Since each channel has the shape of a straight line, 3-D linear regression was performed on the centroid positions. A line in 3-D space can be described by the following vector equation:
where 
is a point
vector belonging to the line, 
a vector parallel to the line
and t a free parameter allowing a position on the line to be
specified. 
, 
and 
are the usual unit vectors
in the x, y and z directions respectively. If line is
written as:
standard formulae of 2-D linear regression can be used, given N 3-D
points 
, 
to be fitted, to
find 
. The vectorial equation of the line is then:
