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Annotate Video Using Detections in Vehicle Coordinates

Configure and use a monoCamera object to display information provided in vehicle coordinates on a video display.


Displaying data recorded in vehicle coordinates on a recorded video is an integral part of ground truth labeling and analyzing tracking results. Using a two-dimensional bird's-eye view can help you understand the overall environment, but it is sometimes hard to correlate the video with the bird's-eye view display. In particular, this problem becomes worse when using a third-party sensor where you cannot access the raw video captured by the sensor, and you need to use a video captured by a separate camera.

Automated Driving System Toolbox™ provides the monoCamera object that facilitates the conversion between vehicle coordinates and image coordinates. This example reads data recorded by a video sensor installed on a test vehicle. Then it displays the data on a video captured by a separate video camera installed on the same car. The data and video were recorded at the following rates:

  • Reported lane information: 20 times per second

  • Reported vision objects: 10 times per second

  • Video frame rate: 20 frames per second

Display a Frame with Video Annotations

The selected frame corresponds to 5.9 seconds into the video clip, when there are several objects to show on the video.

% Set up video reader and player
videoFile      = '01_city_c2s_fcw_10s.mp4';
videoReader = VideoReader(videoFile);
videoPlayer = vision.DeployableVideoPlayer;

% Jump to the desired frame
time = 5.9;
videoReader.CurrentTime = time;
frameWithoutAnnotations = readFrame(videoReader);

title('Original Video Frame')

Get the corresponding recorded data.

recordingFile  = '01_city_c2s_fcw_10s_sensor.mat';
[visionObjects, laneReports, timeStep, numSteps] = readDetectionsFile(recordingFile);
currentStep = round(time / timeStep) + 1;
videoDetections = processDetections(visionObjects(currentStep));
laneBoundaries = processLanes(laneReports(currentStep));

% Set up the monoCamera object for on-video display
sensor = setupMonoCamera(videoReader);

frameWithAnnotations = updateDisplay(frameWithoutAnnotations, sensor, videoDetections, laneBoundaries);

title('Annotated Video Frame')

Display a Clip with Video Annotations

To display the video clip with annotations, simply repeat the annotation frame-by-frame. The video shows that the car pitches slightly up and down, which changes the pitch angle. No attempt has been made to compensate for this pitch motion. As a result, the conversion from vehicle coordinates to image coordinates is a little inaccurate on some of the frames.

% Reset the time back to zero
currentStep = 0;                % Reset the recorded data timestep
videoReader.CurrentTime = 0;    % Reset the video reader time
while currentStep < numSteps && hasFrame(videoReader)
    % Update scenario counters
    currentStep = currentStep + 1;

    % Get the current time

    % Prepare the detections to the tracker
    videoDetections = processDetections(visionObjects(currentStep), videoDetections);

    % Process lanes
    laneBoundaries = processLanes(laneReports(currentStep));

    % Update video frame with annotations from the reported objects
    frameWithoutAnnotations = readFrame(videoReader);
    frameWithAnnotations = updateDisplay(frameWithoutAnnotations, sensor, videoDetections, laneBoundaries);

    % The recorded data was obtained at a rate of 20 frames per second.
    % Pause for 50 milliseconds for a more realistic display rate. If you
    % process data and form tracks in this loop, you do not need this
    % pause.
    pause(0.05 - toc);

    % Display annotated frame

Create the Mono Camera for On-Video Display

The setupMonoCamera function returns a monoCamera sensor object, which is used for converting positions in vehicle coordinates to image coordinates.

Knowing the camera's intrinsic and extrinsic calibration parameters is critical to accurate conversion between pixel and vehicle coordinates.

Start by defining the camera intrinsic parameters. The parameters in this function were estimated based on the camera model. To obtain the parameters for your camera, use the cameraCalibrator app.

Because the data in this example has little distortion, this function ignores the lens distortion coefficients. The parameters are next stored in a cameraIntrinsics object.

Next, define the camera extrinsics. Camera extrinsics relate to the way the camera is mounted on the car. The mounting includes the following properties:

  • Height: Mounting height above the ground, in meters.

