  Farin proposed a method for designing Bezier curves with monotonic curvature
and torsion. Such curves are relevant in design due to their aesthetic shape.
The method relies on applying a matrix M to the first edge of the control
polygon of the curve in order to obtain by iteration the remaining edges. With
this method, sufficient conditions on the matrix $M$ are provided, which lead
to the definition of Class A curves, generalising a previous result by Mineur
et al for plane curves with M being the composition of a dilatation and a
rotation. However, Cao and Wang have shown counterexamples for such conditions.
In this paper, we revisit Farin's idea of using the subdivision algorithm to
relate the curvature at every point of the curve to the curvature at the
initial point in order to produce a closed formula for the curvature of planar
curves in terms of the eigenvalues of the matrix M and the seed vector for the
curve, the first edge of the control polygon. Moreover, we give new conditions
in order to produce planar curves with monotonic curvature. The main difference
is that we do not require our conditions on the eigenvalues to be preserved
under subdivision of the curve. This facilitates giving a unified derivation of
the existing results and obtain more general results in the planar case.
