Convolutional Neural Networks

Overview

The aim is to design a simple convolutional Neural Network using TensorFlow. The tutorial is aimed to sketch a startup model to the the two following:

1. Define an organization for the network architecture, training and evaluation phases.
2. Provides a template framework for constructing larger and more complicated models.

Model Architecture

Two simple convolutional layers`(each have max pooling) followed by two `fully-cnnected layers conisdered. The number of output units for the last fully-connected layer is equal to the number of classes becasue a softmax has been implemented for the classification task.

Code Organization

The source code is embedded in code folder.

Input

The input format is HDF5 for this implemetation but it basically can be anything as long as it satisfies the shape properties. For each TRAIN and TEST data, there are attributes call cube and label. The cube is the data of the shape (Number_Samples,height,width,Number_Channels) and the label is of the form (Number_Samples,1) in which each row has the class label. The label matrix should be transform to the form of (Number_Samples,Number_Classes) for which each row is associated with a sample and all of the columns are zero except for the one which belongs to the class number. Method Reform_Fn does this in the begining.

Training

As conventional procedure, updating the gradient is done with batches of the data. Moreover Batch Normalization has been implemented for each convolutional layer. No Batch Normalization is done for the fully-connected layers. For all the convolutional and fully-connected layers, the drop-out has been used with the same parameter however this parameter can be customized for each layer in the code. Traditional GradientDescentOptimizer has been used.

Loss Function

Cross-entropy loss function has been chosen for the cost of system. The definition is as follows:

cross_entropy = tf.reduce_mean(-tf.reduce_sum(y_ * tf.log(y), reduction_indices=[1]))

However by considering [TensorFlow official documention](https://www.tensorflow.org/versions/r0.11/tutorials/mnist/beginners/index.html#mnist-for-ml-beginners) it is numerically unstable. The reason can be due to the presence of the log. If the output of the network provide a very bad prediction and by normalizing that prediction we get zero for y value, then the loss goes to infinity and this is unstable. Another issue can be the explosion of the exponential. If the output of any of the neurons is large, since in the softmax we do the exponentiation, then the numerator and denominator of the softmax operation can be very large. So a trick can be add a number to all of the unscaled outputs. all the unit output values can be added by -max{fi{i=0,...,n}} which is the ngative sign of maximum of all output values. For further reading refer to [CNN for Visual Recognition](http://cs231n.github.io/linear-classify/) Course by stanford. Also please refer to [softmax_regression](http://ufldl.stanford.edu/wiki/index.php/Softmax_Regression) for further details.

Instead the tf.nn.softmax_cross_entropy_with_logits on the unnormalized logits (i.e., softmax_cross_entropy_with_logits is called on tf.matmul(x, W) + b), this function computes the softmax activation internally which makes it more stable. It's good to take a look at the source code of TensorFlow for that however there is a traditional idea to overcome this problem which is add an epsilon number with the absolute value of y and take it as y.