Figure 9. Momentum of inertia (I) applied to vertically orientate the particle on the computer monitor. The vector r_i denotes the vector from the point of rotation (=center of gravity) to a pixel with grey level G_i at image coordinates x_i and y_i. In a binarized image G_i is 255 (white) inside the object and 0 (black) outside the object. Alpha is the angle between the longest axis of the object and the x-axis of the image. The momentum of inertia is a periodic function of alpha: It is minimal if alpha is 0°, 90°, 180°, 270°, etc. (i.e., if the longest axis is parallel to one of the coordinate axes). The momentum of inertia I reaches a maximum if alpha is 45°, 135°, etc. (i.e., if the longest axis is in a diagonal position). Due to the pre-orientation of the shells in the slide, the longest axis is always located in the upper sector between the two red lines.