The UVW coordinates of samples at a location in a texture projection.
If you get this value on a regular UV projection, the values of (U, V, 0.0) are returned. If you set this attribute on a regular UV projection, the W value is ignored.
If you get or set this value on a homogeneous (camera) projection, the values correspond to (U, V, q) where q is the distance to the camera.
