Object detection with Android

This tutorial shows you how to build an Android app using TensorFlow Lite to continuously detect objects in frames captured by a device camera. This application is designed for a physical Android device. If you are updating an existing project, you can use the code sample as a reference and skip ahead to the instructions for modifying your project.

Object detection overview

Object detection is the machine learning task of identifying the presence and location of multiple classes of objects within an image. An object detection model is trained on a dataset that contains a set of known objects.

The trained model receives image frames as input and attempts to categorize items in the images from the set of known classes it was trained to recognize. For each image frame, the object detection model outputs a list of the objects it detects, the location of a bounding box for each object, and a score that indicates the confidence of the object being correctly classified.

Models and dataset

This tutorial uses models that were trained using the COCO dataset. COCO is a large-scale object detection dataset that contains 330K images, 1.5 million object instances, and 80 object categories.

You have the option to use one of the following pre-trained models:

  • EfficientDet-Lite0 [Recommended] - a lightweight object detection model with a BiFPN feature extractor, shared box predictor, and focal loss. The mAP (mean Average Precision) for the COCO 2017 validation dataset is 25.69%.

  • EfficientDet-Lite1 - a medium-sized EfficientDet object detection model. The mAP for the COCO 2017 validation dataset is 30.55%.

  • EfficientDet-Lite2 - a larger EfficientDet object detection model. The mAP for the COCO 2017 validation dataset is 33.97%.

  • MobileNetV1-SSD - an extremely lightweight model optimized to work with TensorFlow Lite for object detection. The mAP for the COCO 2017 validation dataset is 21%.

For this tutorial, the EfficientDet-Lite0 model strikes a good balance between size and accuracy.

Downloading, extraction, and placing the models into the assets folder is managed automatically by the download.gradle file, which is run at build time. You don't need to manually download TFLite models into the project.

Setup and run example

To setup the object detection app, download the sample from GitHub and run it using Android Studio. The following sections of this tutorial explore the relevant sections of the code example, so you can apply them to your own Android apps.

System requirements

  • Android Studio version 2021.1.1 (Bumblebee) or higher.
  • Android SDK version 31 or higher
  • Android device with a minimum OS version of SDK 24 (Android 7.0 - Nougat) with developer mode enabled.

Get the example code

Create a local copy of the example code. You will use this code to create a project in Android Studio and run the sample application.

To clone and setup the example code:

  1. Clone the git repository
    git clone https://github.com/tensorflow/examples.git
    
  2. Optionally, configure your git instance to use sparse checkout, so you have only the files for the object detection example app:
    cd examples
    git sparse-checkout init --cone
    git sparse-checkout set lite/examples/object_detection/android
    

Import and run the project

Create a project from the downloaded example code, build the project, and then run it.

To import and build the example code project:

  1. Start Android Studio.
  2. From the Android Studio, select File > New > Import Project.
  3. Navigate to the example code directory containing the build.gradle file (.../examples/lite/examples/object_detection/android/build.gradle) and select that directory.
  4. If Android Studio requests a Gradle Sync, choose OK.
  5. Ensure that your Android device is connected to your computer and developer mode is enabled. Click the green Run arrow.

If you select the correct directory, Android Studio creates a new project and builds it. This process can take a few minutes, depending on the speed of your computer and if you have used Android Studio for other projects. When the build completes, the Android Studio displays a BUILD SUCCESSFUL message in the Build Output status panel.

Optional: To fix build errors by updating the Android plugin version:

  1. Open the build.gradle file in the project directory.
  2. Change the Android tools version as follows:

    // from: classpath
    'com.android.tools.build:gradle:4.2.2'
    // to: classpath
    'com.android.tools.build:gradle:4.1.2'
    
  3. Sync the project by selecting: File > Sync Project with Gradle Files.

To run the project:

  1. From Android Studio, run the project by selecting Run > Run….
  2. Select an attached Android device with a camera to test the app.

The next sections show you the modifications you need to make to your existing project to add this functionality to your own app, using this example app as a reference point.

Add project dependencies

In your own application, you must add specific project dependencies to run TensorFlow Lite machine learning models, and access utility functions that convert data such as images, into a tensor data format that can be processed by the model you are using.

The example app uses the TensorFlow Lite Task library for vision to enable execution of the object detection machine learning model. The following instructions explain how to add the required library dependencies to your own Android app project.

The following instructions explain how to add the required project and module dependencies to your own Android app project.

To add module dependencies:

  1. In the module that uses TensorFlow Lite, update the module's build.gradle file to include the following dependencies. In the example code, this file is located here: ...examples/lite/examples/object_detection/android/app/build.gradle (code reference)

    dependencies {
      ...
      implementation 'org.tensorflow:tensorflow-lite-task-vision:0.4.0'
      // Import the GPU delegate plugin Library for GPU inference
      implementation 'org.tensorflow:tensorflow-lite-gpu-delegate-plugin:0.4.0'
      implementation 'org.tensorflow:tensorflow-lite-gpu:2.9.0'
    }
    

    The project must include the Vision task library (tensorflow-lite-task-vision). The graphics processing unit (GPU) library (tensorflow-lite-gpu-delegate-plugin) provides the infrastructure to run the app on GPU, and Delegate (tensorflow-lite-gpu) provides the compatibility list.

