Fig 1. A wound shape with a marked small area and the center of gravity of this shape.

The small area is at a distance y₁ from the line segment a, and at a distance y₂ from the line segment b. The center of gravity is at a distance y_a from the line segment a, and at a distance y_b from the line segment b. These line segments are used for calculation the calibration coefficients k_a and k_b as number of pixels per 1 cm at ruler A and B, respectively.