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Table of Contents

  1. Introduction
  2. Data Preprocessing
  3. Shallow CNN Model
  4. Ensemble Models
  5. Wider CNN Model
  6. Residual Neural Network
  7. Web Application

1 Introdcution

Deep learning has revitalized computer vision and enhanced the accuracy of machine interpretation on images in recent years. Especially convolutional neural networks (CNN) is so far the go-to machine learning algorithm with great preformance. In this project, I build CNN models to classify the facial expression into six facial emotion categories such as happy (😄) , sad (😢), fear (😨) , surprise (😲), neutral (😐) and angry (😠).

label_dict={
 1: ['Angry', '😠'],
 2: ['Fear', '😨'],
 3: ['Happy', '😄'],
 4: ['Sad', '😢'],
 5: ['Surprise', '😲'],
 6: ['Neutral', '😐']
 }

The dataset is taken from Kaggle Challenge. I train three different CNN models: 2-layer model, 3-layer model with dropout to prevent overfitting, and 34-layer residual networks model.

Facial expression recognition can bring the relationship between the human and machine closer. It has gained significant interest in social behavioral science, clinical and security fields.

2 Data Preprocessing

The dataset 35,887 images with the resolution of 48x48 pixels. Each image is flatten to a single row with 2,304 columns and so there are 35,887 rows with 2,304 columns in a single 'csv' file. The following figure is the preview of first 16 images with the class label below each image.

Figure 1. First 16 images with their class label

As you can see from the first few images, this dataset needs some preprocessing. The third image from the left on the first row is even hard for a human to label the facial expression. Here we are only concerns about facial expression. We are only accounting for the facial landmarks. For instance, the second image from the left on the first row can be cropped around the face and we have all the information for the facial expression recognition.

I will realign and crop the face out on some of the images as first preprocessing steps. Figure 2 illustrates how the process is carried out. This image is not in the dataset. This is just for demonstration. The first on the left is the initial preprocess data and I extract the facial region using openface preprocessing alignment script based on dlib library. The middle image in the Figure 2. is a post-process image and the last image is how the machine interprets an image. You can see the darker color around the important landmarks and their locations can give the vital information about facial expression. The computer is comparing and analyzing these facial features and it is reasonable to align these landmarks from images to images so that eye, nose and mouth align for all images. This process is demonstrated in Figure 4.

Get_face.ipynb file contains the preprocessing steps in details while Get_face-result.ipynb shows the visualization of some post-process images.

Figure 3. Extract the facial region.

Figure 4. Align the facial region.

The original dataset contains seven different classes including 'disgust'. However, when we look at the distribution among different categories, 'disgust' only accounts account for about 1.5% of the total dataset as shown in Figure 2. Therefore, I am going to drop this category. Here the modified dataset is the dataset after the data preprocessing step.

Figure 5. Data distribution among categories

Figure 6. Total Number of categories

3 Shallow CNN Model

The first convolutional neural networks model is a 4-layer simple model. There are two convolutional layers and two densely (fully) connected layers. For the simplicity, there is no regularization and dropout to avoid overfitting in this model. The average accuracy for this simple model is around 62 % between 3000 to 10000 iterations.

Layer Name Output Size Process
conv1 24 x 24 filter =5x5 , stride =2, channels =16
conv1 12 x 12 filter =5x5 , stride =2, channels =36
dc1 128
dc2 6 softmax,max pool
Flops [ Ignore biases] 1.1 x 10^6

Figure 6. Architecture of a simple CNN

This is a very shallow network and relatively very easy to train. Here is the loss curve and confusion matrix for this model. As we can conclude from this model, the prediction accuracy is high on Happy,Surprise and Neutral category. This CNN algorithm has hard time classifying Angry, Sad and Fear category and are quite often gotten confused with Neutral as illustrated in the confusion matrix. This model can be reimplemented by running First Convolutional Neural Net-fv.ipynb.

Figure 7. Loss curve and accuracy on the validation dataset on the left and the confusion matrix on the right figure.

4 Ensemble Models

Instead of applying the best single model to predict the categories, I will use multiple models and combine all the predictions together and determine the prediction based on the highest score. So when I train the model in First Convolutional Neural Net-fv.ipynb, I save the model every 1000 iterations. Now each model will give a six probabilistic predictions with respect to each category on an image and I will combine these prediction per category across 9 models. By judging from Figure , ensemble learning has boosted the accuracy on categories such that Sad, Fear and Angry. As expected the accuracy of the prediction is increased by ~2% with 63.7% accuracy.

Figure 8. Confusion matrix for ensemble models

5 Wider CNN Model

In the previous model, we don't consider dropout and regularization to avoid overfitting. In this model, we go one layer deeper and also use 30% dropout.

Layer Name Output Size Process
conv1 24 x 24 filter =5x5 , conv_stride=1,max_pool _stride =2, channels =32
conv2 12 x 12 filter =5x5 , conv_stride=1,max_pool _stride =2, channels =64
conv3 6 x 6 filter =5x5 , conv_stride=1,max_pool _stride =2, channels =64
dc1 128
dc2 6 softmax,max pool
Flops [ Ignore biases] 1.2 x 10^7

The second model is slightly better than the first one with an average accuracy of around 63.6% compared to ~62 % from the first model. But in terms of the amount of computation, only 1.6% increasement is not a good improvement. Therefore, I look into the ensemble learning and the accuracy of the ensemble learning is about 65.9 %. It is about 2.2% improvement on accuracy compared to the first shallow CNN model. Accuracy on differentiating between Sad and Neutral gains a boost in this model.

Figure 7. Confusion matrix for ensemble deeper CNN models

This model is demonstrated in Second Convolutional Neural Net Model.ipynb. Let take a look at a few images in which the algorithm couldn't give the correct prediction. It is even hard for human to predict the category of some of the images shown in Figure 8.

Figure 8. Images where the model predicted incorrectly. In the caption below each image, the label is on the left and the prediction is on the right.

6 Residual Neural Network

Deep Convolutional Neural Net Model.ipynb contains how to setup and run 34-layer residual neural network introduced by Kaiming He.

Figure 9. Loss curve and accuracy on the validation dataset on the left and the confusion matrix on the right figure.

7 Web Application

For the deployment, I need three major dependencies: tensorflow, dlib, and opencv. To meet all the package requirements, I packed all python dependencies and scripts into a docker image and deploy it to Google Container Registry. Then, transform the docker image to a web application using Google App Engine. The web application can be accessed at https://facial-expression-recognition.appspot.com . One can upload the image and the application looks for faces in the image by detecting the facial landmark with dlib. Then, use residual neural network to predict the emotional state and plot a probability bar graph.