Schematic pictures of photon propagation in the jet with the velocity v c (left) and v  ~  c (right). When v c, the scatterings of the photons are approximately isotropic in the observer frame, and the surface of τ_* = 1 is located far from the surface of τ_a = 1, where τ_a is an optical depth in the absence of the scatterings. When v  ~  c, the photons are concentrated to the direction of the flow. Thus, the surface of τ_* = 1 is located close to the surface of τ_a = 1.