Litchi Missions vs DJI Fly Missions

Two waypoint missions built from the same set of coordinates can produce very different flight paths depending on whether they're created in Litchi or DJI Fly. The difference comes from the mathematical method each system uses to turn waypoints into a smooth, flyable curve

Historically:

• Litchi curved missions produced a path with rounded corners that did not pass through interior waypoints.
• DJI Fly missions generated a curve that always passes through every waypoint.

However, this distinction is no longer absolute. The latest version of Litchi's Mission Hub now supports both styles (depending on the drone being used), allowing you to choose:

• Traditional Litchi curved paths (straight lines, rounded corners, stays inside the convex hull)
• New "Smooth Curve" paths (DJI-style interpolation passing through every waypoint)

This update means Litchi can now replicate DJI Fly's behavior when desired, whil still offering its original straight-line with curves method.

Even when two missions share the same set of waypoints, their paths may differ. Some of these differences include:

• Traditional curved Litchi missions do not pass through the interior waypoints while DJI Fly waypoint missions do pass through all waypoints.
• Litchi supports both curved and straight-line missions while DJI Fly only directly supports curved missions. The WPML language used by DJI does supports both straight line missions and curved missions. DJI drones can fly straight line and curved mission. However, DJI Fly's implementation only supports the creation of curved missions.
• Litchi enables the curved portions of the missions to be adjusted while the curves in DJI Fly missions have a fixed curvature.
• The "Convex Hull" property of Litchi missions means that the mission path is guaranteed to be confined within the convex hull of the waypoints. DJI Fly missions do not have this property and therefore, DJI's path can travel outside of the convex hull defined by the waypoints.

Perhaps the most important of those differences is the fact that DJI Fly missions (or Litchi missions using the DJI Engine) pass through the waypoints while traditional curved Litchi missions do not.

Obviously, the two methods are using different technologies to create flight paths from a set of waypoints. So, the question becomes: "Why?" or "How?". I cannot answer the "Why?". But I can answer the "How?".

In Litchi's new Mission Hub, for appropriate drones, Litchi also provides a "Path Mode" named "Smooth Curves". This allows DJI-style curved missions to be designed in the Mission Hub in addition to the traditional curved missions described here.

Turning a series of control points into a smooth curve is a classic math problem, typically solved using splines. There are many spline types, each with its own properties. Some of the more common include:

• Quadratic Bézier Curves
  • Requires three control points
  • Does not pass through the middle control point
• Cubic Bézier Curves
  • Requires four control points
  • Passes through the first and last points only
  • Can be chained into long, composite curves which do pass through control points
• Cardinal Splines
  • Requires a minimum of three control points
  • Pass through all control points
  • Have a tension parameter
• Catmull-Rom Splines
  • A special case of cardinal splines (with the tension set to 0.5)
  • Requires a minimum of three control points
  • Always pass through every control point
  • Comes in "uniform", "chordal", and "centripetal" variants
• Quadratic B-Splines
  • Requires a minimum of three control points
  • Do not pass through all control points
  • Entire curve is contained within the convex hull of the control points
• Cubic B-Splines
  • Requires a minimum of four control points
  • Do not pass through all control points
  • Entire curve is contained within the convex hull of the control points
• The Continuity of Splines (An excellent video describing splines. If you only click on one link on this page, this link should be the one.)

Litchi's traditional curved mission system is not a pure spline, even though it may look like one. Instead, Litchi:

1. Connects waypoints with straight lines
2. Replaces each interior corner with a quadratic Bézier curve or B-spline
3. Lets the user control the curve size at each waypoint

This approach has several consequences:

• The flight path does not pass through interior waypoints
• The flight path always stays inside the convex hull of the control points
• The curvature at each waypoint can be adjusted which is something that cannot be done with a composite spline

Details

• A Matrix Representation for Uniform B-Spline Curves
• Bézier Curves