This is a collection of some of the more frequent construction problems in interpolation of glyph outlines. The animation illustrates an interpolation between the shapes on the left and right. A small version of the interpolation is drawn at the top in black. The starting point of a contour is marked in red. Oncurve points are brown, off curve points are blue. The points and contours are numbered so you can get an idea of where they go to.
Let's start with an interpolation where everything more or less works as it should. The same number of contours (1) and the same number of points(34). Contour direction ok. Point structure ok. Well placed curves. Pretty.
The masters totally different structures. It is not clear which shape needs to interpolate with which. So nothing happens, you need to fix this.
Kinking happens when points that are co-linear in the masters somehow produce unexpected results with interpolations wjere the points are no longer on the same line. See how path 0 becomes a shapes with 5 edges even when both its masters only have 4. Path 2 does it better: in both masters the angle of the line is the same. Point 1 can then travel while remaining on the line. Path 1 illustrates that if the ratio between in incoming and outgoing segments in point 11 are the same, then the angle can vary.
Summary: either the angle or the ratio needs to be the same in the masters.
This K, the overlaps nicely removed, shows kinking between points 8 and 11.
This problem is avoided when drawing the two strokes as separate shapes.
In this example the contour interpolates. But the contour direction is wrong and the interpolation folds in on itself.
A special case of the problem above: when the starting point is on the inside (on the left master) and on the outside (on the right master). This means that the resulting shape can actually resemble a letter. The interpolation still looks like a C, but it gets really thin. You need to make sure your proofing catches such problems.
Another example of a misplaced starting point. The shape collapses.
You can create motion in interpolations by placing the contours in different parts of the glyph. Path 0 moves up and down. Path 1 becomes a lot smaller. This is great to create animation effects.
Small amounts of rotation will work. Path 0 swings from left to right and it looks like rotation. But if you make the rotation reall extreme, like in path 1, you see that the shape inverts in itself. It does not look like a rotation at all.
Overlaps can be used in useful ways. See how the strokes merge from a disconnected state to a single shape.
Special care needs to be taken with emerging features: details that are not present in one of the masters. Some interpolation tools will interpolation nicely between straight and curved segments (path 0). But maybe you want more control over how those curves develop. Then you need to make sure that both segments have the right off-curves.