Go to console

Detect and Track Objects with ML Kit on Android

You can use ML Kit to detect and track objects across frames of video.

When you pass ML Kit images, ML Kit returns, for each image, a list of up to five detected objects and their position in the image. When detecting objects in video streams, every object has an ID that you can use to track the object across images. You can also optionally enable coarse object classification, which labels objects with broad category descriptions.

See the ML Kit Material Design showcase app and the ML Kit quickstart sample on GitHub for examples of this API in use.

Before you begin

  1. If you haven't already, add Firebase to your Android project.
  2. In your project-level build.gradle file, make sure to include Google's Maven repository in both your buildscript and allprojects sections.
  3. Add the dependencies for the ML Kit Android libraries to your module (app-level) Gradle file (usually app/build.gradle):
    dependencies {
      // ...
    
      implementation 'com.google.firebase:firebase-ml-vision:21.0.0'
      implementation 'com.google.firebase:firebase-ml-vision-object-detection-model:17.0.0'
    }
    

1. Configure the object detector

To start detecting and tracking objects, first create an instance of FirebaseVisionObjectDetector, optionally specifying any detector settings you want to change from the default.

  1. Configure the object detector for your use case with a FirebaseVisionObjectDetectorOptions object. You can change the following settings:

    Object Detector Settings
    Detection mode STREAM_MODE (default) | SINGLE_IMAGE_MODE

    In STREAM_MODE (default), the object detector runs with low latency, but might produce incomplete results (such as unspecified bounding boxes or category labels) on the first few invocations of the detector. Also, in STREAM_MODE, the detector assigns tracking IDs to objects, which you can use to track objects across frames. Use this mode when you want to track objects, or when low latency is important, such as when processing video streams in real time.

    In SINGLE_IMAGE_MODE, the object detector waits until a detected object's bounding box and (if you enabled classification) category label are available before returning a result. As a consequence, detection latency is potentially higher. Also, in SINGLE_IMAGE_MODE, tracking IDs are not assigned. Use this mode if latency isn't critical and you don't want to deal with partial results.

    Detect and track multiple objects false (default) | true

    Whether to detect and track up to five objects or only the most prominent object (default).

    Classify objects false (default) | true

    Whether or not to classify detected objects into coarse categories. When enabled, the object detector classifies objects into the following categories: fashion goods, food, home goods, places, plants, and unknown.

    The object detection and tracking API is optimized for these two core use cases:

    • Live detection and tracking of the most prominent object in the camera viewfinder
    • Detection of multiple objects from a static image

    To configure the API for these use cases:

    Java

    // Live detection and tracking
    FirebaseVisionObjectDetectorOptions options =
            new FirebaseVisionObjectDetectorOptions.Builder()
                    .setDetectorMode(FirebaseVisionObjectDetectorOptions.STREAM_MODE)
                    .enableClassification()  // Optional
                    .build();
    
    // Multiple object detection in static images
    FirebaseVisionObjectDetectorOptions options =
            new FirebaseVisionObjectDetectorOptions.Builder()
                    .setDetectorMode(FirebaseVisionObjectDetectorOptions.SINGLE_IMAGE_MODE)
                    .enableMultipleObjects()
                    .enableClassification()  // Optional
                    .build();
    

    Kotlin

    // Live detection and tracking
    val options = FirebaseVisionObjectDetectorOptions.Builder()
            .setDetectorMode(FirebaseVisionObjectDetectorOptions.STREAM_MODE)
            .enableClassification()  // Optional
            .build()
    
    // Multiple object detection in static images
    val options = FirebaseVisionObjectDetectorOptions.Builder()
            .setDetectorMode(FirebaseVisionObjectDetectorOptions.SINGLE_IMAGE_MODE)
            .enableMultipleObjects()
            .enableClassification()  // Optional
            .build()
    
  2. Get an instance of FirebaseVisionObjectDetector:

    Java

    FirebaseVisionObjectDetector objectDetector =
            FirebaseVision.getInstance().getOnDeviceObjectDetector();
    
    // Or, to change the default settings:
    FirebaseVisionObjectDetector objectDetector =
            FirebaseVision.getInstance().getOnDeviceObjectDetector(options);
    

    Kotlin

    val objectDetector = FirebaseVision.getInstance().getOnDeviceObjectDetector()
    
    // Or, to change the default settings:
    val objectDetector = FirebaseVision.getInstance().getOnDeviceObjectDetector(options)
    

2. Run the object detector

To detect and track objects, pass images to the FirebaseVisionObjectDetector instance's processImage() method.

