TF Lite Developer Guide

Using a TensorFlow Lite model in your mobile app requires multiple considerations: you must choose a pre-trained or custom model, convert the model to a TensorFLow Lite format, and finally, integrate the model in your app.

1. Choose a model

Depending on the use case, you can choose one of the popular open-sourced models, such as InceptionV3 or MobileNets, and re-train these models with a custom data set or even build your own custom model.

Use a pre-trained model

MobileNets is a family of mobile-first computer vision models for TensorFlow designed to effectively maximize accuracy, while taking into consideration the restricted resources for on-device or embedded applications. MobileNets are small, low-latency, low-power models parameterized to meet the resource constraints for a variety of uses. They can be used for classification, detection, embeddings, and segmentation—similar to other popular large scale models, such as Inception. Google provides 16 pre-trained ImageNet classification checkpoints for MobileNets that can be used in mobile projects of all sizes.

Inception-v3 is an image recognition model that achieves fairly high accuracy recognizing general objects with 1000 classes, for example, "Zebra", "Dalmatian", and "Dishwasher". The model extracts general features from input images using a convolutional neural network and classifies them based on those features with fully-connected and softmax layers.

On Device Smart Reply is an on-device model that provides one-touch replies for incoming text messages by suggesting contextually relevant messages. The model is built specifically for memory constrained devices, such as watches and phones, and has been successfully used in Smart Replies on Android Wear. Currently, this model is Android-specific.

These pre-trained models are available for download

Re-train Inception-V3 or MobileNet for a custom data set

These pre-trained models were trained on the ImageNet data set which contains 1000 predefined classes. If these classes are not sufficient for your use case, the model will need to be re-trained. This technique is called transfer learning and starts with a model that has been already trained on a problem, then retrains the model on a similar problem. Deep learning from scratch can take days, but transfer learning is fairly quick. In order to do this, you need to generate a custom data set labeled with the relevant classes.

The TensorFlow for Poets codelab walks through the re-training process step-by-step. The code supports both floating point and quantized inference.

Train a custom model

A developer may choose to train a custom model using Tensorflow (see the TensorFlow tutorials for examples of building and training models). If you have already written a model, the first step is to export this to a tf.GraphDef file. This is required because some formats do not store the model structure outside the code, and we must communicate with other parts of the framework. See Exporting the Inference Graph to create .pb file for the custom model.

TensorFlow Lite currently supports a subset of TensorFlow operators. Refer to the TensorFlow Lite & TensorFlow Compatibility Guide for supported operators and their usage. This set of operators will continue to grow in future Tensorflow Lite releases.

2. Convert the model format

The model generated (or downloaded) in the previous step is a standard Tensorflow model and you should now have a .pb or .pbtxt tf.GraphDef file. Models generated with transfer learning (re-training) or custom models must be converted—but, we must first freeze the graph to convert the model to the Tensorflow Lite format. This process uses several model formats:

  • tf.GraphDef (.pb) —A protobuf that represents the TensorFlow training or computation graph. It contains operators, tensors, and variables definitions.
  • CheckPoint (.ckpt) —Serialized variables from a TensorFlow graph. Since this does not contain a graph structure, it cannot be interpreted by itself.
  • FrozenGraphDef —A subclass of GraphDef that does not contain variables. A GraphDef can be converted to a FrozenGraphDef by taking a CheckPoint and a GraphDef, and converting each variable into a constant using the value retrieved from the CheckPoint.
  • SavedModel —A GraphDef and CheckPoint with a signature that labels input and output arguments to a model. A GraphDef and CheckPoint can be extracted from a SavedModel.
  • TensorFlow Lite model (.tflite) —A serialized FlatBuffer that contains TensorFlow Lite operators and tensors for the TensorFlow Lite interpreter, similar to a FrozenGraphDef.

Freeze Graph

To use the GraphDef .pb file with TensorFlow Lite, you must have checkpoints that contain trained weight parameters. The .pb file only contains the structure of the graph. The process of merging the checkpoint values with the graph structure is called freezing the graph.

You should have a checkpoints folder or download them for a pre-trained model (for example, MobileNets).

