1 and Merge1 until the pipe highlights.
When it does, release the mouse button to insert Merge2 on that pipe ( FIGURE
4.22 ) .
Figure 4.22. Inserting a node on an existing pipe.