  2. In Android Studio, sync the project dependencies by selecting: File > Sync Project with Gradle Files.

Initialize the ML model

In your Android app, you must initialize the TensorFlow Lite machine learning model with parameters before running predictions with the model. These initialization parameters are consistent across object detection models and can include settings such as minimum accuracy thresholds for predictions.

A TensorFlow Lite model includes a .tflite file containing the model code and frequently includes a labels file containing the names of the classes predicted by the model. In the case of object detection, classes are objects such as a person, dog, cat, or car.

This example downloads several models that are specified in download_models.gradle, and the ObjectDetectorHelper class provides a selector for the models:

val modelName =
  when (currentModel) {
    MODEL_MOBILENETV1 -> "mobilenetv1.tflite"
    MODEL_EFFICIENTDETV0 -> "efficientdet-lite0.tflite"
    MODEL_EFFICIENTDETV1 -> "efficientdet-lite1.tflite"
    MODEL_EFFICIENTDETV2 -> "efficientdet-lite2.tflite"
    else -> "mobilenetv1.tflite"
  }

To initialize the model in your app:

  1. Add a .tflite model file to the src/main/assets directory of your development project, such as: EfficientDet-Lite0.
  2. Set a static variable for your model's file name. In the example app, you set the modelName variable to MODEL_EFFICIENTDETV0 to use the EfficientDet-Lite0 detection model.
  3. Set the options for model, such as the prediction threshold, results set size, and optionally, hardware acceleration delegates:

    val optionsBuilder =
      ObjectDetector.ObjectDetectorOptions.builder()
        .setScoreThreshold(threshold)
        .setMaxResults(maxResults)
    
  4. Use the settings from this object to construct a TensorFlow Lite ObjectDetector object that contains the model:

    objectDetector =
      ObjectDetector.createFromFileAndOptions(
        context, modelName, optionsBuilder.build())
    

The setupObjectDetector sets up the following model parameters:

  • Detection threshold
  • Maximum number of detection results
  • Number of processing threads to use (BaseOptions.builder().setNumThreads(numThreads))
  • Actual model (modelName)
  • ObjectDetector object (objectDetector)

Configure hardware accelerator

When initializing a TensorFlow Lite model in your application, you can use hardware acceleration features to speed up the prediction calculations of the model.

TensorFlow Lite delegates are software modules that accelerate execution of machine learning models using specialized processing hardware on a mobile device, such as Graphics Processing Units (GPUs), Tensor Processing Units (TPUs), and Digital Signal Processors (DSPs). Using delegates for running TensorFlow Lite models is recommended, but not required.

The object detector is initialized using the current settings on the thread that is using it. You can use CPU and NNAPI delegates with detectors that are created on the main thread and used on a background thread, but the thread that initialized the detector must use the GPU delegate.

The delegates are set within the ObjectDetectionHelper.setupObjectDetector() function:

when (currentDelegate) {
    DELEGATE_CPU -> {
        // Default
    }
    DELEGATE_GPU -> {
        if (CompatibilityList().isDelegateSupportedOnThisDevice) {
            baseOptionsBuilder.useGpu()
        } else {
            objectDetectorListener?.onError("GPU is not supported on this device")
        }
    }
    DELEGATE_NNAPI -> {
        baseOptionsBuilder.useNnapi()
    }
}

For more information about using hardware acceleration delegates with TensorFlow Lite, see TensorFlow Lite Delegates.

Prepare data for the model

In your Android app, your code provides data to the model for interpretation by transforming existing data such as image frames into a Tensor data format that can be processed by your model. The data in a Tensor you pass to a model must have specific dimensions, or shape, that matches the format of data used to train the model.

The EfficientDet-Lite0 model used in this code example accepts Tensors representing images with a dimension of 320 x 320, with three channels (red, blue, and green) per pixel. Each value in the tensor is a single byte between 0 and 255. So, to run predictions on new images, your app must transform that image data into Tensor data objects of that size and shape. The TensorFlow Lite Task Library Vision API handles the data transformation for you.

The app uses an ImageAnalysis object to pull images from the camera. This object calls the detectObject function with bitmap from the camera. The data is automatically resized and rotated by ImageProcessor so that it meets the image data requirements of the model. The image is then translated into a TensorImage object.

To prepare data from the camera subsystem to be processed by the ML model:

  1. Build an ImageAnalysis object to extract images in the required format:

    imageAnalyzer =
        ImageAnalysis.Builder()
            .setTargetAspectRatio(AspectRatio.RATIO_4_3)
            .setTargetRotation(fragmentCameraBinding.viewFinder.display.rotation)
            .setBackpressureStrategy(ImageAnalysis.STRATEGY_KEEP_ONLY_LATEST)
            .setOutputImageFormat(OUTPUT_IMAGE_FORMAT_RGBA_8888)
            .build()
            ...
    
