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Get Started with the Video Labeler

The Video Labeler app provides an easy way to mark axis-aligned or rotated rectangular region of interest (ROI) labels, polyline ROI labels, pixel ROI labels, and scene labels in a video or image sequence.

You can use labeled data to validate or train algorithms such as image classifiers, object detectors, and semantic and instance segmentation networks. Consider your application when choosing a labeling drawing tool to create ROI labels. For more details on how to select the right label type and drawing tool for your application, see ROI Labels, Sublabels, and Attributes.

This example gets you started using the app by showing you how to:

  • Manually label an image frame from a video.

  • Automatically label across image frames using an automation algorithm.

  • Export the labeled ground truth data.

ROI and Scene Label Definitions

  • An ROI label corresponds to either an axis-aligned or rotated rectangular, polyline, pixel, or polygon region of interest. These labels contain two components: the label name, such as "cars," and the region you create.

  • A Scene label describes the nature of a scene, such as "sunny." You can associate this label with a frame.

Load Unlabeled Data

Programmatically open the app and load a video. Videos must be in a file format readable by VideoReader.

You can also load a video from the app. Open the app from the Apps tab, under Image Processing and Computer Vision. Click Import to load a video or image sequence, or click Open Session to load a saved session.

Explore the video. Click the Play button to play the entire video, or use the slider to navigate between frames.

To load an image sequence with corresponding timestamps, select Import > Image Sequence. The app supports all image file formats supported by imread. To read additional file formats, you can create an imageDatastore and use the ReadFcn property.

To load a custom data source that cannot be read by VideoReader or imread, see Use Custom Image Source Reader for Labeling.

Create Label Definitions

Define the labels you intend to draw. In this example, you define labels directly within the app. To define labels from the MATLAB® command line instead, use the labelDefinitionCreator.

Set Time Interval

You can label the entire video or start with a portion of the video. In this example, you label a five-second time range within the loaded video. In the text boxes below the video, enter these times in seconds:

  1. In the Current Time box, type 5 and press Enter.

  2. In the Start Time box, type 5 so that the slider is at the start of the time range.

  3. In the End Time box, type 10.

Optionally, to make adjustments to the time range, click and drag the red interval flags.

The entire app is now set up to focus on this specific time range. The video plays only within this interval, and labeling and automation algorithms apply only to this interval. You can change the interval at any time by moving the flags.

To expand the time range to fill the entire playback section, click Zoom in Time Range.

Create ROI Label

An ROI label is a label that corresponds to a region of interest (ROI) in an image. You can define these ROI label types.

  • Rectangle — Draw 2-D rectangular bounding box labels around objects in an image, such as vehicles, boats, buildings.

  • Rotated Rectangle — Draw 2-D rotated-rectangular bounding box labels around objects in an image, such as vehicles, boats, buildings.

  • Projected cuboid — Draw 3-D bounding box labels around objects in an image, such as vehicles, boats, buildings.

  • Line — Draw linear ROIs to label lines, such as lane boundaries.

  • Pixel label — Draw pixels to label various classes, such as road or sky, for semantic segmentation. For more information about pixel labeling, see Label Pixels for Semantic Segmentation.

  • Polygon — Draw polygon labels around objects. You can label distinct instances of the same class. For more information on drawing polygon ROI labels for instance and semantic segmentation networks, see Label Objects Using Polygons

For more details about these ROI label definitions, see ROI Labels, Sublabels, and Attributes.

In this example, you define a vehicle group for labeling types of vehicles, and then create two Rectangle ROI labels, Car and Truck. After creating labels, you can use the Show ROI Labels drop-down menu to select On Hover, Always, or Never to control how the ROI label names appear during labeling. By default, the names will appear when you hover on an ROI.

  1. In the ROI Labels pane on the left, click Label.

  2. Create a Rectangle label named Car.

  3. Optionally, change the label color by clicking the preview color.

  4. From the Group drop-down menu, select New Group and name the group Vehicle

  5. Click OK.

    The Vehicle group name appears in the ROI Labels pane with the label Car created. You can move a label in the list to a different position or group in the list by left-clicking and dragging the label up or down.

