In each frame, length of a MT is estimated by the Euclidean distance between a fixed point on the MT, called the origin, and the MT tip, (a). Shortening length between two consecutive frames is calculated as the difference of respective lengths. This estimate may not reflect the actual shortening as shown in (b).
Altınok et al. BMC Cell Biology 2007 8(Suppl 1):S4 doi:10.1186/1471-2121-8-S1-S4