To freeze the graph, use the following command (changing the arguments):

freeze_graph --input_graph=/tmp/mobilenet_v1_224.pb \
  --input_checkpoint=/tmp/checkpoints/mobilenet-10202.ckpt \
  --input_binary=true \
  --output_graph=/tmp/frozen_mobilenet_v1_224.pb \
  --output_node_names=MobileNetV1/Predictions/Reshape_1

The input_binary flag must be enabled so the protobuf is read and written in a binary format. Set the input_graph and input_checkpoint files.

The output_node_names may not be obvious outside of the code that built the model. The easiest way to find them is to visualize the graph, either with TensorBoard or graphviz.

The frozen GraphDef is now ready for conversion to the FlatBuffer format (.tflite) for use on Android or iOS devices. For Android, the Tensorflow Optimizing Converter tool supports both float and quantized models. To convert the frozen GraphDef to the .tflite format:

toco --input_file=$(pwd)/mobilenet_v1_1.0_224/frozen_graph.pb \
  --input_format=TENSORFLOW_GRAPHDEF \
  --output_format=TFLITE \
  --output_file=/tmp/mobilenet_v1_1.0_224.tflite \
  --inference_type=FLOAT \
  --input_type=FLOAT \
  --input_arrays=input \
  --output_arrays=MobilenetV1/Predictions/Reshape_1 \
  --input_shapes=1,224,224,3

The input_file argument should reference the frozen GraphDef file containing the model architecture. The frozen_graph.pb file used here is available for download. output_file is where the TensorFlow Lite model will get generated. The input_type and inference_type arguments should be set to FLOAT, unless converting a quantized model. Setting the input_array, output_array, and input_shape arguments are not as straightforward. The easiest way to find these values is to explore the graph using Tensorboard. Reuse the arguments for specifying the output nodes for inference in the freeze_graph step.

It is also possible to use the Tensorflow Optimizing Converter with protobufs from either Python or from the command line (see the toco_from_protos.py example). This allows you to integrate the conversion step into the model design workflow, ensuring the model is easily convertible to a mobile inference graph. For example:

import tensorflow as tf

img = tf.placeholder(name="img", dtype=tf.float32, shape=(1, 64, 64, 3))
val = img + tf.constant([1., 2., 3.]) + tf.constant([1., 4., 4.])
out = tf.identity(val, name="out")

with tf.Session() as sess:
  tflite_model = tf.contrib.lite.toco_convert(sess.graph_def, [img], [out])
  open("converteds_model.tflite", "wb").write(tflite_model)

For usage, see the Tensorflow Optimizing Converter command-line examples.

Refer to the Ops compatibility guide for troubleshooting help, and if that doesn't help, please file an issue.

The development repo contains a tool to visualize TensorFlow Lite models after conversion. To build the visualize.py tool:

bazel run tensorflow/contrib/lite/tools:visualize -- model.tflite model_viz.html

This generates an interactive HTML page listing subgraphs, operations, and a graph visualization.

3. Use the TensorFlow Lite model for inference in a mobile app

After completing the prior steps, you should now have a .tflite model file.

Android

Since Android apps are written in Java and the core TensorFlow library is in C++, a JNI library is provided as an interface. This is only meant for inference—it provides the ability to load a graph, set up inputs, and run the model to calculate outputs.

The open source Android demo app uses the JNI interface and is available on GitHub. You can also download a prebuilt APK. See the Android demo guide for details.

The Android mobile guide has instructions for installing TensorFlow on Android and setting up bazel and Android Studio.

iOS

To integrate a TensorFlow model in an iOS app, see the TensorFlow Lite for iOS guide and iOS demo guide.

Core ML support

Core ML is a machine learning framework used in Apple products. In addition to using Tensorflow Lite models directly in your applications, you can convert trained Tensorflow models to the CoreML format for use on Apple devices. To use the converter, refer to the Tensorflow-CoreML converter documentation.

Raspberry Pi

Compile Tensorflow Lite for a Raspberry Pi by following the RPi build instructions This compiles a static library file (.a) used to build your app. There are plans for Python bindings and a demo app.