MORPHING OPTIONS COMPARATOR
We compare 4 inbetweening techniques morphing between a line segment A and a semi-circle B.
Closest Projection: Linear motion between a point on B and its closest projection on A (note that swapping the roles of A and B would make this morph impossible.
Arc Length: Linear motion between corresponding points on A and B that are distributed along each curve at uniform arc-length intervals.
Ball Arc: Where a point moves along a circular arc that is normal to both curves.
Ball Line: Where points move along the line segment connecting the Ball Arc ends.
To better compare them, press 'k', 'f', 'p' to hide all except the animation and press 'm' to speed it up.
Note what happens near the final shpae B (the semi-circle): The two left techniques seem to be reaching it via a vertical growth, rather than expanding towards it as a circle would.
Also note how the two bottom morphs seem to flatten the curve on each side.
We suggest that the Ball Arc (top-right) is the best of these four schemes.