# Manipulating Trajectories¶

Once a trajectory has been generated, you can retrieve information from it using certain methods. These methods will be useful when writing code to follow these trajectories.

## Getting the total duration of the trajectory¶

Because all trajectories have timestamps at each point, the amount of time it should take for a robot to traverse the entire trajectory is pre-determined. The TotalTime() (C++) / getTotalTimeSeconds() (Java) method can be used to determine the time it takes to traverse the trajectory.

// Get the total time of the trajectory in seconds
double duration = trajectory.getTotalTimeSeconds();


## Sampling the trajectory¶

The trajectory can be sampled at various timesteps to get the pose, velocity, and acceleration at that point. The Sample(units::second_t time) (C++) / sample(double timeSeconds) (Java) method can be used to sample the trajectory at any timestep. The parameter refers to the amount of time passed since 0 seconds (the starting point of the trajectory). This method returns a Trajectory::Sample with information about that sample point.

// Sample the trajectory at 1.2 seconds. This represents where the robot
// should be after 1.2 seconds of traversal.
Trajectory.Sample point = trajectory.sample(1.2);


The Trajectory::Sample struct has several pieces of information about the sample point:

• t: The time elapsed from the beginning of the trajectory up to the sample point.

• velocity: The velocity at the sample point.

• acceleration: The acceleration at the sample point.

• pose: The pose (x, y, heading) at the sample point.

• curvature: The curvature (rate of change of heading with respect to distance along the trajectory) at the sample point.

Note: The angular velocity at the sample point can be calculated by multiplying the velocity by the curvature.

## Getting all states of the trajectory (advanced)¶

A more advanced user can get a list of all states of the trajectory by calling the States() (C++) / getStates() (Java) method. Each state represents a point on the trajectory. When the trajectory is created using the TrajectoryGenerator::GenerateTrajectory(...) method, a list of trajectory points / states are created. When the user samples the trajectory at a particular timestep, a new sample point is interpolated between two existing points / states in the list.