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.

Before you begin

  1. If you haven't already, add Firebase to your Android project.
  2. Add the dependencies for the ML Kit Android libraries to your module (app-level) Gradle file (usually app/build.gradle): 
    apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'

dependencies {
  // ...

  implementation 'com.google.firebase:firebase-ml-vision:24.0.3'
  implementation 'com.google.firebase:firebase-ml-vision-object-detection-model:19.0.6'
}

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+KTX 

    // 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+KTX 

    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 media.Image object, such as when capturing an image from a device's camera, pass the media.Image object and the image's rotation to FirebaseVisionImage.fromMediaImage().

      If you use the CameraX library, the OnImageCapturedListener and ImageAnalysis.Analyzer classes calculate the rotation value for you, so you just need to convert the rotation to one of ML Kit's ROTATION_ constants before calling FirebaseVisionImage.fromMediaImage():

      Java

      private class YourAnalyzer implements ImageAnalysis.Analyzer {

    private int degreesToFirebaseRotation(int degrees) {
        switch (degrees) {
            case 0:
                return FirebaseVisionImageMetadata.ROTATION_0;
            case 90:
                return FirebaseVisionImageMetadata.ROTATION_90;
            case 180:
                return FirebaseVisionImageMetadata.ROTATION_180;
            case 270:
                return FirebaseVisionImageMetadata.ROTATION_270;
            default:
                throw new IllegalArgumentException(
                        "Rotation must be 0, 90, 180, or 270.");
        }
    }

    @Override
    public void analyze(ImageProxy imageProxy, int degrees) {
        if (imageProxy == null || imageProxy.getImage() == null) {
            return;
        }
        Image mediaImage = imageProxy.getImage();
        int rotation = degreesToFirebaseRotation(degrees);
        FirebaseVisionImage image =
                FirebaseVisionImage.fromMediaImage(mediaImage, rotation);
        // Pass image to an ML Kit Vision API
        // ...
    }
}

      Kotlin+KTX

      private class YourImageAnalyzer : ImageAnalysis.Analyzer {
    private fun degreesToFirebaseRotation(degrees: Int): Int = when(degrees) {
        0 -> FirebaseVisionImageMetadata.ROTATION_0
        90 -> FirebaseVisionImageMetadata.ROTATION_90
        180 -> FirebaseVisionImageMetadata.ROTATION_180
        270 -> FirebaseVisionImageMetadata.ROTATION_270
        else -> throw Exception("Rotation must be 0, 90, 180, or 270.")
    }

    override fun analyze(imageProxy: ImageProxy?, degrees: Int) {
        val mediaImage = imageProxy?.image
        val imageRotation = degreesToFirebaseRotation(degrees)
        if (mediaImage != null) {
            val image = FirebaseVisionImage.fromMediaImage(mediaImage, imageRotation)
            // Pass image to an ML Kit Vision API
            // ...
        }
    }
}

      If you don't use a camera library that gives you the image's rotation, you can calculate it from 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;
}
      VisionImage.java

      Kotlin+KTX

      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
}
      VisionImage.kt

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

      Java

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

      Kotlin+KTX

      val image = FirebaseVisionImage.fromMediaImage(mediaImage, rotation)
      VisionImage.kt
    • To create a FirebaseVisionImage object from a file URI, pass the app context and file URI to FirebaseVisionImage.fromFilePath(). This is useful when you use an ACTION_GET_CONTENT intent to prompt the user to select an image from their gallery app.

      Java

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

      Kotlin+KTX

      val image: FirebaseVisionImage
try {
    image = FirebaseVisionImage.fromFilePath(context, uri)
} catch (e: IOException) {
    e.printStackTrace()
}
      VisionImage.kt
    • To create a FirebaseVisionImage object from a ByteBuffer or a byte array, first calculate the image rotation as described above for media.Image input.

      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();
      VisionImage.java

      Kotlin+KTX

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

      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);
      VisionImage.java

      Kotlin+KTX

      val image = FirebaseVisionImage.fromByteBuffer(buffer, metadata)
// Or: val image = FirebaseVisionImage.fromByteArray(byteArray, metadata)
      VisionImage.kt
    • To create a FirebaseVisionImage object from a Bitmap object:

      Java

      FirebaseVisionImage image = FirebaseVisionImage.fromBitmap(bitmap);
      VisionImage.java

      Kotlin+KTX

      val image = FirebaseVisionImage.fromBitmap(bitmap)
      VisionImage.kt
      The image represented by the Bitmap object must be upright, with no additional rotation required.

  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+KTX 

    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 classification. 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+KTX 

    // 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][showcase-link]{: .external } 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.

  • 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.