  6. To add the second label, click Label. Name the label Truck and make sure the Vehicle group is selected. Click OK.

  7. Use the mouse to draw rectangular Car ROIs around the two vehicles.

Create Sublabels

A sublabel is a type of ROI label that corresponds to a parent ROI label. Each sublabel must belong to, or be a child of, a specific label defined in the ROI Labels pane. For example, in a driving scene, a vehicle label might have sublabels for headlights, license plates, or wheels. For more details about sublabels, see ROI Labels, Sublabels, and Attributes.

Define a sublabel for headlights.

  1. In the ROI Labels pane on the left, click the Car label.

  2. Click Sublabel.

  3. Create a Rectangle sublabel named headlight and optionally write a description. Click OK.

    The headlight sublabel appears in the ROI Labels pane. The sublabel is nested under the selected ROI label, Car, and has the same color as its parent label.

    You can add multiple sublabels under a label. You can also drag-and-drop the sublabels to reorder them in the list. Right-click any label for additional edits.

  4. In the ROI Labels pane, select the headlight sublabel.

  5. In the image frame, select the Car label. The label turns yellow when selected. You must select the Car label (parent ROI) before you can add a sublabel to it.

    Draw headlight sublabels for each of the cars.

  6. Repeat the previous steps to label the headlights of the other car. To draw the labels more precisely, use the pan and zoom options located in the upper-right corner of the labeling window.

Sublabels can only be used with rectangular or polyline ROI labels and cannot have their own sublabels. For more details on working with sublabels, see Use Sublabels and Attributes to Label Ground Truth Data.

Show or Hide Labels and Sublabels

You can show or hide the labels or sublabels in a labeled video by using the Eye icon icon on the ROI Labels pane. The Eye icon appears only after you define a label or sublabel. By default, the app displays all the labels and the sublabels.

To hide a label or sublabel, click on the Eye icon icon along side the label or sublabel name. The app hides the corresponding label or sublabel and displays the Hide icon icon.

Show labels

Hide labels


Create Attributes

An ROI attribute specifies additional information about an ROI label or sublabel. For example, in a driving scene, attributes might include the type or color of a vehicle. In the ocean scene, attributes might include the type of sailboat or number of sails. You can define ROI attributes of these types.

  • Numeric Value — Specify a numeric scalar attribute, such as the number of doors on a labeled vehicle or the number of sails on a sailboat.

  • String — Specify a string scalar attribute, such as the color of a vehicle or boat.

  • Logical — Specify a logical true or false attribute, such as whether a vehicle is in motion or a boat is making way.

  • List — Specify a drop-down list attribute of predefined strings, such as make or model of a vehicle or boat.

For more details about these attribute types, see ROI Labels, Sublabels, and Attributes.

Add an attribute for the vehicle type.

  1. In the ROI Labels pane on the left, select the Car label and click Attribute.

  2. In the Attribute Name box, type carType. Set the attribute type to List.

  3. In the List Items section, type different types of cars, such as Sedan, Hatchback, SUV, and Wagon, each on its own line. Optionally give the attribute a description, and click OK. You can hover the information icon that appears next to the attribute field to display the added description.

  4. Select a Car ROI label in the image frame. In the Attributes and Sublabels pane, select the appropriate carType attribute value for that vehicle.

  5. Repeat the previous step to assign a carType attribute to the other vehicle.

You can also add attributes to sublabels. Add an attribute for the headlight sublabel that tells whether the headlight is on.

  1. In the ROI Labels pane on the left, select the headlight sublabel and click Attribute.

  2. In the Attribute Name box, type isOn. Set the attribute type to Logical. Leave the Default Value set to Empty, optionally write a description, and click OK.

  3. Select a headlight in the video frame. Set the appropriate isOn attribute value, or leave the attribute value set to Empty.

  4. Repeat the previous step to set the isOn attribute for the other headlights.

To delete an attribute, right-click an ROI label or sublabel, and select the attribute to delete. Deleting the attribute removes attribute information from all previously created ROI label annotations.

Create Scene Labels

A scene label defines additional information for the entire scene. Use scene labels to describe conditions, such as lighting and weather, or events, such as lane changes.

Create a scene label to apply to an image.

  1. In the left pane of the app, select the Scene Labels tab next to the ROI Labels tab.

  2. Click Define new scene label, and in the Label Name box, enter a scene label named daytime.

  3. Change the color of the label definition to light blue to reflect the nature of the scene label. Under the Color parameter, click the color preview and select the standard light blue colors. Then, click OK to close the color selection window.

  4. Leave the Group parameter set to the default of None and click OK. The Scene Labels pane shows the scene label definition.

