This fact together with some kind of support constraint can be used in order to iteratively calculate the phases.
Unlike the error reduction algorithm which alternately applies Fourier and object constraints the HIO "skips" the object domain step and replaces it with negative feedback acting upon the previous solution.
The HIO differs from error reduction only in one step but this is enough to reduce this problem significantly.
In crystallography, the scientist is seldom interested in the atomic coordinates relative to any other reference than the molecule itself and is therefore more than happy with a solution that is upside-down of flipped from the actual image.
Other methods of phasing a coherent diffraction pattern include difference map algorithm and "relaxed averaged alternating reflections" or RAAR.