[ft-devel] preliminary analysis of the problem with cubic spline optimisation

[Graham Asher]

The aim is to draw a straight line between the start end end of a cubic 
spline, instead of rendering the curve correctly, if the curve deviates 
by less than a certain value from the straight line. The code in 
gray_render_cubic in ftgrays.c checks only the midpoint of the curve. 
However, this is not necessarily the furthest point from the straight 
line. If the sequence start, control1, control2, end is not monotonic in 
one of the dimensions x, y, an s-shaped curve is produced with points 
further from the line than the midpoint, which may even lie on the line. 
This situation occurs in 's' for the sample font.

With this information I can easily produce a fix, simply by testing for 
non-monotonicity and disabling the optimisation in such circumstances, 
but I shall see if I can up with something slightly better.

Graham