Fig. 3

Displacement measurements determined using Adobe Photoshop software. a The measurement scale; P₁, the intersection point of a marker line and the edge vertex of a scale line, with a length of 1 cm on the steel ruler; P₂, the intersection point of a marker line and the edge vertex of a scale line, with a length of 2 cm on the steel ruler. b The center point; 0, the origin point; 1, the upper edge of the positioning needle; 2, the left edge of the needle on the left vertical ruler; 3, tangent to the lower edge of the needle; 4, the horizontal centerline; 5, the left vertical ruler tangent to the right edge of the needle; 6, the vertical centerline; Q, the center of the circular needle. c The measured distance between two points. d The overall view