Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 76

You need to move the red channel of the mask image so you'll have a copy of it in the
alpha channel. Using the check boxes, you can tell the Shuffle node to output red into the
alpha.
1. Select Read3, and in the Channel toolbox click Shuffle.
A new node called Shuffle1 has been inserted between Read3 and Merge2
( FIGURE 2.47 ) . Shuffle1's Properties panel was automatically loaded into the
Properties Bin.
Figure 2.47. Shuffle1 is now inserted after Read3.
2. In Shuffle1's Properties panel, check all the red boxes on the very left column, as
shown in FIGURE 2.48 . This places the R channel in the R, G, B, and alpha chan-
nels.
Figure 2.48. Setting up the Shuffle node to copy the red channel into all the
four channels.