Similarly, each point that is adter the last edge will be shifted by a similar distance computed for the last edge. Strong interpolation works wonders when it interpolates sections of glyphs that are contained within two edges. However, it can unpleasantly transform sections that span over more than two edges. When only one point of the contour is touched, then the whole contour is shifted. When no point of the contour is touched, the contour doesn't move from its scaled position.
Weak interpolation works wonders when all extrema points lie on segments and edges.
However, it has many weaknesses when this condition isn't met. This is due to the fact that only one of their points is touched for each of them. Unfortunately, each inside contour has a vertical extremum that seems to need to be strong-interpolated. Weak interpolation is thus a technique that cannot be used alone. To be exact, we select which interpolation to apply depending on each point's status, given that not all points are equal.