Object detection guide for Android

The Object Detector task lets you detect the presence and location of multiple classes of objects. For example, an object detector can locate dogs within in an image. These instructions show you how to use the Object Detector task on Android. The code sample described in these instructions is available on GitHub. You can see this task in action by viewing this Web demo. For more information about the capabilities, models, and configuration options of this task, see the Overview.

Code example

The MediaPipe Tasks example code is a simple implementation of a Object Detector app for Android. The example uses the camera on a physical Android device to continuously detect objects, and can also use images and videos from the device gallery to statically detect objects.

You can use the app as a starting point for your own Android app, or refer to it when modifying an existing app. The Object Detector example code is hosted on GitHub.

Download the code

The following instructions show you how to create a local copy of the example code using the git command line tool.

To download the example code:

1. Clone the git repository using the following command:

   git clone https://github.com/google-ai-edge/mediapipe-samples
   

2. Optionally, configure your git instance to use sparse checkout, so you have only the files for the Object Detector example app:

   cd mediapipe
   git sparse-checkout init --cone
   git sparse-checkout set examples/object_detection/android
   

After creating a local version of the example code, you can import the project into Android Studio and run the app. For instructions, see the Setup Guide for Android.

Key components

The following files contain the crucial code for the Object Detector example application:

• ObjectDetectorHelper.kt - Initializes the object detector and handles the model and delegate selection
• MainActivity.kt - Implements the application and assembles the user interface components
• OverlayView.kt - Handles and displays the results

Setup

This section describes key steps for setting up your development environment and code projects to use Object Detector. For general information on setting up your development environment for using MediaPipe tasks, including platform version requirements, see the Setup guide for Android.

Dependencies

Object Detector uses the com.google.mediapipe:tasks-vision library. Add this dependency to the build.gradle file of your Android app development project. Import the required dependencies with the following code:

dependencies {
    implementation 'com.google.mediapipe:tasks-vision:latest.release'
}

Model

The MediaPipe Object Detector task requires a trained model that is compatible with this task. For more information on available trained models for Object Detector, see the task overview Models section.

Select and download the model, and then store it within your project directory:

<dev-project-root>/src/main/assets

Use the BaseOptions.Builder.setModelAssetPath() method to specify the path used by the model. For a code example, see the next section.

Create the task

You can use the createFromOptions function to create the task. The createFromOptions function accepts configuration options including running mode, display names locale, max number of results, confidence threshold, category allowlist and denylist. If a configuration option is not specified, the default value will be used. For more information on configuration options, see Configuration Overview.

The Object Detector task supports 3 input data types: still images, video files and live video streams. You need to specify the running mode corresponding to your input data type when creating the task. Choose the tab corresponding to your input data type to see how to create the task and run inference.

ObjectDetectorOptions options =
  ObjectDetectorOptions.builder()
    .setBaseOptions(BaseOptions.builder().setModelAssetPath(‘model.tflite’).build())
    .setRunningMode(RunningMode.IMAGE)
    .setMaxResults(5)
    .build();
objectDetector = ObjectDetector.createFromOptions(context, options);
    

ObjectDetectorOptions options =
  ObjectDetectorOptions.builder()
    .setBaseOptions(BaseOptions.builder().setModelAssetPath(‘model.tflite’).build())
    .setRunningMode(RunningMode.VIDEO)
    .setMaxResults(5)
    .build();
objectDetector = ObjectDetector.createFromOptions(context, options);
    

ObjectDetectorOptions options =
  ObjectDetectorOptions.builder()
    .setBaseOptions(BaseOptions.builder().setModelAssetPath(‘model.tflite’).build())
    .setRunningMode(RunningMode.LIVE_STREAM)
    .setMaxResults(5)
    .setResultListener((result, inputImage) -> {
      // Process the detection result here.
    })
    .setErrorListener((result, inputImage) -> {
      // Process the classification errors here.
    })
   .build();
objectDetector = ObjectDetector.createFromOptions(context, options);
    

The Object Detector example code implementation allows the user to switch between processing modes. The approach makes the task creation code more complicated and may not be appropriate for your use case. You can see this code in the ObjectDetectorHelper class setupObjectDetector() function.

