b481e886e5
This comes out of considering a one-dimensional transition in weight on a rectangular cloth grid. At the transition face loop, one side of each rectangular face would be scaled by k1, and the opposite one by k2, thus turning the rectangle into a trapezoid. Averaging would be used to choose the scale factor for the remaining two sides. If Geometrical Mean, i.e. sqrt(k1*k2) is used, it so happens that the diagonals of the trapezoid also end up scaled by sqrt(k1*k2) compared to the original rectangle. This means that the same scale factor is correct for both structural and shear springs, which is not the case with simple average.