  In phase unwrapping, the locations and densities of residues are indicative
of the severity of the unwrapping problem. The residues are used to detect and
evade inconsistent phase areas. Gdeisat et al. proposed an algorithm to
increase the number of residues in a wrapped-phase map to improve the results
of phase unwrapping. But this method will take much time to make the Fourier
transform, inverse Fourier transform, select and shift the spectral components,
and there is no theoretical analysis on why the frequency shift can increase
the number of residues. In view of the above problems, we proposed an algorithm
to increase the number of residues in a wrapped-phase map, which only uses a
simple multiply operation in spatial domain to realize frequency shift by
taking advantage of the frequency shift property of Fourier transform. Besides
that, we discuss the relationship between the number of residues and frequency
shift. Finally, the experimental evaluation is conducted to prove the validity
of the proposed method. Experimental results demonstrated that the proposed
method can speed up more than 50%.
