s by converting everything to a pixel representation (i.e., turn our vector graphics layer into raster graphics). What’s going on here is that the second argument to <code>with_mask()</code> is allowed to be a “registered filter”, or it has to actually be a raster object. I’m not doing either one! I’m trying to pass it a raw ggplot layer. That’s where the <code>as_reference()</code> function comes in. Its role is to take a layer and “register” it as a filter (the <code>id</code> argument is 