For each frame of video or image in a sequence, do the following:

  1. Create a FirebaseVisionImage object from your image.

    • To create a FirebaseVisionImage object from a Bitmap object:

      Java

      FirebaseVisionImage image = FirebaseVisionImage.fromBitmap(bitmap);

      Kotlin

      val image = FirebaseVisionImage.fromBitmap(bitmap)
      The image represented by the Bitmap object must be upright, with no additional rotation required.
    • To create a FirebaseVisionImage object from a media.Image object, such as when capturing an image from a device's camera, first determine the angle the image must be rotated to compensate for both the device's rotation and the orientation of camera sensor in the device:

      Java

      private static final SparseIntArray ORIENTATIONS = new SparseIntArray();
      static {
          ORIENTATIONS.append(Surface.ROTATION_0, 90);
          ORIENTATIONS.append(Surface.ROTATION_90, 0);
          ORIENTATIONS.append(Surface.ROTATION_180, 270);
          ORIENTATIONS.append(Surface.ROTATION_270, 180);
      }
      
      /**
       * Get the angle by which an image must be rotated given the device's current
       * orientation.
       */
      @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
      private int getRotationCompensation(String cameraId, Activity activity, Context context)
              throws CameraAccessException {
          // Get the device's current rotation relative to its "native" orientation.
          // Then, from the ORIENTATIONS table, look up the angle the image must be
          // rotated to compensate for the device's rotation.
          int deviceRotation = activity.getWindowManager().getDefaultDisplay().getRotation();
          int rotationCompensation = ORIENTATIONS.get(deviceRotation);
      
          // On most devices, the sensor orientation is 90 degrees, but for some
          // devices it is 270 degrees. For devices with a sensor orientation of
          // 270, rotate the image an additional 180 ((270 + 270) % 360) degrees.
          CameraManager cameraManager = (CameraManager) context.getSystemService(CAMERA_SERVICE);
          int sensorOrientation = cameraManager
                  .getCameraCharacteristics(cameraId)
                  .get(CameraCharacteristics.SENSOR_ORIENTATION);
          rotationCompensation = (rotationCompensation + sensorOrientation + 270) % 360;
      
          // Return the corresponding FirebaseVisionImageMetadata rotation value.
          int result;
          switch (rotationCompensation) {
              case 0:
                  result = FirebaseVisionImageMetadata.ROTATION_0;
                  break;
              case 90:
                  result = FirebaseVisionImageMetadata.ROTATION_90;
                  break;
              case 180:
                  result = FirebaseVisionImageMetadata.ROTATION_180;
                  break;
              case 270:
                  result = FirebaseVisionImageMetadata.ROTATION_270;
                  break;
              default:
                  result = FirebaseVisionImageMetadata.ROTATION_0;
                  Log.e(TAG, "Bad rotation value: " + rotationCompensation);
          }
          return result;
      }

      Kotlin

      private val ORIENTATIONS = SparseIntArray()
      
      init {
          ORIENTATIONS.append(Surface.ROTATION_0, 90)
          ORIENTATIONS.append(Surface.ROTATION_90, 0)
          ORIENTATIONS.append(Surface.ROTATION_180, 270)
          ORIENTATIONS.append(Surface.ROTATION_270, 180)
      }
      /**
       * Get the angle by which an image must be rotated given the device's current
       * orientation.
       */
      @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
      @Throws(CameraAccessException::class)
      private fun getRotationCompensation(cameraId: String, activity: Activity, context: Context): Int {
          // Get the device's current rotation relative to its "native" orientation.
          // Then, from the ORIENTATIONS table, look up the angle the image must be
          // rotated to compensate for the device's rotation.
          val deviceRotation = activity.windowManager.defaultDisplay.rotation
          var rotationCompensation = ORIENTATIONS.get(deviceRotation)
      
          // On most devices, the sensor orientation is 90 degrees, but for some
          // devices it is 270 degrees. For devices with a sensor orientation of
          // 270, rotate the image an additional 180 ((270 + 270) % 360) degrees.
          val cameraManager = context.getSystemService(CAMERA_SERVICE) as CameraManager
          val sensorOrientation = cameraManager
                  .getCameraCharacteristics(cameraId)
                  .get(CameraCharacteristics.SENSOR_ORIENTATION)!!
          rotationCompensation = (rotationCompensation + sensorOrientation + 270) % 360
      
          // Return the corresponding FirebaseVisionImageMetadata rotation value.
          val result: Int
          when (rotationCompensation) {
              0 -> result = FirebaseVisionImageMetadata.ROTATION_0
              90 -> result = FirebaseVisionImageMetadata.ROTATION_90
              180 -> result = FirebaseVisionImageMetadata.ROTATION_180
              270 -> result = FirebaseVisionImageMetadata.ROTATION_270
              else -> {
                  result = FirebaseVisionImageMetadata.ROTATION_0
                  Log.e(TAG, "Bad rotation value: $rotationCompensation")
              }
          }
          return result
      }

      Then, pass the media.Image object and the rotation value to FirebaseVisionImage.fromMediaImage():

      Java

      FirebaseVisionImage image = FirebaseVisionImage.fromMediaImage(mediaImage, rotation);

      Kotlin

      val image = FirebaseVisionImage.fromMediaImage(mediaImage, rotation)
    • To create a FirebaseVisionImage object from a ByteBuffer or a byte array, first calculate the image rotation as described above.

