Fig. 2

Three features extracted from double peaks (waves) at a base position. L, M and R indicate the positions of the left bottom, the middle top and the right bottom of a wave, respectively. a The horizontal distance (|x ₁ − x ₄|). b The vertical height ratio (|y ₁|/|y ₄|). c The half-wave width ratio [(|y ₁ − y ₂| + |y ₁ − y ₃|)/(|y ₄ − y ₅| + |y ₄ − y ₆|)]