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Track Pedestrians from a Moving Car

This example shows how to track pedestrians using a camera mounted in a moving car.


This example shows how to perform automatic detection and tracking of people in a video from a moving camera. It demonstrates the flexibility of a tracking system adapted to a moving camera, which is ideal for automotive safety applications. Unlike the vehicle tracking example, Track Multiple Vehicles Using a Camera, this example contains additional algorithmic steps. These steps include people detection and customized cost for assigning detections to tracks.

The tracking workflow consists of the following steps:

  1. Define camera intrinsics and camera mounting position.

  2. Load and configure a pretrained people detector.

  3. Set up a multi-object tracker.

  4. Run the detector for each video frame.

  5. Compute costs for detections-to-tracks assignments.

  6. Update the tracker with detection results.

  7. Display the tracking results in a video.

Configure People Detector and Multi-object Tracker

In this example, we use a pretrained ACF people detector, and configure this detector to incorporate camera information. By default, the detector scans the entire image at multiple scales. By knowing the camera parameters, we can configure the detector to only detect people on the ground plane at reasonable scales.

% Load the monoCamera object that contains the camera information.
d = load('TrackingDemoMonoCameraSensor.mat', 'sensor');

% Load a pretrained ACF people detector. The ACF detector uses "Aggregate
% Channel Features", which is fast to compute in practice. The 'caltech'
% model is trained on caltech pedestrian dataset, which can detect people
% at the minimum resolution of 50x21 pixels.
detector = peopleDetectorACF('caltech');

% Configure the detector using the sensor information. The detector only
% tries to find people at image regions above the ground plane. This can
% reduce computation and prevent spurious detections.

% The width of common pedestrian is between 0.5 to 1.5 meters. Only a
% bounding box of width within this range is considered as a detection
% candidate in image.
pedWidth = [0.5, 1.5];

% Configure the detector using the monoCamera sensor and desired width.
detector = configureDetectorMonoCamera(detector, d.sensor, pedWidth);

% Initialize an multi-object tracker including setting the filter,
% the detection-to-track assignment threshold, the coasting and
% confirmation parameters. You can find the |setupTracker| function at the
% end of this example.
[tracker, positionSelector] = setupTracker();

Track People in a Video

At each time step, run the detector, compute detection-to-track assignment costs, update the tracker with detection results, and display the tracking results in a video.

% Setup Video Reader and Player
videoFile = 'pedtracking.mp4';
videoReader = VideoReader(videoFile);
videoPlayer = vision.DeployableVideoPlayer();

currentStep = 0;
snapshot = [];
snapTimeStamp = 107;
cont = hasFrame(videoReader);
while cont
    % Update frame counters.
    currentStep = currentStep + 1;

    % Read the next frame.
    frame = readFrame(videoReader);

    % Run the detector and package the returned results into an object
    % required by multiObjectTracker.  You can find the |detectObjects|
    % function at the end of this example.
    detections = detectObjects(detector, frame, currentStep);

    % Using the list of objectDetections, return the tracks, updated for
    % 'currentStep' time. After the first frame, a helper function
    % |detectionToTrackCost| is called to compute the customized cost
    % matrix between all tracks and all detections.
    if currentStep == 1
        costMatrix = zeros(0, numel(detections));
        [confirmedTracks,~,allTracks] = updateTracks(tracker, detections, currentStep, ...
        costMatrix = detectionToTrackCost(allTracks, detections, ...
            positionSelector, tracker.AssignmentThreshold);
        [confirmedTracks,~,allTracks] = updateTracks(tracker, detections, currentStep, ...

    % Remove the tracks for people that are far away.
    confirmedTracks = removeNoisyTracks(confirmedTracks, positionSelector, d.sensor.Intrinsics.ImageSize);

    % Insert tracking annotations.
    frameWithAnnotations = insertTrackBoxes(frame, confirmedTracks, positionSelector, d.sensor);

    % Display the annotated frame.

    % Take snapshot for publishing at snapTimeStamp seconds
    if currentStep == snapTimeStamp
        snapshot = frameWithAnnotations;

    % Exit the loop if the video player figure is closed by user.
    cont = hasFrame(videoReader) && isOpen(videoPlayer);

Show the tracked people and display the distance to the ego car.

if ~isempty(snapshot)

The tracking workflow presented here can be easily integrated into the Visual Perception Using Monocular Camera, where the people or vehicle detection step can be enhanced with the tracker. To learn about additional tracking capabilities in the Automated Driving System Toolbox®, please take a look at the monoCamera and multiObjectTracker.

Supporting Functions

setupTracker function creates a multiObjectTracker to track multiple objects with Kalman filters. When creating a multiObjectTracker consider the following:

  • FilterInitializationFcn: The likely motion and measurement models. In this case, the objects are expected to have a constant velocity motion. See the 'Define a Kalman filter' section.