  5. Click Add Label to apply the daytime label to the scene. A checkmark appears next to the scene label.

  6. To edit or delete a scene label, right-click on the label and select either Edit Label or Delete Label.

Label Ground Truth

So far, you have labeled only one frame in the video. To label the remaining frames, choose one of these options.

Label Ground Truth Manually

When you click the right arrow key to advance to the next frame, the ROI labels from the previous frame do not carry over. Only the sunny scene label applies to each frame, because this label was applied over the entire time range.

Advance frame by frame and draw the label and sublabel ROIs manually. Also update the attribute information for these ROIs.

Label Ground Truth Using Automation Algorithm

To speed up the labeling process, you can use an automation algorithm within the app. You can either define your own automation algorithm, see Create Automation Algorithm for Labeling and Temporal Automation Algorithms, or use a built-in automation algorithm. In this example, you label the ground truth using a built-in point tracking algorithm.

In this example, you automate the labeling of only the Car ROI labels. The built-in automation algorithms do not support sublabel and attribute automation.

  1. Select the labels you want to automate. In the first frame of the video, press Ctrl and click to select the two Car label annotations. The labels are highlighted in yellow.

  2. From the app toolstrip, select Select Algorithm > Point Tracker. This algorithm tracks one or more rectangle ROIs over short intervals using the Kanade-Lucas-Tomasi (KLT) algorithm.

  3. (optional) Configure the automation settings. For example, make sure that Import selected ROIs is selected so that the Car labels you selected are imported into the automation session.

  4. Click Automate to open an automation session. The algorithm instructions appear in the right pane, and the selected labels are available to automate.

  5. Click Run to track the selected ROIs over the interval.

  6. Examine the results of running the algorithm.

    The vehicles that enter the scene later are unlabeled. The unlabeled vehicles did not have an initial ROI label, so the algorithm did not track them. Click Undo Run. Use the slider to find the frames where each vehicle first appears. Draw vehicle ROIs around each vehicle, and then click Run again.

  7. Advance frame by frame and manually move, resize, delete, or add ROIs to improve the results of the automation algorithm.

    When you are satisfied with the algorithm results, click Accept. Alternatively, to discard labels generated during the session and label manually instead, click Cancel. The Cancel button cancels only the algorithm session, not the app session.

Optionally, you can now manually label the remaining frames with sublabel and attribute information.

To further evaluate your labels, you can view a visual summary of the labeled ground truth. From the app toolstrip, select View Label Summary. Use this summary to compare the frames, frequency of labels, and scene conditions. For more details, see View Summary of Ground Truth Labels. This summary does not support sublabels or attributes.

Export Labeled Ground Truth

You can export the labeled ground truth to a MAT-file or to a variable in the MATLAB workspace. In both cases, the labeled ground truth is stored as a groundTruth object. You can use this object to train a deep-learning-based computer vision algorithm. For more details, see Training Data for Object Detection and Semantic Segmentation.


If you export pixel data, the pixel label data and ground truth data are saved in separate files but in the same folder. For considerations when working with exported pixel labels, see How Labeler Apps Store Exported Pixel Labels.

In this example, you export the labeled ground truth to the MATLAB workspace. From the app toolstrip, select Export Labels > To Workspace. The exported MATLAB variable is gTruth.

Display the properties of the exported groundTruth object. The information in your exported object might differ from the information shown here.

gTruth = 

  groundTruth with properties:

          DataSource: [1×1 groundTruthDataSource]
    LabelDefinitions: [2x6 table]
           LabelData: [531×3 timetable]

Data Source

DataSource is a groundTruthDataSource object containing the path to the images or video and timestamps. Display the properties of this object.

ans = 

groundTruthDataSource for a video file with properties

        Source: ...matlab\toolbox\vision\visiondata\visiontraffic.avi
    TimeStamps: [531×1 duration]

Label Definitions

LabelDefinitions is a table containing information about the label definitions. This table does not contain information about the labels that are drawn on the video frames. To save the label definitions in their own MAT-file, from the app toolstrip, select Export Labels > To File. You can then import these label definitions into another app session by selecting Import Files.

