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. 2021 Jan 13;25(7):4635–4642. doi: 10.1007/s00784-021-03777-z

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© The Author(s) 2021

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Fig. 2 — Representation of the measurement of linear distances (D1_2, D1_3, D1_4, D2_3, D2_4, and D3_4) between the centers of the four spheres (left) and measurement of the angle between the normal vectors (right)