Conversion of TensorFlow Detection Models and Launch with OpenCV Python

Original author	Anastasia Murzova
Compatibility	OpenCV >= 4.5

Goals

In this tutorial you will learn how to:

• obtain frozen graphs of TensorFlow (TF) detection models
• run converted TensorFlow model with OpenCV Python API

We will explore the above-listed points by the example of SSD MobileNetV1.

Introduction

Let's briefly view the key concepts involved in the pipeline of TensorFlow models transition with OpenCV API. The initial step in the conversion of TensorFlow models into cv.dnn.Net is obtaining the frozen TF model graph. A frozen graph defines the combination of the model graph structure with kept values of the required variables, for example, weights. The frozen graph is saved in protobuf (.pb) files. There are special functions for reading .pb graphs in OpenCV: cv.dnn.readNetFromTensorflow and cv.dnn.readNet.

Requirements

To be able to experiment with the below code you will need to install a set of libraries. We will use a virtual environment with python3.7+ for this:

virtualenv -p /usr/bin/python3.7 <env_dir_path>
source <env_dir_path>/bin/activate

For OpenCV-Python building from source, follow the corresponding instructions from the Introduction to OpenCV.

Before you start the installation of the libraries, you can customize the requirements.txt, excluding or including (for example, opencv-python) some dependencies. The below line initiates requirements installation into the previously activated virtual environment:

pip install -r requirements.txt

Practice

In this part we are going to cover the following points:

1. create a TF classification model conversion pipeline and provide the inference
2. provide the inference, process prediction results

Model Preparation

The code in this subchapter is located in the samples/dnn/dnn_model_runner module and can be executed with the below line:

python -m dnn_model_runner.dnn_conversion.tf.detection.py_to_py_ssd_mobilenet

The following code contains the steps of the TF SSD MobileNetV1 model retrieval:

tf_model_name = 'ssd_mobilenet_v1_coco_2017_11_17'
graph_extraction_dir = "./"
frozen_graph_path = extract_tf_frozen_graph(tf_model_name, graph_extraction_dir)
print("Frozen graph path for {}: {}".format(tf_model_name, frozen_graph_path))

In extract_tf_frozen_graph function we extract the provided in model archive frozen_inference_graph.pb for its further processing:

# define model archive name
tf_model_tar = model_name + '.tar.gz'
# define link to retrieve model archive
model_link = DETECTION_MODELS_URL + tf_model_tar
 
tf_frozen_graph_name = 'frozen_inference_graph'
 
try:
    urllib.request.urlretrieve(model_link, tf_model_tar)
except Exception:
    print("TF {} was not retrieved: {}".format(model_name, model_link))
    return
 
print("TF {} was retrieved.".format(model_name))
 
tf_model_tar = tarfile.open(tf_model_tar)
frozen_graph_path = ""
 
for model_tar_elem in tf_model_tar.getmembers():
    if tf_frozen_graph_name in os.path.basename(model_tar_elem.name):
        tf_model_tar.extract(model_tar_elem, extracted_model_path)
        frozen_graph_path = os.path.join(extracted_model_path, model_tar_elem.name)
        break
tf_model_tar.close()

After the successful execution of the above code we will get the following output:

TF ssd_mobilenet_v1_coco_2017_11_17 was retrieved.
Frozen graph path for ssd_mobilenet_v1_coco_2017_11_17: ./ssd_mobilenet_v1_coco_2017_11_17/frozen_inference_graph.pb

To provide model inference we will use the below double-decker bus photo (under Pexels license):

Double-decker bus

To initiate the test process we need to provide an appropriate model configuration. We will use ssd_mobilenet_v1_coco.config from TensorFlow Object Detection API. TensorFlow Object Detection API framework contains helpful mechanisms for object detection model manipulations.

We will use this configuration to provide a text graph representation. To generate .pbtxt we will use the corresponding samples/dnn/tf_text_graph_ssd.py script:

python tf_text_graph_ssd.py --input ssd_mobilenet_v1_coco_2017_11_17/frozen_inference_graph.pb --config ssd_mobilenet_v1_coco_2017_11_17/ssd_mobilenet_v1_coco.config --output ssd_mobilenet_v1_coco_2017_11_17.pbtxt

After successful execution ssd_mobilenet_v1_coco_2017_11_17.pbtxt will be created.

Before we run object_detection.py, let's have a look at the default values for the SSD MobileNetV1 test process configuration. They are located in models.yml:

ssd_tf:
  model: "ssd_mobilenet_v1_coco_2017_11_17.pb"
  config: "ssd_mobilenet_v1_coco_2017_11_17.pbtxt"
  mean: [0, 0, 0]
  scale: 1.0
  width: 300
  height: 300
  rgb: true
  classes: "object_detection_classes_coco.txt"
  sample: "object_detection"

To fetch these values we need to provide frozen graph ssd_mobilenet_v1_coco_2017_11_17.pb model and text graph ssd_mobilenet_v1_coco_2017_11_17.pbtxt:

python object_detection.py ssd_tf --input ../data/pexels_double_decker_bus.jpg

This line is equivalent to:

python object_detection.py --model ssd_mobilenet_v1_coco_2017_11_17.pb --config  ssd_mobilenet_v1_coco_2017_11_17.pbtxt  --input ../data/pexels_double_decker_bus.jpg --width 300 --height 300 --classes ../data/dnn/object_detection_classes_coco.txt

The result is:

OpenCV SSD bus result

There are several helpful parameters, which can be also customized for result corrections: threshold (--thr) and non-maximum suppression (--nms) values.