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+The pivot, p, is defined as the point, within the convex hull of the
+contacts, which, after rotation and scaling, leaves the transformed
+contacts as close to the actual positions as possible.
+
+Let r_i be the starting points and s_i the actual ending points in a
+transformation. Let D be the scaling, and R the rotation. Then, minimizing
+
+L(p) = sum_i |D R (r_i - p) + p - s_i|^2 / N
+
+yields the pivot. Let
+
+rm = sum_i r_i / N,
+p = rm + u,
+q_i = s_i - rm - D R (r_i - rm),
+
+and we get
+
+L(p) = sum_i |(1 - D R) u - q_i|^2 / N.
+
+With
+
+L0 = sum_i norm2(q_i) / N,
+T = (1 - D R)' (1 - D R),
+m = sum_i q_i / N,
+
+we can write this as
+
+L(p) = L0 + u' T u - 2 m' (1 - D R) u.
+
+To handle the constraint, we can approximate the hull with a circle
+centered at rm. If we pick the average radius, P, the constraint becomes
+
+|u| < P.
+
+Relaxing [1] the expression (h >= 0) yields
+
+L(p, h) = L0 + u' T u - 2 m' (1 - D R) u + h (|u|^2 - P^2),
+
+leading to the linear equation
+
+(T + h) u = (1 - D R)' m.
+
+Further,
+
+sm = sum_i s_i / N,
+m = sum_i (s_i - rm - D R (r_i - rm)) / N = sm - rm,
+
+thus m is the average displacement. In words, the pivot is the average
+position plus a correction depending on the average displacement.
+
+*
+
+Some algebra solves the equation,
+
+D' = D,
+[D, R] = 0,
+R = S + C,
+S' = -S,
+C' = C,
+R + R' = 2 C,
+T = (1 - D R)' (1 - D R) = 1 + D^2 - 2 D C,
+
+which is a simple diagonal scaling operator. With
+
+a = 1 - D C,
+b = D S,
+
+we can write this as
+
+T = (1 - DC)^2 + D^2(1 - C^2) = (1 - DC)^2 + D^2 S^2 = a^2 + b^2.
+
+Similarly, we can write
+
+(1 - D R)' = ((a, b), (-b, a)),
+
+and thusly,
+
+u = Q(h) m,
+
+with
+
+Q(h) = ((a, b), (-b, a)) / (a^2 + b^2 + h).
+
+When D R = 1, it follows that a^2 + b^2 = 0, and the relaxation ensures
+that u is finite.
+
+*
+
+The drag is found by minimizing
+
+E(d) = sum_i | D R (r_i - p) + p + d - s_i |^2 / N,
+E(d) = d^2 + 2 d' ((1 - D R) u - m) + E0,
+
+which leads to the linear equation
+
+d = m - (1 - D R) u.
+
+Explicitly,
+
+d = m - (a ux - b uy, a uy + b ux).
+
+Inserting the expression for u yields, after some algebra,
+
+d = m (1 - (a^2 + b^2) / (a^2 + b^2 + h)).
+
+When h = 0, d = 0, as expected.
+
+When a^2 + b^2 = 0, d = m, also as expected.
+
+For constrained cases, the drag is a fraction of the average displacement.
+
+*
+
+Time to look at measures for the relaxation parameter. Since d depends on
+h, we can write the correction u(h) in terms of d instead. After som
+algebra,
+
+|u(h)| = (|m| - |d|) / sqrt(a^2 + b^2).
+
+Conversely, d(h) can be written in terms of the constrained u(h) as
+
+d(h) = m (1 - sqrt(a^2 + b^2) |u(h)| / |m|).
+
+Since |u(0)| = |m| / sqrt(a^2 + b^2), we obtain
+
+d(h) = m (1 - |u(h)| / |u(0)|).
+
+*
+
+We can now write down an explicit recipe for determining the pivot (p) and
+drag (d), given the transformation parameters a and b.
+
+w = (a mx + b my, a my - b mx).
+
+If |w| = 0, then u = 0. Consequently p = rm, d = m, and we are done. Else,
+
+u = w |m|^2 / |w|^2,
+
+t = P / |u|.
+
+If t >= 1, then p = rm + u, d = 0, and we are done. Else,
+
+p = rm + t u,
+d = (1 - t) m.
+
+[1] See Lagrange relaxation