  We present a convariant formulation for radiative transfer in curved space
time and demonstrate some applications in the black-hole systems. We calculate
the emission from semi-transparent accretion tori around black holes, for
opacity provided by the Fe K lines and for opacity dominated by electron
scattering. We also calculate the emission from radiative inefficient accretion
flow in black holes with opacity provided by electron-positron annihilation
lines. Finally we show shadows cast by accreting black holes with different
spins and with different distribution of warm material around them.