  2. Connect the analyzer to the camera subsystem and create a bitmap buffer to contain the data received from the camera:

    .also {
      it.setAnalyzer(cameraExecutor) {
        image -> if (!::bitmapBuffer.isInitialized)
        { bitmapBuffer = Bitmap.createBitmap( image.width, image.height,
        Bitmap.Config.ARGB_8888 ) } detectObjects(image)
        }
      }
    
  3. Extract the specific image data needed by the model, and pass the image rotation information:

    private fun detectObjects(image: ImageProxy) {
      //Copy out RGB bits to the shared bitmap buffer
      image.use {bitmapBuffer.copyPixelsFromBuffer(image.planes[0].buffer) }
        val imageRotation = image.imageInfo.rotationDegrees
        objectDetectorHelper.detect(bitmapBuffer, imageRotation)
      }
    
  4. Complete any final data transformations and add the image data to a TensorImage object, as shown in the ObjectDetectorHelper.detect() method of the example app:

    val imageProcessor = ImageProcessor.Builder().add(Rot90Op(-imageRotation / 90)).build()
    // Preprocess the image and convert it into a TensorImage for detection.
    val tensorImage = imageProcessor.process(TensorImage.fromBitmap(image))
    

Run predictions

In your Android app, once you create a TensorImage object with image data in the correct format, you can run the model against that data to produce a prediction, or inference.

In the fragments/CameraFragment.kt class of the example app, the imageAnalyzer object within the bindCameraUseCases function automatically passes data to the model for predictions when the app is connected to the camera.

The app uses the cameraProvider.bindToLifecycle() method to handle the camera selector, preview window, and ML model processing. The ObjectDetectorHelper.kt class handles passing the image data into the model. To run the model and generate predictions from image data:

  • Run the prediction by passing the image data to your predict function:

    val results = objectDetector?.detect(tensorImage)
    

The TensorFlow Lite Interpreter object receives this data, runs it against the model, and produces a list of predictions. For continuous processing of data by the model, use the runForMultipleInputsOutputs() method so that Interpreter objects are not created and then removed by the system for each prediction run.

Handle model output

In your Android app, after you run image data against the object detection model, it produces a list of predictions which your app code must handle by executing additional business logic, displaying results to the user, or taking other actions.

The output of any given TensorFlow Lite model varies in terms of the number of predictions it produces (one or many), and the descriptive information for each prediction. In the case of an object detection model, predictions typically include data for a bounding box that indicates where an object is detected in the image. In the example code, the results are passed to the onResults function in CameraFragment.kt, which is acting as a DetectorListener on the object detection process.

interface DetectorListener {
  fun onError(error: String)
  fun onResults(
    results: MutableList<Detection>?,
    inferenceTime: Long,
    imageHeight: Int,
    imageWidth: Int
  )
}

For the model used in this example, each prediction includes a bounding box location for the object, a label for the object, and a prediction score between 0 and 1 as a Float representing the confidence of the prediction, with 1 being the highest confidence rating. In general, predictions with a score below 50% (0.5) are considered inconclusive. However, how you handle low-value prediction results is up to you and the needs of your application.

To handle model prediction results:

  1. Use a listener pattern to pass results to your app code or user interface objects. The example app uses this pattern to pass detection results from the ObjectDetectorHelper object to the CameraFragment object:

    objectDetectorListener.onResults(
    // instance of CameraFragment
        results,
        inferenceTime,
        tensorImage.height,
        tensorImage.width)
    
  2. Act on the results, such as displaying the prediction to the user. The example draws an overlay on the CameraPreview object to show the result:

    override fun onResults(
      results: MutableList<Detection>?,
      inferenceTime: Long,
      imageHeight: Int,
      imageWidth: Int
    ) {
        activity?.runOnUiThread {
            fragmentCameraBinding.bottomSheetLayout.inferenceTimeVal.text =
                String.format("%d ms", inferenceTime)
    
            // Pass necessary information to OverlayView for drawing on the canvas
            fragmentCameraBinding.overlay.setResults(
                results ?: LinkedList<Detection>(),
                imageHeight,
                imageWidth
            )
    
            // Force a redraw
            fragmentCameraBinding.overlay.invalidate()
        }
    }
    

Once the model has returned a prediction result, your application can act on that prediction by presenting the result to your user or executing additional logic. In the case of the example code, the application draws a bounding box around the identified object and displays the class name on screen.

Next steps

  • Explore various uses of TensorFlow Lite in the examples.
  • Learn more about using machine learning models with TensorFlow Lite in the Models section.
  • Learn more about implementing machine learning in your mobile application in the TensorFlow Lite Developer Guide.