Path And Pathfollower

Path and PathFollower in FlatRedBall2

Use when working with Path, PathFollower, patrol routes, scripted movement, arc geometry, or trajectory rendering.

Key Types

• FlatRedBall2.Math.Path — defines the path geometry and renders it
• FlatRedBall2.Movement.PathFollower — moves an entity along a Path

Building a Path

var path = new Path()
    .MoveTo(-200, 0)         // pen-up move; sets starting point
    .LineTo(200, 0)          // straight segment
    .ArcTo(200, -100, -MathF.PI / 2)  // arc to endpoint, sweeping angle
    .LineBy(0, -50);         // relative line

path.IsLooped = true;        // closes back to start; affects TotalLength

All builder methods return this for chaining. By variants are relative; To variants are absolute.

Arc Angle Convention (Gotcha)

ArcTo(endX, endY, signedAngleRadians):

• Positive = CCW arc, Negative = CW arc (Y+ up world space)
• For a CCW arc going left-to-right, the path bows downward — the center is above the chord, and the minor arc sweeps below it. Negate the angle to bow upward.
• A full circle requires two ArcTo(π) calls — a single arc from a point to itself has zero chord length and degenerates to nothing.

Querying Position and Tangent

Vector2 pos     = path.PointAtLength(distance);   // by world-unit distance
Vector2 pos     = path.PointAtRatio(0.5f);        // by normalized ratio [0, 1]
Vector2 tangent = path.TangentAtLength(distance); // unit direction of travel
float   total   = path.TotalLength;               // includes closing segment when IsLooped

Rendering

path.Color         = Color.Cyan;
path.LineThickness = 3f;
path.IsVisible     = true;
screen.Add(path);  // registers for rendering like any other IRenderable

Rendering and following are independent — add to screen only when you want it visible.

PathFollower

var follower = new PathFollower(path)
{
    Speed        = 200f,   // world units per second
    Loops        = true,   // wraps at end (independent of Path.IsLooped)
    FaceDirection = true,  // sets entity.Rotation to face direction of travel
};

follower.WaypointReached += segmentIndex => { /* crossed into segment N */ };
follower.PathCompleted   += () => { /* reached end of open path */ };

Call in CustomActivity:

_follower.Activity(this, time.DeltaSeconds);

PathFollower only sets entity.X, entity.Y, and optionally entity.Rotation. It does not affect velocity or physics. Call follower.Reset() to restart from the beginning.

FaceDirection Rotation Convention

FaceDirection = true sets entity.Rotation = Angle.FromRadians(atan2(tangent.Y, tangent.X)), which maps the movement tangent to FlatRedBall2's standard math convention (0 = facing right, positive = CCW).