DJI uses a Centripetal Catmull-Rom spline to define the flight path. A Catmull-Rom spline has C1 continuity. If you are not familiar with parametric continuity you can read about it here:
en.wikipedia.org
When compared to other types of splines, the Centripetal Catmull-Rom spline provides some features that are well-suited for drone flight paths. If a Catmull-Rom spline was implemented in 2D and altitude transitions were made linear, it would result in a path that has C0 continuity. This means that the 1st derivative at the waypoints would not be continuous and would require infinite acceleration at those points. This would be similar to a straight-line mission where the flight path abruptly changes at the waypoints. Litchi and DJI handle such cases by having the drone stop momentarily at each waypoint. The WPML language used by DJI calls this waypoint mode "toPointAndStopWithDiscontinuityCurvature".
Additionally, if a spline with C0 continuity was used to define a flight path in which video was captured, the resulting video would have abrupt changes at the waypoints rendering the video less than pleasing.
What DJI has implemented is not "lazy programming" with regard to the use of a spline. It is the correct implementation of a Centripetal Catmull-Rom spline applied to waypoints in 3D space. What DJI could do is to provide some stats such as the minimum (and perhaps maximum) heights of the resulting flight path. DJI's implementation of waypoints has always seemed like more of a proof-of-concept where only the most basic waypoint functions are provided. Applications such as DroneLink or Litchi provide more advanced waypoint design features making the creation of waypoint missions much more efficient.