  • Pitch: Pitch of the camera, in degrees, where positive is angled below the horizon and toward the ground. In most cases, the camera is pitched slightly below the horizon.

  • Roll: Roll of the camera about its axis. For example, if the video is flipped upside down, use roll = 180.

  • Yaw: Angle of the camera sideways, where positive is in the direction of the positive y-axis (to the left). For example, a forward-facing camera has a yaw angle of 0 degrees, and a backward-facing camera has a yaw angle of 180 degrees.

function sensor = setupMonoCamera(vidReader)
% Define the camera intrinsics from the video information
focalLength    = [1260 1100];                    % [fx, fy]              % pixels
principalPoint = [360 245];                      % [cx, cy]              % pixels
imageSize = [vidReader.height, vidReader.width]; % [numRows, numColumns] % pixels
intrinsics = cameraIntrinsics(focalLength, principalPoint, imageSize);

% Define the camera mounting (camera extrinsics)
mountingHeight = 1.45;   % height in meters from the ground
mountingPitch  = 1.25;   % pitch of the camera in degrees
mountingRoll   = 0.15;   % roll of the camera in degrees
mountingYaw    = 0;      % yaw of the camera in degrees
sensor = monoCamera(intrinsics, mountingHeight, ...
    'Pitch', mountingPitch, ...
    'Roll',  mountingRoll, ...
    'Yaw',   mountingYaw);

Using the Mono Camera Object to Update the Display

The updateDisplay function displays all the object annotations on top of the video frame.

The display update includes the following steps:

  1. Using the monoCamera sensor to convert reported detections into bounding boxes and annotating the frame.

  2. Using the insertLaneBoundary method of the parabolicLaneBoundary object to insert the lane annotations.

function frame = updateDisplay(frame, sensor, videoDetections, laneBoundaries)

% Allocate memory for bounding boxes
bboxes = zeros(numel(videoDetections), 4);

% Create the bounding boxes
for i = 1:numel(videoDetections)
    % Use monoCamera sensor to convert the position in vehicle coordinates
    % to the position in image coordinates.
    % Notes:
    %   1. The width of the object is reported and is used to calculate the
    %      size of the bounding box around the object (half width on each
    %      side). The height of the object is not reported. Instead, the
    %      function uses a height/width ratio of 0.85 for cars and 3 for
    %      pedestrians.
    %   2. The reported location is at the center of the object at ground
    %      level, i.e., the bottom of the bounding box.
    xyLocation1 = vehicleToImage(sensor, videoDetections(i).positions' + [0,videoDetections(i).widths/2]);
    xyLocation2 = vehicleToImage(sensor, videoDetections(i).positions' - [0,videoDetections(i).widths/2]);
    dx = xyLocation2(1) - xyLocation1(1);

    % Define the height/width ratio based on object class
    if strcmp(videoDetections(i).labels, 'Car')
        dy = dx * 0.85;
    elseif  strcmp(videoDetections(i).labels, 'Pedestrian')
        dy = dx * 3;
        dy = dx;

    % Estimate the bounding box around the vehicle. Subtract the height of
    % the bounding box to define the top-left corner.
    bboxes(i,:) =[(xyLocation1 - [0, dy]), dx, dy];

% Add labels
labels = {videoDetections(:).labels}';

% Add bounding boxes to the frame
if ~isempty(labels)
    frame = insertObjectAnnotation(frame, 'rectangle', bboxes, labels,...
        'Color', 'yellow', 'FontSize', 10, 'TextBoxOpacity', .8, 'LineWidth', 2);

% Display the lane boundary on the video frame
xRangeVehicle = [1, 100];
xPtsInVehicle = linspace(xRangeVehicle(1), xRangeVehicle(2), 100)';
frame = insertLaneBoundary(frame, laneBoundaries(1), sensor, xPtsInVehicle, ...
    'Color', 'red');
frame = insertLaneBoundary(frame, laneBoundaries(2), sensor, xPtsInVehicle, ...
    'Color', 'green');


This example showed how to create a monoCamera sensor object and use it to display objects described in vehicle coordinates on a video captured by a separate camera. Try using recorded data and a video camera of your own. Try calibrating your camera to create a monoCamera that allows for transformation from vehicle to image coordinates, and vice versa.

