invisiobj
    // The invisobj class will still be tangible, but just
    // not visible. That is, it can only be seen by certain
    // mobs, but it will still interact with the overworld
    // if it is dense.

    parent_type = /obj

    var
        // The list of mobs this is displayed to. They are
        // actually displayed to clients, but we'll just use
        // this for simplicity.
        list/displayed[0]

        image/image

    New(atom/loc)
        ..(loc)

        image = new /image(src.icon, loc, src.icon_state)

        icon = null
        icon_state = null

    Move(...)
        ..()

        // Everytime the /invisiobj moves via the Move()
        // method, the image's data is updated to support
        // this change.

        image.loc = src.loc
        image.pixel_x = src.pixel_x
        image.pixel_y = src.pixel_y
        image.pixel_z = src.pixel_z
        image.dir = src.dir

    proc
        // Now some events and event handler stuff.

        Display(mob/m)
            src.displayed += m
            m << src.image

            src.OnDisplay(mob/m)

        UnDisplay(mob/m)
            src.displayed -= m
            if(m.client)
                m.client.images -= src.image

            src.OnUnDisplay(mob/m)

        OnDisplay(mob/m)

        OnUnDisplay(mob/m)

mob
    Overlay/my_overlay
    
    Login()
        ..()
        
        // attach an overlay to the mob
        my_overlay = overlay("icon state")
        
        // make it only shown to this mob
        my_overlay.ShowTo(src)