Configuration options

This task has the following configuration options for Android apps:

Prepare data

You need to convert the input image or frame to a com.google.mediapipe.framework.image.MPImage object before passing it to the Object Detector.

The following examples explain and show how to prepare data for processing for each of the available data types:

import com.google.mediapipe.framework.image.BitmapImageBuilder;
import com.google.mediapipe.framework.image.MPImage;

// Load an image on the user’s device as a Bitmap object using BitmapFactory.

// Convert an Android’s Bitmap object to a MediaPipe’s Image object.
Image mpImage = new BitmapImageBuilder(bitmap).build();
    

import com.google.mediapipe.framework.image.BitmapImageBuilder;
import com.google.mediapipe.framework.image.MPImage;

// Load a video file on the user's device using MediaMetadataRetriever

// From the video’s metadata, load the METADATA_KEY_DURATION and
// METADATA_KEY_VIDEO_FRAME_COUNT values. Use these values
// to calculate the timestamp of each frame later.

// Loop through the video and load each frame as a Bitmap object.

// Convert the Android’s Bitmap object to a MediaPipe’s Image object.
Image mpImage = new BitmapImageBuilder(frame).build();
    

import com.google.mediapipe.framework.image.MediaImageBuilder;
import com.google.mediapipe.framework.image.MPImage;

// Create a CameraX’s ImageAnalysis to continuously receive frames
// from the device’s camera. Configure it to output frames in RGBA_8888
// format to match with what is required by the model.

// For each Android’s ImageProxy object received from the ImageAnalysis,
// extract the encapsulated Android’s Image object and convert it to
// a MediaPipe’s Image object.
android.media.Image mediaImage = imageProxy.getImage()
MPImage mpImage = new MediaImageBuilder(mediaImage).build();
    

In the Object Detector example code, the data preparation is handled in the ObjectDetectorHelper class within the detectImage(), detectVideoFile(), detectLivestreamFrame() functions.

Run the task

Depending on the type of data your are working with, use the ObjectDetector.detect...() method that is specific to that data type. Use detect() for individual images, detectForVideo() for frames in video files, and detectAsync() for video streams. When you are performing detections on a video stream, make sure you run the detections on a separate thread to avoid blocking the user interface thread.

The following code samples show simple examples of how to run Object Detector in these different data modes:

ObjectDetectorResult detectionResult = objectDetector.detect(image);
    

// Calculate the timestamp in milliseconds of the current frame.
long frame_timestamp_ms = 1000 * video_duration * frame_index / frame_count;

// Run inference on the frame.
ObjectDetectorResult detectionResult =
    objectDetector.detectForVideo(image, frameTimestampMs);
    

// Run inference on the frame. The detection results will be available
// via the `resultListener` provided in the `ObjectDetectorOptions` when
// the object detector was created.
objectDetector.detectAsync(image, frameTimestampMs);
    

The Object Detector code example shows the implementations of each of these modes in more detail detect(), detectVideoFile(), and detectAsync(). The example code allows the user to switch between processing modes which may not be required for your use case.

Note the following:

• When running in the video mode or the live stream mode, you must also provide the timestamp of the input frame to the Object Detector task.
• When running in the image or the video mode, the Object Detector task will block the current thread until it finishes processing the input image or frame. To avoid blocking the current thread, execute the processing in a background thread.
• When running in the live stream mode, the Object Detector task doesn’t block the current thread but returns immediately. It will invoke its result listener with the detection result every time it has finished processing an input frame. If the detect function is called when the Object Detector task is busy processing another frame, the new input frame will be ignored.

Handle and display results

Upon running inference, the Object Detector task returns an ObjectDetectorResult object which describes the objects that it has found in the input image.

The following shows an example of the output data from this task:

ObjectDetectorResult:
 Detection #0:
  Box: (x: 355, y: 133, w: 190, h: 206)
  Categories:
   index       : 17
   score       : 0.73828
   class name  : dog
 Detection #1:
  Box: (x: 103, y: 15, w: 138, h: 369)
  Categories:
   index       : 17
   score       : 0.73047
   class name  : dog

The following image shows a visualization of the task output:

The Object Detector example code demonstrates how to display the detection results returned from the task, see the OverlayView class for more details.