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Siamese Neural Network ( With Pytorch Code Example )

28 Jan, 2019 / WHIZ.AI By: WHIZ.AI

Author : D. Robin Reni , AI Research Intern

Classification of Items based on their similarity is one of the major challenge of Machine Learning and Deep Learning problems.But we have seen good results in Deep Learning comparing to ML thanks to Neural Networks , Large Amounts of Data and Computational Power. We have mostly seen that Neural Networks are used for Image Detection and Recognition etc . But I am not going to explain how we execute it. But in brief, we can say that “ Neural Networks are extracting features by finding the patterns in large set of images using mathematical computation for detecting and recognizing“.

But why Siamese Neural Networks ? and What are Siamese Neural Networks ? . For many of them it is a new word. Don’t worry! I will make it clear and easy for you. I will also explain the same with a small mini project. But before that, you should understand some terminologies and topics behind it. Lets dive in!

Siamese Neural Network Definition : 

A Siamese Neural Network is a class of neural network architectures that contain two or more identical sub networks. identical here means, they have the same configuration with the same parameters and weights. Parameter updating is mirrored across both sub networks. It is used to find the similarity of the inputs by comparing its feature vectors.

One-shot Learning :

It is an object categorization problem, found mostly in Computer Vision. Where, most Deep Learning based object categorization algorithms require training on hundreds or thousands of samples/images and very large datasets, one-shot learning aims to learn information about object categories from one, or only a few, training samples/images.
More about One-Shot Learning : https://www.youtube.com/watch?v=FIjy3lV_KJU

Contrastive Loss or Lossless Triplet Loss: 

Like any distance-based loss, it tries to ensure that semantically similar examples are embedded close together. It is calculated on Pairs (other popular distance-based Loss functions are Triplet & Center Loss, calculated on Triplets and Point wise respectively)

Reasons to Use Siamese Neural Network : 

  1. Needs less training Examples to classify images because of One-Shot Learning
  2. Learn by Embedding of the image so that it can learn Semantic Similarity
  3. It helps in ensemble to give the best classifiers because of its correlation properties.
  4. Mainly used for originality verification .
That’s it! These are the main terminologies that you should know before you get started with Siamese Networks . Now we can learn about architecture and inner working of the network .

Siamese Neural Network Architecture : 

Don’t get panic by seeing its architecture. Its an elaborated structure of two identical CNN which is placed parallel. All the layer definitions of the CNN are depends upon the developer and the domain for what they are developing . The only thing you have to note from this architecture is Two Identical CNN’s placed in parallel.
If you are not familiar about Convolutional Neural Network read this blog :https://medium.com/@phidaouss/convolutional-neural-networks-cnn-or-convnets-d7c688b0a207 .

Working with Siamese Neural Network : 

In general, we learn image representations via a supervised metric-based approach with siamese neural networks, then reuse that network’s features for one-shot learning without any retraining. Also we use large Siamese Convolutional Neural Networks because learning generic image features, easily trained and can be used irrespective of the domain.
  • Preprocessing :First step, Collect and Preprocess your data .
  • Modelling : Then design your CNN architecture mostly large according to your domain its better to use standardized CNN for siamese which you can find in many research papers .
  • Feature Extraction : Next create sequence model for the same architecture which is to pass Input 1 and Input 2. It is the step where Siamese theory is implemented and final layer of both the architecture should return a feature vector of the passed input images Feature Vector of Image 1 and Feature Vector of Image 2 .
  • Similarity Score : Then to calculate the similarity of the the two feature vectors we use some similarity functions such as Cosine Similarity , Euclidean Distance etc and this function gives similarity score of the feature vectors and based upon the threshold of the values classification is done .
  • Loss Function : To find the loss on the Validation Set , we use triplet loss function , contrastive loss, regularized cross entropy etc to find out the loss and calculate the accuracy .
  • Optimization :So , to improve the accuracy we will backpropagate the network and optimize the loss using optimization techniques such as RMSprop, Mini Batch Gradient Descent , Adam Optimizer etc.
  • Training : Now our complete flow for training is set . Train the model with the preprocessed images until a fixed epoch. Test your accuracy if its low then tryHyper Parameter Optimization to improve it. Also increasing your data and image augmentation may help you in increasing the accuracy.
  • One-Shot Learning : Now we have a mastered trained Siamese Network for classification or Verification. We have a test image X and we wish to classify into one of C categories. For each C categories we have Xc= { X0 , X1 , X2 , …. , Xc-1 }  images.Calculate the similarity score for X and Xc images.Thenpredict the class corresponding to the maximum similarity . This step is known as One Shot Learning.
That’s it ! These are the important working principles which you have to know when you are to implement Siamese Neural Network for your problem. Its time to make your own Siamese Neural Network. Lets get your hands dirty with some python codes.

