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Path and Trajectory Generation

While the TrajectoryBuilder API is great for making continuous and connected paths, it does not allow as much customization as manually creating the Path and Trajectory objects.

Hermes actually has two types of path: PosePath and PositionPath<Param> (the Param type parameter will be explained in a different guide). The main difference is that PositionPaths are only for, well, positions, while PosePaths also include headings. An example of a PositionPath is a Line, which simply connects two Vector2ds.

Types of PositionPath

The primary types of PositionPath are 1. Lines, which are straight lines that connect Vector2ds, 2. Quintic hermite splines, which are used by splineTo and its variants, and 3. Bézier curves, which can only be created manually.

Creating a Line is easy! Simply pass the two points you are creating into its constructor:

val line = Line(Vector2d(0.0, 0.0), Vector2d(10.0, 10.0))

PositionPath<Arclength> line = 
    new Line(new Vector2d(0.0, 0.0), new Vector2d(10.0, 10.0));

Splines and Bézier curves, however, require a little more work. Both curve types are implemented parametrically, meaning the x and y curves are separate objects that both depend on a parameter t. In addition, the QuinticSpline1d and BezierCurve1d classes expect t to range solely from 0.0 to 1.0. This means more math (integration, for those who know calculus) is required to allow t to range through the full length of the curve.

The library conveniently provides a function to generate a Bézier curve using Vector2ds as control points, allowing end users to skip the extra steps described above:

val curve = fromPoints(
    Vector2d(0.0, 0.0),
    Vector2d(10.0, 10.0),
    Vector2d(10.0, 30.0)
)

The fromPoints function comes from the file BezierCurves.kt.

PositionPath<Arclength> curve = BezierCurves.fromPoints(
        new Vector2d(0.0, 0.0),
        new Vector2d(10.0, 10.0),
        new Vector2d(10.0, 30.0)
);

The official type returned by fromPoints is ArcLengthReparamCurve2d, which is essentially a fancy name for a curve that ranges from 0.0 to its full length.

Due to the nature of hermite splines, it is difficult to create them by themselves. We recommend using the trajectoryBuilder API's splineTo and related methods instead.

Types of PosePath

Hermes also includes three default functions in the PositionPath interface to add heading paths, which creates PosePath objects!

The four major types of heading paths include: 1. Tangent heading, where the heading is based off the tangent of the curve, 2. Constant heading, 3. Linear heading interpolation, and 4. Spline heading interpolation, which actually uses a QuinticSpline1d to smoothly interpolate headings

The link to the KDoc provides an explanation on how to add heading paths to a PositionPath to create a PosePath.

Turning A PosePath into a Trajectory

A trajectory is simply a path and a profile!

Motion profiling in Hermes is based on your tuning, so the easiest way to create a profile is with the createProfile(path: PosePath) function of Drive. It uses your tuned constants and parameters to generate a forward profile of your path, and returns a DisplacementProfile object. To create a TimeProfile object, you can simply pass the displacement-based profile into TimeProfile's constructor.

Similarly, the createTrajectory(path: PosePath) function of Drive uses your tuned constants to create a profile of your path, and then returns a DisplacementTrajectory object based on it.