Figure 4
The figure has two panels labeled “(a)” and “(b)”. In panel (a), two transparent vertical cylinders are shown. In the left cylinder, a group of yellow points is scattered in the upper half and a group of blue points in the lower half, illustrating two separated point sets. In the right cylinder, a straight gray centerline is drawn from top to bottom with a vertical double‑headed arrow beside it labeled “Spatial Error”. Curved yellow and blue lines wrap around this centerline, suggesting the measured positions of the two-point sets relative to the ideal axis. Panel (b) is a Cartesian plot with the horizontal axis labeled “Points Distribution [meters]”, ranging from 0.275 to 0.475 with an interval of 0.025, and the vertical axis labeled “Point Count”, ranging from 0.0 to 15.0 with an interval of 2.5. Two smooth bell‑shaped curves are plotted: a yellow curve, spanning between (0.277, 0.0) and (0.457, 0.0), peaked to the left at (0.370, 13), and a blue curve, spanning between (0.325, 0.0) and (0.475, 0.0), peaked to the right at (0.400, 16.9). A horizontal double‑headed line labeled “Spatial Error” spans the distance between the peaks of the two curve.Implementation of M3C2, (a) shows the selection of points and the projection of points on the surface normal to compute spatial error; (b) illustrates the computation of spatial error