  Wavefunction-splitting or mask method, widely used in the non-relativistic
calculations of the photoelectron angular distributions, is extended to the
relativistic domain within the dipole approximation. Since the closed-form
expressions for the relativistic Volkov states are not available within the
dipole approximation, we build such states numerically solving a single
second-order differential equation. We calculate the photoelectron energy
spectra and angular distributions for highly charged ions under different
ionization regimes with both the direct and the relativistic mask methods. We
show that the relativistic mask method works very well and reproduces the
electron energy and angular distributions calculated by the direct method in
the energy range where both methods can be used. On the other hand, the
relativistic mask method can be applied for longer laser pulses and/or higher
photoelectron energies where the direct method may have difficulties.
