Screenspace mapping or to be more geeky Normalized Device Coordinates (NDC) mapping allows you to map a texture according the screenspace coordinates rather than use traditional UV coordinates.
The example below shows traditional UV mapping on the left and screenspace mapping on the right applied to a flat plane inside Maya (see middle for what the camera is seeing).
This technique was used in ye olden’ days (it started getting phased out around 2006-2008) inside Renderman shader to composite occlusion renders with beauty renders. The occlusion would be rendered out in a prepass and then composited during the beauty render.
You could also use this technique to do 2d compositing or even just general purpose image processing inside Maya.
The method of doing this is slightly different between Maya Software and Mental Ray, in order to do this in Mental Ray you need to use a mib_texture_vector and mib_texture_filter_lookup, the shading network looks like this…
Note that the camera should be the one your rendering from if you want the mapping in screenspace, otherwise this will act like a camera projection (it is a projection node). One final caveat with Maya Software is that you’ll need to delete the UVs on the geometry in order for this to work. If you want to switch between UVs and no UVs, apply a Delete UVs node and set the node behaviour to HasNoEffect when you want UVs and set it to Normal when you don’t want UVs.