Tangents #

Picture a robot traveling along a path. The direction that the robot is moving at any given point is the tangent at the point. Lines have a fixed tangent direction; splines have a variable tangent.

By default, the heading of the robot follows the tangent. In fact, tank robots are required to have the heading match the tangent (if you want to impress your friends, this is a nonholonomic constraint). Mecanum robots are more flexible and have a decoupled heading and tangent.

It’s important to separate heading and tangent in one’s mind. The tangent of this path

    .lineToX(48.0)

is the same as this path

    .lineToXLinearHeading(48.0, Math.PI / 2)

despite the heading changing.

When using TrajectoryActionBuilder, the begin tangent of any new path segment is chosen to match the end tangent of the last segment (this helps maintain continuity). But the tangent can still be changed manually using setTangent(). (The heading cannot be changed, however, because that would require the robot to teleport instead of merely coming to a stop.)

Finally, reversing the robot is the same as setting the tangent to be 180 degrees from where it currently is. Every call to setReversed() is secretly a call to setTangent().