The particle picking centering method
The so-called center of gravity method where the image is scanned and
the intensity peaks calculated to find the center of the particle 'mass'.
The image is then shifted by the correct amounts to maximize the number
of peaks near the center.
Another centering method averages all the picked particles together, and
then cross correlates each individual particle to the average. The cross
correlation function is used to determine by how much to shift each particle
when trying to center it. When all the particles have been cross correlated
to the average and shifted, a new average is generated. Once again, all the
particles are compared to the new average, and shifted as necessary to
center them as best as possible. This iterative process is repeated until no
significant shift is necessary for all the particles Crosscorrelating to a
global average is but one variation on this theme. Similar methods also use
an external model or a rotational average of the particle itself as the
centering reference. Unfortunately, it can be difficult to obtain a
reasonable external refernce, so a global average or a rotational average
are most often used.