Radiation Transfer of Emission Lines in Curved Space-Time
  We present a general formulation for ray-tracing calculations in curved space-time. The formulation takes full account of relativistic effects in the photon transport and the relative motions of the emitters and the light-of-sight absorbing material. We apply the formulation to calculate the emission from accretion disks and tori around rotating black holes. 
Steven V. Fuerst and Kinwah Wu