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17_the_trajectory_system.md

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The Trajectory system

The Trajectory system lies at the very heart of Pathfinder's movement. The concept of a Trajectory is relatively simple - it's a movement-oriented and stateful object that guides your robot along a certain path at a certain speed.

What does a trajectory do?

Well, it guides your robot along a specified path at a specified speed. The Trajectory interface is extremely minimal and it only has three methods:

  • PointXYZ nextMarker(PointXYZ current)
  • boolean isDone(PointXYZ current)
  • double speed(PointXYZ current)

Each of those methods accepts a single PointXYZ parameter, representing the robot's current position. Based on the robot's position, the Trajectory should be capable of instructing the robot on what to do next.

How are trajectories processed?

Through Followers. Beyond the Follower interface, you won't need to know much about how Pathfinder actually follows trajectories - if you're curious, check out FollowerExecutor.

The Follower interface

In order for Pathfinder to be able to follow a Trajectory, you need to use a Follower. As the name would suggest, a Follower... well, it follows a trajectory, of course!

I can't think of any reasons you'd want to use a Follower implementation other than the default GenericFollower, but you have the option to do so if you so desire.

FollowerExecutor

Followers are executed with follower executors. I genuinely cannot conceive a single reason why you'd ever need to know how a FollowerExecutor works, but you can feel free to check out the FollowerExecutor class.

Trajectory implementations

Several implementations of the Trajectory interface are provided within Pathfinder.

  • LinearTrajectory: The simplest trajectory implementation. Moves to a single point at a linear speed. This will dynamically correct for any movement error.
  • MutableLinearTrajectory: Just like LinearTrajectory, but it's mutable - that's really all I can say.
  • FastTrajectory: Similar to LinearTrajectory in the sense that it moves to a single target point at a linear speed, but FastTrajectory will correct if it overshoots the target. Additionally, FastTrajectory does not require the robot's heading to be correct in order for the trajectory to be finished.
  • TimedTrajectory: A trajectory with time constraints.
  • TaskTrajectory: A trajectory that executes an action not related to movement.
  • SplineTrajectory: A trajectory that utilizes a Spline to guide the robot.
  • AdvancedSplineTrajectory: The most advanced form of Trajectory provided in Pathfinder, AdvancedSplineTrajectory has separate spline interpolation functions responsible for controlling the trajectory's X/Y target, desired heading, and even speed.
  • ArcTrajectory: A trajectory based on Circle that follows an arc.
  • EmptyTrajectory: A trajectory that does absolutely nothing.
  • ControlledTrajectory: A trajectory that utilizes a Controller to determine speed.
  • MultiSegmentTrajectory: A Trajectory composed of several other trajectories. Each of these trajectories will be executed sequentially.
  • MultiTargetTrajectory: Sort of like a MultiSegmentTrajectory of many linear trajectories: a MultiTargetTrajectory will guide the robot to a series of points.

Creating a custom trajectory implementation

There's a whole guide available here.

Further reading