Display the label definitions table. Each row contains information about an ROI label definition or a scene label definition. If you exported pixel label data, the LabelDefinitions table also includes a PixelLabelID column containing the ID numbers for each pixel label definition.

ans =
  3×6 table

      Name         Type        LabelColor        Group       Description     Hierarchy  
    _________    _________    ____________    ___________    ___________    ____________

    {'Car'  }    Rectangle    {1×3 double}    {'Vehicle'}    {0×0 char}     {1×1 struct}
    {'Truck'}    Rectangle    {1×3 double}    {'Vehicle'}    {0×0 char}     {0×0 double}
    {'Sunny'}    Scene        {1×3 double}    {'Weather'}    {0×0 char}     {0×0 double} 

Within LabelDefinitions, the Hierarchy column stores information about the sublabel and attribute definitions of a parent ROI label.

Display the sublabel and attribute information for the Car label.

ans = 

  struct with fields:
       numDoors: [1×1 struct]
          color: [1×1 struct]
       inMotion: [1×1 struct]
        carType: [1×1 struct]
      headlight: [1×1 struct]
           Type: Rectangle
    Description: ''

Display information about the headlight sublabel.

ans = 

  struct with fields:
           Type: Rectangle
    Description: ''
          Color: [0.5862 0.8276 0.3103]
           isOn: [1×1 struct]

Display information about the carType attribute.

ans = 

  struct with fields:

      ListItems: {3×1 cell}
    Description: ''

Label Data

LabelData is a timetable containing information about the ROI labels drawn at each timestamp, across the entire video. The timetable contains one column per label.

Display the first few rows of the timetable. The first few timestamps indicate that no vehicles were detected and that the sunny scene label is false. These results are because this portion of the video was not labeled. Only the time range of 5–10 seconds was labeled. This code shows the interval labeled with the sunny scene label.

labelData = gTruth.LabelData;
gTruthInterval = labelData(timerange('00:00:05','00:00:10'),:);
ans =

  8×3 timetable

       Time           Car            Truck        sunny
    __________    ____________    ____________    _____

    5.005 sec     [1×2 struct]    [1×0 struct]    true 
    5.0384 sec    [1×2 struct]    [1×0 struct]    true 
    5.0717 sec    [1×2 struct]    [1×0 struct]    true 
    5.1051 sec    [1×2 struct]    [1×0 struct]    true 
    5.1385 sec    [1×2 struct]    [1×0 struct]    true 
    5.1718 sec    [1×2 struct]    [1×0 struct]    true 
    5.2052 sec    [1×2 struct]    [1×0 struct]    true 
    5.2386 sec    [1×2 struct]    [1×0 struct]    true 

Display the first few timetable rows from the 5-10 second interval that contains labels.

gTruthInterval = labelData(timerange('00:00:05','00:00:10'),:);
ans =

  8×3 timetable

       Time           Car            Truck        sunny
    __________    ____________    ____________    _____

    5.005 sec     [1×2 struct]    [1×0 struct]    true 
    5.0384 sec    [1×2 struct]    [1×0 struct]    true 
    5.0717 sec    [1×2 struct]    [1×0 struct]    true 
    5.1051 sec    [1×2 struct]    [1×0 struct]    true 
    5.1385 sec    [1×2 struct]    [1×0 struct]    true 
    5.1718 sec    [1×2 struct]    [1×0 struct]    true 
    5.2052 sec    [1×2 struct]    [1×0 struct]    true 
    5.2386 sec    [1×2 struct]    [1×0 struct]    true 

For each Car label, the structure includes the position of the bounding box and information about its sublabels and attributes.

Display the bounding box positions for the vehicles at the start of the time range. Your bounding box positions might differ from the ones shown here.

gTruthInterval(1,:).Car{1}.Position % [x y width height], in pixels
ans =

  1×4 single row vector

  415.8962   82.4737  130.8474  129.3805

ans =

  1×4 single row vector

  235.2182    1.0000  117.0611   55.3500

Save App Session

From the app toolstrip, select Save and save a MAT-file of the app session. The saved session includes the data source, label definitions, and labeled ground truth. It also includes your session preferences, such as the layout of the app. To change layout options, select Layout.

At any time during a session, you can select New Session to start a new session. You have the option of saving the current session or cancelling.

The app session MAT-file is separate from the ground truth MAT-file that is exported when you select Export > From File. To share labeled ground truth data, as a best practice, share the ground truth MAT-file containing the groundTruth object, not the app session MAT-file. For more details, see Share and Store Labeled Ground Truth Data.

See Also



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