The combination of the boxes checked in the channel matrix create the list of channels that are output to the layer selected in the top dropdown menu on the right.
This four channel stream acts as the second set of outputs from the node. It allows you to output another four channels from the node, for a total of eight channels of output to match the possible eight channels of input.
In this case, this second set of outputs has not been utilized.
TIP: While not required, it’s good practice to use the first set of outputs for swapping channels in the current data stream and the second set of outputs for creating new channels. This protects the default rgba set from unintentional overwriting, and makes it easier for other artists to understand the workings of your script.
The basic process then for swapping channels is to first select your incoming layers from the 1 in and 2 in (or, in the case of the Shuffle node, the in 1 and in 2) dropdown menus. Then, select your outgoing layers from the dropdown menus on the right. Then make the actual channel swaps by clicking on the resulting matrix.
For example, to take the simplest case, suppose, you wanted to copy the red channel of an rgba set into its alpha channel. You would click on the matrix to create the following configuration.
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| Configuring the channel matrix. |
You can see that the matrix makes use of the r channel (standing for red) twice. It goes out once as the red channel, and another time as the alpha channel.