Hands-On with Siamese : 

We know that Siamese is basically for classification using similarity score. 
In this blog we just represent the main part of Siamese Network. We considered Offline Signature Classification based upon Similarity Score as proof of concept. The Siamese architecture is inspired by Signet Paper. The dataset we used is ICDAR 2011 Dataset since its the classic and valid open source data.

Siamese  Code Structure 

class SiameseNetwork(nn.Module):
     def __init__(self):
          super(SiameseNetwork, self).__init__()
          # Setting up the Sequential of CNN Layers
          self.cnn1 = nn.Sequential( 
          nn.Conv2d(1, 96, kernel_size=11,stride=1),
          nn.MaxPool2d(3, stride=2),

          nn.Conv2d(96, 256, kernel_size=5,stride=1,padding=2),
          nn.MaxPool2d(3, stride=2),

          nn.Conv2d(256,384 , kernel_size=3,stride=1,padding=1),
          nn.Conv2d(384,256 , kernel_size=3,stride=1,padding=1),
          nn.MaxPool2d(3, stride=2),

          # Defining the fully connected layers
          self.fc1 = nn.Sequential(
          # First Dense Layer
          nn.Linear(30976, 1024),
          # Second Dense Layer
          nn.Linear(1024, 128),
          # Final Dense Layer

     def forward_once(self, x):
          # Forward pass 
          output = self.cnn1(x)
          output = output.view(output.size()[0], -1)
          output = self.fc1(output)
          return output

    def forward(self, input1, input2):
         # forward pass of input 1
         output1 = self.forward_once(input1)
         # forward pass of input 2
         output2 = self.forward_once(input2)
         # returning the feature vectors of two inputs
         return output1, output2

Contrastive Loss Definition

class ContrastiveLoss(torch.nn.Module):

      def __init__(self, margin=2.0):
            super(ContrastiveLoss, self).__init__()
            self.margin = margin

      def forward(self, output1, output2, label):
            # Find the pairwise distance or eucledian distance of two output feature vectors
            euclidean_distance = F.pairwise_distance(output1, output2)
            # perform contrastive loss calculation with the distance
            loss_contrastive = torch.mean((1-label) * torch.pow(euclidean_distance, 2) +
            (label) * torch.pow(torch.clamp(self.margin - euclidean_distance, min=0.0), 2))

            return loss_contrastive

Oneshot Learning :

Oneshot learning is like extracting the feature vectors of the input images from the trained model to say how much both of the images are dissimilar without training the images of large datasets.
def oneshot(model,img1,img2):
       # Gives you the feature vector of both inputs
       output1,output2 = model(img1.cuda(),img2.cuda())
       # Compute the distance 
       euclidean_distance = F.pairwise_distance(output1, output2)
       #with certain threshold of distance say its similar or not
       if eucledian_distance > 0.5:
               print("Orginal Signature")
               print("Forged Signature")
Thats it from codingside ! For making the model into production you can follow lot of articles and production documents.

Major Applications Siamese Neural Network :

  • Face Recognition based upon Similarity
  • Object Classifier 
  • Detect minute changes in Documents
  • Blood Cell Classification 

To View Full Code and Model , Run this in your Google Colab Account :https://colab.research.google.com/drive/1FsixLon5Zz3_rFA0xIHzc8Tvnnw8FLr8