# MomentOfInertia Plugin¶

MomentOfInertia plugin keeps up-to-date tensor of inertia for every cell. Internally, it uses parallel axis theorem to calculate most up-to-date tensor of inertia. While, the plugin can be called directly:

<Plugin Name="MomentOfInertia"/>


most commonly it is called indirectly by other plugins like e.g. LengthConstraint plugin.

MomentOfInertia plugin gives users access (via Python scripting) to current lengths of cell’s semiaxes. Examples in Demos/PluginDemos/MomentOfInertia demonstrate how to get lengths of semiaxes. For example, to get semiaxes lengths for a given cell, in Python we would type:

axes=self.momentOfInertiaPlugin.getSemiaxes(cell)


axes is a 3-component vector with 0th element being length of minor axis, 1st – length of median axis (which is set to 0 in 2D) and 2nd element indicating the length of major semiaxis.

Note

Important: Because calculating lengths of semiaxes involves quite a few of floating point operations it may happen (usually on hexagonal lattice) that for cells composed of 1, 2, or 3 pixels one moment the square of one of the semiaxes may end up being slightly negative leading to NaN (not a number) length. This is due to round-off error and whenever CC3D detects very small absolute value of square of the length of semiaxes (10-6) it sets length of this semiaxes to 0.0 regardless whether the squared value is positive or negative. However, it is a good practice to test whether the length of semiaxis is sane by adding a simple if statement as shown below (here we show how to test for a NaN):

if length != length:
print "length is NaN":
else:
print "length is a proper floating point number"