      Then, create a FirebaseVisionImageMetadata object that contains the image's height, width, color encoding format, and rotation:

      Java

      FirebaseVisionImageMetadata metadata = new FirebaseVisionImageMetadata.Builder()
              .setWidth(480)   // 480x360 is typically sufficient for
              .setHeight(360)  // image recognition
              .setFormat(FirebaseVisionImageMetadata.IMAGE_FORMAT_NV21)
              .setRotation(rotation)
              .build();

      Kotlin

      val metadata = FirebaseVisionImageMetadata.Builder()
              .setWidth(480) // 480x360 is typically sufficient for
              .setHeight(360) // image recognition
              .setFormat(FirebaseVisionImageMetadata.IMAGE_FORMAT_NV21)
              .setRotation(rotation)
              .build()

      Use the buffer or array, and the metadata object, to create a FirebaseVisionImage object:

      Java

      FirebaseVisionImage image = FirebaseVisionImage.fromByteBuffer(buffer, metadata);
      // Or: FirebaseVisionImage image = FirebaseVisionImage.fromByteArray(byteArray, metadata);

      Kotlin

      val image = FirebaseVisionImage.fromByteBuffer(buffer, metadata)
      // Or: val image = FirebaseVisionImage.fromByteArray(byteArray, metadata)
    • To create a FirebaseVisionImage object from a file, pass the app context and file URI to FirebaseVisionImage.fromFilePath():

      Java

      FirebaseVisionImage image;
      try {
          image = FirebaseVisionImage.fromFilePath(context, uri);
      } catch (IOException e) {
          e.printStackTrace();
      }

      Kotlin

      val image: FirebaseVisionImage
      try {
          image = FirebaseVisionImage.fromFilePath(context, uri)
      } catch (e: IOException) {
          e.printStackTrace()
      }
  2. Pass the image to the processImage() method:

    Java

    objectDetector.processImage(image)
            .addOnSuccessListener(
                    new OnSuccessListener<List<FirebaseVisionObject>>() {
                        @Override
                        public void onSuccess(List<FirebaseVisionObject> detectedObjects) {
                            // Task completed successfully
                            // ...
                        }
                    })
            .addOnFailureListener(
                    new OnFailureListener() {
                        @Override
                        public void onFailure(@NonNull Exception e) {
                            // Task failed with an exception
                            // ...
                        }
                    });
    

    Kotlin

    objectDetector.processImage(image)
            .addOnSuccessListener { detectedObjects ->
                // Task completed successfully
                // ...
            }
            .addOnFailureListener { e ->
                // Task failed with an exception
                // ...
            }
    
  3. If the call to processImage() succeeds, a list of FirebaseVisionObjects is passed to the success listener.

    Each FirebaseVisionObject contains the following properties:

    Bounding box A Rect indicating the position of the object in the image.
    Tracking ID An integer that identifies the object across images. Null in SINGLE_IMAGE_MODE.
    Category The coarse category of the object. If the object detector doesn't have classification enabled, this is always FirebaseVisionObject.CATEGORY_UNKNOWN.
    Confidence The confidence value of the object classificaton. If the object detector doesn't have classification enabled, or the object is classified as unknown, this is null.

    Java

    // The list of detected objects contains one item if multiple object detection wasn't enabled.
    for (FirebaseVisionObject obj : detectedObjects) {
        Integer id = obj.getTrackingId();
        Rect bounds = obj.getBoundingBox();
    
        // If classification was enabled:
        int category = obj.getClassificationCategory();
        Float confidence = obj.getClassificationConfidence();
    }
    

    Kotlin

    // The list of detected objects contains one item if multiple object detection wasn't enabled.
    for (obj in detectedObjects) {
        val id = obj.trackingId       // A number that identifies the object across images
        val bounds = obj.boundingBox  // The object's position in the image
    
        // If classification was enabled:
        val category = obj.classificationCategory
        val confidence = obj.classificationConfidence
    }
    

Improving usability and performance

For the best user experience, follow these guidelines in your app:

  • Successful object detection depends on the object's visual complexity. Objects with a small number of visual features might need to take up a larger part of the image to be detected. You should provide users with guidance on capturing input that works well with the kind of objects you want to detect.
  • When using classification, if you want to detect objects that don't fall cleanly into the supported categories, implement special handling for unknown objects.

Also, check out the ML Kit Material Design showcase app and the Material Design Patterns for machine learning-powered features collection.

When using streaming mode in a real-time application, follow these guidelines to achieve the best framerates:

  • Don't use multiple object detection in streaming mode, as most devices won't be able to produce adequate framerates.

  • Disable classification if you don't need it.

  • Throttle calls to the detector. If a new video frame becomes available while the detector is running, drop the frame.
  • If you are using the output of the detector to overlay graphics on the input image, first get the result from ML Kit, then render the image and overlay in a single step. By doing so, you render to the display surface only once for each input frame. See the CameraSourcePreview and GraphicOverlay classes in the quickstart sample app for an example.
  • If you use the Camera2 API, capture images in ImageFormat.YUV_420_888 format.

    If you use the older Camera API, capture images in ImageFormat.NV21 format.

Known issues

  • On Android API level 18 and older, the object detector recycles Bitmap objects passed to it. Pass a copy of the Bitmap if you need it later.
  • In streaming mode, if the category of an object changes, the detector assigns it a new tracking ID. This can happen as an object comes into focus, when the camera angle changes and makes the object clearer, and so on.