  • AssignmentThreshold: how far detections may fall from tracks. If there are detections that are not assigned to tracks, but should be, increase this value. If there are detections that get assigned to tracks that are too far, decrease this value. This example uses bounding box overlap ratio as the assignment cost, with the threshold set to 0.999.

  • NumCoastingUpdates: How many times a track is coasted before deletion. Coasting is a term used for updating the track without an assigned detection (predicting). The default value for this parameter is 5.

  • ConfirmationParameters: The parameters for confirming a track. A new track is initialized with every unassigned detection. Some of these detections may be false, so all the tracks are initialized as Tentative. To confirm a track, it has to be detected at least M times in N tracker updates. The choice of M and N depends on the visibility of the objects. This example uses the default of 3 detections out of 5 updates.

  • HasCostMatrixInput: True if cost matrix is provided as input. Set this property to true to pass a cost matrix as part of the inputs to updateTracks.

The outputs of setupTracker are:

  • tracker - The multiObjectTracker that is configured for this case.

  • positionSelector - A matrix that specifies which elements of the State vector are the position: position = positionSelector * State

function [tracker, positionSelector] = setupTracker()
    % Create the tracker object.
    tracker = multiObjectTracker('FilterInitializationFcn', @initBboxFilter, ...
        'AssignmentThreshold', 0.999, ...
        'NumCoastingUpdates', 5, ...
        'ConfirmationParameters', [3 5], ...
        'HasCostMatrixInput', true);

    % The State vector is: [x; vx; y; vy; w; vw; h; vh]
    % [x;y;w;h] = positionSelector * State
    positionSelector = [1 0 0 0 0 0 0 0; ...
                        0 0 1 0 0 0 0 0; ...
                        0 0 0 0 1 0 0 0; ...
                        0 0 0 0 0 0 1 0];

initBboxFilter function defines a Kalman filter to filter bounding box measurement.

function filter = initBboxFilter(Detection)
% Step 1: Define the motion model and state.
%   Use a constant velocity model for a bounding box on the image.
%   The state is [x; vx; y; vy; w; wv; h; hv]
%   The state transition matrix is:
%       [1 dt 0  0 0  0 0  0;
%        0  1 0  0 0  0 0  0;
%        0  0 1 dt 0  0 0  0;
%        0  0 0  1 0  0 0  0;
%        0  0 0  0 1 dt 0  0;
%        0  0 0  0 0  1 0  0;
%        0  0 0  0 0  0 1 dt;
%        0  0 0  0 0  0 0  1]
%   Assume dt = 1. This example does not consider time-variant transition
%   model for linear Kalman filter.
    dt = 1;
    cvel =[1 dt; 0 1];
    A = blkdiag(cvel, cvel, cvel, cvel);

% Step 2: Define the process noise.
%   The process noise represents the parts of the process that the model
%   does not take into account. For example, in a constant velocity model,
%   the acceleration is neglected.
    G1d = [dt^2/2; dt];
    Q1d = G1d*G1d';
    Q = blkdiag(Q1d, Q1d, Q1d, Q1d);

% Step 3: Define the measurement model.
%   Only the position ([x;y;w;h]) is measured.
%   The measurement model is
    H = [1 0 0 0 0 0 0 0; ...
         0 0 1 0 0 0 0 0; ...
         0 0 0 0 1 0 0 0; ...
         0 0 0 0 0 0 1 0];

% Step 4: Map the sensor measurements to an initial state vector.
%   Because there is no measurement of the velocity, the v components are
%   initialized to 0:
    state = [Detection.Measurement(1); ...
             0; ...
             Detection.Measurement(2); ...
             0; ...
             Detection.Measurement(3); ...
             0; ...
             Detection.Measurement(4); ...

% Step 5: Map the sensor measurement noise to a state covariance.
%   For the parts of the state that the sensor measured directly, use the
%   corresponding measurement noise components. For the parts that the
%   sensor does not measure, assume a large initial state covariance. That way,
%   future detections can be assigned to the track.
    L = 100; % Large value
    stateCov = diag([Detection.MeasurementNoise(1,1), ...
                     L, ...
                     Detection.MeasurementNoise(2,2), ...
                     L, ...
                     Detection.MeasurementNoise(3,3), ...
                     L, ...
                     Detection.MeasurementNoise(4,4), ...

