This youtube clip says that the original Invisible Man (1930s) had parts of his body (like face) wrapped in black velvet, and was filmed against a black background, in order to render those parts invisible.
I don't understand how this short description works. What really is going on in post-editing for them to have achieved this partial invisibility effect? Is it just that the black-velvet against a black background is invisible. But what explains the actual scene's background being an actual scene setting (not a black background)
And how does this trick correspond step-by-step in editing to the use of "green men" as done in Invisible Man (2020), also described but at the start of the clip