* Create two channels (Vertical and Horizontal, here) and fill them with a gradient.
* Right-click one of the channels, and "Channel to selection"
* RIght-click the other, and "Add|Subtract |Intersect to selection"
* "Select > Save to channel". You now have a thumbnail image of the result.
Attachment:
ChannelOps.png [ 21.3 KiB | Viewed 700 times ]
If you do the same with a pair of layers, and change the blend mode of the top layer, you get similar results, where
* Channel addition is layer addition
* Channel subtraction is layer subtraction
* Channel intersection is "Darken only"
Note however that layer blend modes are computed after being converted to "linear light" values while it is possible that the Channel values are applied directly (which is why you get similar shapes, but not the exact same result).