Hasta La Motion!

Perhaps I’m jumping the gun by asking this question, as I’ve only been using nuke for a few days, but why don’t the read and constant nodes have mask inputs? I’m coming from an After Effects/Fusion background and doing something like cutting out a solid color or an imported image with a mask is very straight forward. I was able to successfully cut out a ‘rectangle’ node with a bezier, as it does have a mask input, but it looks like any node under the ‘image’ menu is output only. Could someone describe how they might accomplish something like this in Nuke?

Thanks..