How to fade 3d sprite when it approach to camera border

Question

I can't figure out how to modify this code to work correctly, I need that when the player turns the camera and the sprite approaches its border, the sprite gradually fades.I also need it to work regardless of how the camera moves.

    var viewport = get_viewport()
var screen_pos = camera.unproject_position(self.get_transform().origin)
var cent = camera.unproject_position((camera.get_global_transform().origin-Vector3(0,0, 0.8)))
var dist_from_center = screen_pos.distance_to(Vector2(viewport.size/2))

self.modulate.a = 1 - (dist_from_center / 1200)