  The advent of X-ray Free Electron Lasers promises the possibility to
determine the structure of individual particles such as microcrystallites,
viruses and biomolecules from single-shot diffraction snapshots obtained before
the particle is destroyed by the intense femtosecond pulse. This program
requires the ability to determine the orientation of the particle giving rise
to each snapshot at signal levels as low as ~10-2 photons/pixel. Two apparently
different approaches have recently demonstrated this capability. Here we show
they represent different implementations of the same fundamental approach, and
identify the primary factors limiting their performance.
