# Integrate object detectors

Object detectors can identify which of a known set of objects might be present
and provide information about their positions within the given image or a video
stream. An object detector is trained to detect the presence and location of
multiple classes of objects. For example, a model might be trained with images
that contain various pieces of fruit, along with a _label_ that specifies the
class of fruit they represent (e.g. an apple, a banana, or a strawberry), and
data specifying where each object appears in the image. See the
[introduction of object detection](../../models/object_detection/overview.md)
for more information about object detectors.

Use the Task Library `ObjectDetector` API to deploy your custom object detectors
or pretrained ones into your mobile apps.

## Key features of the ObjectDetector API

*   Input image processing, including rotation, resizing, and color space
    conversion.

*   Label map locale.

*   Score threshold to filter results.

*   Top-k detection results.

*   Label allowlist and denylist.

## Supported object detector models

The following models are guaranteed to be compatible with the `ObjectDetector`
API.

*   The
    [pretrained object detection models on TensorFlow Hub](https://tfhub.dev/tensorflow/collections/lite/task-library/object-detector/1).

*   Models created by
    [AutoML Vision Edge Object Detection](https://cloud.google.com/vision/automl/object-detection/docs).

*   Models created by
    [TensorFlow Lite Model Maker for object detector](https://www.tensorflow.org/lite/guide/model_maker).

*   Custom models that meet the
    [model compatibility requirements](#model-compatibility-requirements).

## Run inference in Java

See the
[Object Detection reference app](https://github.com/tensorflow/examples/tree/master/lite/examples/object_detection/android/)
for an example of how to use `ObjectDetector` in an Android app.

### Step 1: Import Gradle dependency and other settings

Copy the `.tflite` model file to the assets directory of the Android module
where the model will be run. Specify that the file should not be compressed, and
add the TensorFlow Lite library to the module’s `build.gradle` file:

```java
android {
    // Other settings

    // Specify tflite file should not be compressed for the app apk
    aaptOptions {
        noCompress "tflite"
    }

}

dependencies {
    // Other dependencies

    // Import the Task Vision Library dependency
    implementation 'org.tensorflow:tensorflow-lite-task-vision:0.2.0'
}
```

Note: starting from version 4.1 of the Android Gradle plugin, .tflite will be
added to the noCompress list by default and the aaptOptions above is not needed
anymore.

### Step 2: Using the model

```java
// Initialization
ObjectDetectorOptions options = ObjectDetectorOptions.builder().setMaxResults(1).build();
ObjectDetector objectDetector = ObjectDetector.createFromFileAndOptions(context, modelFile, options);

// Run inference
List<Detection> results = objectDetector.detect(image);
```

See the
[source code and javadoc](https://github.com/tensorflow/tflite-support/blob/master/tensorflow_lite_support/java/src/java/org/tensorflow/lite/task/vision/detector/ObjectDetector.java)
for more options to configure `ObjectDetector`.

## Run inference in C++

```c++
// Initialization
ObjectDetectorOptions options;
options.mutable_base_options()->mutable_model_file()->set_file_name(model_file);
std::unique_ptr<ObjectDetector> object_detector = ObjectDetector::CreateFromOptions(options).value();

// Run inference
const DetectionResult result = object_detector->Detect(*frame_buffer).value();
```

See the
[source code](https://github.com/tensorflow/tflite-support/blob/master/tensorflow_lite_support/cc/task/vision/object_detector.h)
for more options to configure `ObjectDetector`.

## Example results

Here is an example of the detection results of
[ssd mobilenet v1](https://tfhub.dev/tensorflow/lite-model/ssd_mobilenet_v1/1/metadata/1)
from TensorFlow Hub.

<img src="images/dogs.jpg" alt="dogs" width="50%">

```
Results:
 Detection #0 (red):
  Box: (x: 355, y: 133, w: 190, h: 206)
  Top-1 class:
   index       : 17
   score       : 0.73828
   class name  : dog
 Detection #1 (green):
  Box: (x: 103, y: 15, w: 138, h: 369)
  Top-1 class:
   index       : 17
   score       : 0.73047
   class name  : dog
```

Render the bounding boxes onto the input image:

<img src="images/detection-output.png" alt="detection output" width="50%">

Try out the simple
[CLI demo tool for ObjectDetector](https://github.com/tensorflow/tflite-support/tree/master/tensorflow_lite_support/examples/task/vision/desktop#object-detector)
with your own model and test data.

## Model compatibility requirements

The `ObjectDetector` API expects a TFLite model with mandatory
[TFLite Model Metadata](../../convert/metadata.md). See examples of creating
metadata for object detectors using the
[TensorFlow Lite Metadata Writer API](../../convert/metadata_writer_tutorial.ipynb#object_detectors).

The compatible object detector models should meet the following requirements:

*   Input image tensor: (kTfLiteUInt8/kTfLiteFloat32)

    -   image input of size `[batch x height x width x channels]`.
    -   batch inference is not supported (`batch` is required to be 1).
    -   only RGB inputs are supported (`channels` is required to be 3).
    -   if type is kTfLiteFloat32, NormalizationOptions are required to be
        attached to the metadata for input normalization.

*   Output tensors must be the 4 outputs of a `DetectionPostProcess` op, i.e:

    -   Locations tensor (kTfLiteFloat32)
        -   tensor of size `[1 x num_results x 4]`, the inner array representing
            bounding boxes in the form [top, left, right, bottom].
        -   BoundingBoxProperties are required to be attached to the metadata
            and must specify `type=BOUNDARIES` and `coordinate_type=RATIO.
    -   Classes tensor (kTfLiteFloat32)

        -   tensor of size `[1 x num_results]`, each value representing the
            integer index of a class.
        -   optional (but recommended) label map(s) can be attached as
            AssociatedFile-s with type TENSOR_VALUE_LABELS, containing one label
            per line. The first such AssociatedFile (if any) is used to fill the
            `class_name` field of the results. The `display_name` field is
            filled from the AssociatedFile (if any) whose locale matches the
            `display_names_locale` field of the `ObjectDetectorOptions` used at
            creation time ("en" by default, i.e. English). If none of these are
            available, only the `index` field of the results will be filled.

    -   Scores tensor (kTfLiteFloat32)

        -   tensor of size `[1 x num_results]`, each value representing the
            score of the detected object.

    -   Number of detection tensor (kTfLiteFloat32)

        -   integer num_results as a tensor of size `[1]`.
