# getTrackVelocities

Obtain updated track velocities and velocity covariance matrix

## Syntax

``velocity = getTrackVelocities(tracks,velocitySelector)``
``````[velocity,velocityCovariances] = getTrackVelocities(tracks,velocitySelector)``````

## Description

example

````velocity = getTrackVelocities(tracks,velocitySelector)` returns velocities of tracked objects.```

example

``````[velocity,velocityCovariances] = getTrackVelocities(tracks,velocitySelector)``` also returns the track velocity covariance matrices.```

## Examples

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Create an extended Kalman filter tracker for 3-D constant-acceleration motion.

`tracker = multiObjectTracker('FilterInitializationFcn',@initcaekf);`

Initialize the tracker with one detection.

```detection = objectDetection(0,[10;-20;4],'ObjectClassID',3); tracks = tracker(detection,0);```

Add a second detection at a later time and at a different position.

```detection = objectDetection(0.1,[10.3;-20.2;4],'ObjectClassID',3); tracks = tracker(detection,0.2);```

Obtain the velocity vector from the track state.

```velocitySelector = [0 1 0 0 0 0 0 0 0; 0 0 0 0 1 0 0 0 0; 0 0 0 0 0 0 0 1 0]; velocity = getTrackVelocities(tracks,velocitySelector)```
```velocity = 1×3 1.0093 -0.6728 0 ```

Create an extended Kalman filter tracker for 3-D constant-acceleration motion.

`tracker = multiObjectTracker('FilterInitializationFcn',@initcaekf);`

Initialize the tracker with one detection.

```detection = objectDetection(0,[10;-20;4],'ObjectClassID',3); tracks = tracker(detection,0);```

Add a second detection at a later time and at a different position.

```detection = objectDetection(0.1,[10.3;-20.2;4.3],'ObjectClassID',3); tracks = tracker(detection,0.2);```

Obtain the velocity vector from the track state.

```velocitySelector = [0 1 0 0 0 0 0 0 0; 0 0 0 0 1 0 0 0 0; 0 0 0 0 0 0 0 1 0]; [velocity,velocityCovariance] = getTrackVelocities(tracks,velocitySelector)```
```velocity = 1×3 1.0093 -0.6728 1.0093 ```
```velocityCovariance = 3×3 70.0685 0 0 0 70.0685 0 0 0 70.0685 ```

## Input Arguments

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Object tracks, specified as an array of `objectTrack` objects or an array of structures containing sufficient information to obtain the track velocity information. At a minimum, these structures must contain a `State` column vector field and a positive-definite `StateCovariance` matrix field. For a sample track structure, see `toStruct`.

Velocity selector, specified as a D-by-N real-valued matrix of ones and zeros. D is the number of dimensions of the tracker. N is the size of the state vector. Using this matrix, the function extracts track velocities from the state vector. Multiply the state vector by velocity selector matrix returns velocities. The same selector is applied to all object tracks.

## Output Arguments

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Velocities of tracked objects at last update time, returned as a 1-by-D vector or a real-valued M-by-D matrix. D represents the number of velocity elements. M represents the number of tracks.

Velocity covariance matrices of tracked objects, returned as a real-valued D-by-D-matrix or a real-valued D-by-D-by-M array. D represents the number of velocity elements. M represents the number of tracks. Each D-by-D submatrix is a velocity covariance matrix for a track.

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### Velocity Selector for 2-Dimensional Motion

Show the velocity selection matrix for two-dimensional motion when the state consists of the position and velocity.

`$\left[\begin{array}{cccc}0& 1& 0& 0\\ 0& 0& 0& 1\end{array}\right]$`

### Velocity Selector for 3-Dimensional Motion

Show the velocity selection matrix for three-dimensional motion when the state consists of the position and velocity.

`$\left[\begin{array}{cccccc}0& 1& 0& 0& 0& 0\\ 0& 0& 0& 1& 0& 0\\ 0& 0& 0& 0& 0& 1\end{array}\right]$`

### Velocity Selector for 3-Dimensional Motion with Acceleration

Show the velocity selection matrix for three-dimensional motion when the state consists of the position, velocity, and acceleration.

`$\left[\begin{array}{ccccccccc}0& 1& 0& 0& 0& 0& 0& 0& 0\\ 0& 0& 0& 0& 1& 0& 0& 0& 0\\ 0& 0& 0& 0& 0& 0& 0& 1& 0\end{array}\right]$`

## Extended Capabilities

### C/C++ Code GenerationGenerate C and C++ code using MATLAB® Coder™.

Introduced in R2017a