% Step 6: Create the correct filter.
%   In this example, all the models are linear, so use trackingKF as the
%   tracking filter.
    filter = trackingKF(...
        'StateTransitionModel', A, ...
        'MeasurementModel', H, ...
        'State', state, ...
        'StateCovariance', stateCov, ...
        'MeasurementNoise', Detection.MeasurementNoise, ...
        'ProcessNoise', Q);

detectObjects function detects people in an image.

function detections = detectObjects(detector, frame, frameCount)
    % Run the detector and return a list of bounding boxes: [x, y, w, h]
    bboxes = detect(detector, frame);

    % Define the measurement noise.
    L = 100;
    measurementNoise = [L 0  0  0; ...
                        0 L  0  0; ...
                        0 0 L/2 0; ...
                        0 0  0 L/2];

    % Formulate the detections as a list of objectDetection reports.
    numDetections = size(bboxes, 1);
    detections = cell(numDetections, 1);
    for i = 1:numDetections
        detections{i} = objectDetection(frameCount, bboxes(i, :), ...
            'MeasurementNoise', measurementNoise);

removeNoisyTracks function removes noisy tracks. A track is considered to be noisy if its predicted bounding box is too small. Typically, this implies the person is far away.

function tracks = removeNoisyTracks(tracks, positionSelector, imageSize)

    if isempty(tracks)

    % Extract the positions from all the tracks.
    positions = getTrackPositions(tracks, positionSelector);
    % The track is 'invalid' if the predicted position is about to move out
    % of the image, or the bounding box is too small.
    invalid = ( positions(:, 1) < 1 | ...
                positions(:, 1) + positions(:, 3) > imageSize(2) | ...
                positions(:, 3) <= 5 | ...
                positions(:, 4) <= 10 );
    tracks(invalid) = [];

insertTrackBoxes inserts bounding boxes in an image and displays the track's position in front of the car, in world units.

function I = insertTrackBoxes(I, tracks, positionSelector, sensor)

    if isempty(tracks)

    % Allocate memory.
    labels = cell(numel(tracks), 1);
    % Retrieve positions of bounding boxes.
    bboxes = getTrackPositions(tracks, positionSelector);

    for i = 1:numel(tracks)
        box = bboxes(i, :);

        % Convert to vehicle coordinates using monoCamera object
        xyVehicle = imageToVehicle(sensor, [box(1)+box(3)/2, box(2)+box(4)]);

        labels{i} = sprintf('x=%.1f,y=%.1f',xyVehicle(1),xyVehicle(2));

    I = insertObjectAnnotation(I, 'rectangle', bboxes, labels, 'Color', 'yellow', ...
        'FontSize', 10, 'TextBoxOpacity', .8, 'LineWidth', 2);

detectionToTrackCost computes the customized cost for detections-to-tracks assignment.

Assigning object detections in the current frame to existing tracks is done by minimizing cost. The cost is computed using the bboxOverlapRatio function, and this cost is the overlap ratio between the predicted and the detected bounding box. In this example, the person is assumed to move gradually in consecutive frames, due to the high frame rate of the video and the low motion speed of the person.

First, this example computes the cost of assigning every detection to each track by using the bboxOverlapRatio measure. As people move toward or away from the camera, the centroid point alone cannot accurately describe their motion. The cost takes into account the distance on the image plane, and the scale of the bounding boxes. This prevents assigning detections that are far away from the camera to tracks that are closer to the camera, even if the predicted and detected centroids coincide. The choice of this cost function eases the computation without resorting to a more sophisticated dynamic model. The results are stored in an M by N matrix, where M is the number of tracks, and N is the number of detections.

The value for the cost of not assigning a detection to a track depends on the range of values returned by the cost function. This value must be tuned experimentally. Setting it too low increases the likelihood of creating a new track, and can result in track fragmentation. Setting it too high can result in a single track corresponding to a series of separate moving objects. Possible assignment entries can be excluded from optimization whose value is above the nonassignment cost, and set to Inf.

function costMatrix = detectionToTrackCost(tracks, detections, ...
                                positionSelector, threshold)

    if isempty(tracks) || isempty(detections)
        costMatrix = zeros(length(tracks), length(detections));

    % Compute the overlap ratio between the predicted boxes and the
    % detected boxes, and compute the cost of assigning each detection
    % to each track. The cost is minimum when the predicted bbox is
    % perfectly aligned with the detected bbox (overlap ratio is one).

    % Retrieve positions of bounding boxes.
    trackBboxes = getTrackPositions(tracks, positionSelector);
    % Check that the width and height are positive before computing the box
    % overlap ratio.
    trackBboxes(:, 3) = max(eps, trackBboxes(:, 3));
    trackBboxes(:, 4) = max(eps, trackBboxes(:, 4));

    % Extract the detected bounding box from all the detections.
    allDetections = [detections{:}];
    bboxes = reshape([allDetections(:).Measurement], 4, length(detections))';

    % Compute all pairwise assignment costs.
    costMatrix = 1 - bboxOverlapRatio(trackBboxes, bboxes);
    % Set unrealistic assignment cost to Inf if there is little box
    % overlap.
    costMatrix(costMatrix(:) > threshold) = Inf;
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