Supporting Functions

readDetectionsFile - Reads the recorded sensor data file. The recorded data is in a single structure that is divided into four struct arrays. This example uses only the following two arrays:

  1. laneReports, a struct array that reports the boundaries of the lane. It has these fields: left and right. Each element of the array corresponds to a different timestep. Both left and right are structures with these fields: isValid, confidence, boundaryType, offset, headingAngle, and curvature.

  2. visionObjects, a struct array that reports the detected vision objects. It has the fields numObjects (integer) and object (struct). Each element of the array corresponds to a different timestep. object is a struct array, where each element is a separate object with these fields: id, classification, position (x;y;z), velocity(vx;vy;vz), size(dx;dy;dz). Note: z=vy=vz=dx=dz=0

function [visionObjects, laneReports, timeStep, numSteps] = readDetectionsFile(filename)
A = load(strcat(filename));
visionObjects =;
laneReports = A.lane;

% Prepare some time variables
timeStep = 0.05;                 % Lane data is provided every 50 milliseconds
numSteps = numel(visionObjects); % Number of recorded timesteps

processDetections - Reads the recorded vision detections. This example extracts only the following properties:

  1. Position: A two-dimensional [x, y] array in vehicle coordinates

  2. Width: The width of the object as reported by the video sensor (Note: The sensor does not report any other dimension of the object size.)

  3. Labels: The reported classification of the object

function videoDetections = processDetections(visionData, videoDetections)
% The video sensor reports a classification value as an integer
% according to the following enumeration (starting from 0)
ClassificationValues = {'Unknown', 'Unknown Small', 'Unknown Big', ...
    'Pedestrian', 'Bike', 'Car', 'Truck', 'Barrier'};

% The total number of objects reported by the sensor in this frame
numVideoObjects = visionData.numObjects;

% The video objects are reported only 10 times per second, but the video
% has a frame rate of 20 frames per second. To prevent the annotations from
% flickering on and off, this function returns the values from the previous
% timestep if there are no video objects.
if numVideoObjects == 0
    if nargin == 1 % Returning a result even if there is no previous value
        videoDetections = struct('positions', {}, 'labels', {}, 'widths', {});
    % Prepare a container for the relevant properties of video detections
    videoDetections = struct('positions', [], 'labels', [], 'widths', []);
    for i = 1:numVideoObjects
        videoDetections(i).widths = visionData.object(i).size(2);
        videoDetections(i).positions = visionData.object(i).position(1:2);
        videoDetections(i).labels = ClassificationValues{visionData.object(i).classification + 1};

processLanes - Reads reported lane information and converts it into parabolicLaneBoundary objects.

Lane boundaries are updated based on the laneReports from the recordings. The sensor reports the lanes as parameters of a parabolic model: % $y = ax^2 + bx + c$

function laneBoundaries = processLanes(laneReports)
% Return processed lane boundaries

% Boundary type information
types = {'Unmarked', 'Solid', 'Dashed', 'Unmarked', 'BottsDots', ...
    'Unmarked', 'Unmarked', 'DoubleSolid'};

% Read the recorded lane reports for this frame
leftLane    = laneReports.left;
rightLane   = laneReports.right;

% Create parabolicLaneBoundary objects for left and right lane boundaries
leftParams = cast([leftLane.curvature, leftLane.headingAngle, leftLane.offset], 'double');
leftBoundaries = parabolicLaneBoundary(leftParams);
leftBoundaries.BoundaryType = types{leftLane.boundaryType};

rightParams  = cast([rightLane.curvature, rightLane.headingAngle, rightLane.offset], 'double');
rightBoundaries = parabolicLaneBoundary(rightParams);
rightBoundaries.BoundaryType = types{rightLane.boundaryType};

laneBoundaries = [leftBoundaries, rightBoundaries];
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