OpenCV provides a package called dnn that allows us to implement deep neural network in our application. In this tutorial, I will walk you step by step to implement a retrained Mobile Net to your C# application with EmguCV (.Net wrapper for OpenCV).

1. Retrained Mobile Net model

   Mobile Net is a small efficient convolutional neural network provided by Google. There is a very easy-to-go turotial to retrain a Mobile Net model to classify images of flowers. It's a very good way to make a neural network model for classification even if you've never worked with a tensorflow model before.

2. Tensorflow

   As we're working with Tensorflow model, we need to have tensorflow in our machines. You should install it from source because we will need graph_transforms tool later. If you install it using pip, it's ok but you still need to build graph_transforms separately. All about installing Tensorflow is available in the official Tensorflow website.

3. EmguCV

   Create a C# project and add EmguCV using Nuget Package Management.

1. Make some changes in the Tensorflow model so it can be used in EmguCV.dnn.

   In this part, you can do it either in Windows or Ubuntu. I recommend to do it in Ubuntu because it's simpler and less buggy. Suppose we have a mobile net with input image resolution of 128x128 retrained_mobilenet_1.0_128.pb.

   1.1. Optimize_for_inference

   For Windows

   python -m tensorflow.python.tools.optimize_for_inference.py \
    --input retrained_mobilenet_1.0_128.pb \
    --output opt_graph.pb \
    --frozen_graph True \
    --input_names input \
    --output_names final_result
   

   For Ubuntu

   python ~/tensorflow/tensorflow/python/tools/optimize_for_inference.py \
    --input retrained_mobilenet_1.0_128.pb \
    --output opt_graph.pb \
    --frozen_graph True \
    --input_names input \
    --output_names final_result
   

   1.2. Graph_transforms

   tensorflow/bazel-bin/tensorflow/tools/graph_transforms/transform_graph \
    --in_graph=opt_graph.pb \
    --out_graph=mobilenet_for_dnn.pb \
    --inputs=input \
    --outputs=final_result \
    --transforms="fold_constants remove_nodes(op=Squeeze) remove_nodes(op=PlaceholderWithDefault) strip_unused_nodes(type=float, shape=\"1,128,128,3\") sort_by_execution_order "
   

   Remember to correct the shape parameter as your model's input resolution. Here is 1,128,128,3.

2. Use the model in EmguCV

   Using the Dnn

   using Emgu.CV.Dnn;
   

   Read the model

   Net mobile_net = DnnInvoke.ReadNetFromTensorflow("mobilenet_for_dnn.pb");
   

   Read the labels

   List<string> labels = new List<string>();
   using (System.IO.StreamReader fr = new System.IO.StreamReader("labels.txt"))
   {
       string line = "";
       while ((line = fr.ReadLine()) != null)
       {
           labels.Add(line);
       }
   }
   //"label.txt" contains labels for all classes in the model.
   

   Read image

   Mat m = new Mat("image_to_classify.jpg");
   Mat blob = DnnInvoke.BlobFromImage(m, 1, new Size(128, 128));
   

   Set input and forward the image blob

   tensor_net.SetInput(blob, "input");
   Mat detection = tensor_net.Forward("final_result");
   

   Get the output

   byte[] data = new byte[116];
   detection.CopyTo(data);
   
   List<float> confidence = new List<float>();
   
   for (int i = 0; i < data.Length / 4; i++)
   {
       confidence.Add(BitConverter.ToSingle(data, i * 4));
   
   }
   //The output is a byte array containing the confidence of every class in every 4 bytes. 
   //My model is to classify 29 objects, so the output has 4x29=116 bytes.
   //The BitConverter.ToSingle(data, i * 4) will convert 4 bytes to a float value.
   

   Get the prediction

   int maxIndex = confidence.IndexOf(confidence.Max());
   float sum = confidence.Sum(); // make sure the sum is 1.0
   string prediction = labels[maxIndex];
   

Below image is the implementation in C# of Mobile net in dnn EmguCV for number plate recognition. The inputs for the model are images of characters that I cropped from the original image. More information about plate detection and segmentation is available here. As you can see, the total time for number plate recognition (including plate detection, character segmentation and character recognition) is around 200 ms without GPU. It's promising to run it in a real-time application.