How to use keras ImageDataGenerator with a Siamese

Question

I'm trying to build up both a Siamese neural network and triple neural network on a custom large dataset

Keras has ImageDataGenerator which makes the generation of input data to a regular neural network very easy.

I'm interesting to use ImageDataGenerator or similar ways in order to train a networks with 2(siamese) and 3(triple) inputs.

In mniset keras siamese example, The input generated by a pre-process stage which is done by create_pairs method. I don't think this kind of way fit for a large dataset.

Is it possible to use ImageDataGenerator in this case? What are my other options assuming the data-set is very big?

Answer 1

The idea of DataGenerators is to give fit_generator a stream of data in batches.. hence giving control to you how you want to produce the data, ie whether you load from files or you do some data augmentation like what is done in ImageDataGenerator.

Here I posting the modified version of mniset siamese example with custom DataGenerator, you can work it out from here.

import numpy as np
np.random.seed(1337)  # for reproducibility

import random
from keras.datasets import mnist
from keras.models import Sequential, Model
from keras.layers import Dense, Dropout, Input, Lambda
from keras.optimizers import SGD, RMSprop
from keras import backend as K

class DataGenerator(object):
    """docstring for DataGenerator"""
    def __init__(self, batch_sz):
        # the data, shuffled and split between train and test sets
        (X_train, y_train), (X_test, y_test) = mnist.load_data()
        X_train = X_train.reshape(60000, 784)
        X_test = X_test.reshape(10000, 784)
        X_train = X_train.astype('float32')
        X_test = X_test.astype('float32')
        X_train /= 255
        X_test /= 255

        # create training+test positive and negative pairs
        digit_indices = [np.where(y_train == i)[0] for i in range(10)]
        self.tr_pairs, self.tr_y = self.create_pairs(X_train, digit_indices)

        digit_indices = [np.where(y_test == i)[0] for i in range(10)]
        self.te_pairs, self.te_y = self.create_pairs(X_test, digit_indices)

        self.tr_pairs_0 = self.tr_pairs[:, 0]
        self.tr_pairs_1 = self.tr_pairs[:, 1]
        self.te_pairs_0 = self.te_pairs[:, 0]
        self.te_pairs_1 = self.te_pairs[:, 1]

        self.batch_sz = batch_sz
        self.samples_per_train  = (self.tr_pairs.shape[0]/self.batch_sz)*self.batch_sz
        self.samples_per_val    = (self.te_pairs.shape[0]/self.batch_sz)*self.batch_sz


        self.cur_train_index=0
        self.cur_val_index=0

    def create_pairs(self, x, digit_indices):
        '''Positive and negative pair creation.
        Alternates between positive and negative pairs.
        '''
        pairs = []
        labels = []
        n = min([len(digit_indices[d]) for d in range(10)]) - 1
        for d in range(10):
            for i in range(n):
                z1, z2 = digit_indices[d][i], digit_indices[d][i+1]
                pairs += [[x[z1], x[z2]]]
                inc = random.randrange(1, 10)
                dn = (d + inc) % 10
                z1, z2 = digit_indices[d][i], digit_indices[dn][i]
                pairs += [[x[z1], x[z2]]]
                labels += [1, 0]
        return np.array(pairs), np.array(labels)

    def next_train(self):
        while 1:
            self.cur_train_index += self.batch_sz
            if self.cur_train_index >= self.samples_per_train:
                self.cur_train_index=0
            yield ([    self.tr_pairs_0[self.cur_train_index:self.cur_train_index+self.batch_sz], 
                        self.tr_pairs_1[self.cur_train_index:self.cur_train_index+self.batch_sz]
                    ],
                    self.tr_y[self.cur_train_index:self.cur_train_index+self.batch_sz]
                )

    def next_val(self):
        while 1:
            self.cur_val_index += self.batch_sz
            if self.cur_val_index >= self.samples_per_val:
                self.cur_val_index=0
            yield ([    self.te_pairs_0[self.cur_val_index:self.cur_val_index+self.batch_sz], 
                        self.te_pairs_1[self.cur_val_index:self.cur_val_index+self.batch_sz]
                    ],
                    self.te_y[self.cur_val_index:self.cur_val_index+self.batch_sz]
                )

def euclidean_distance(vects):
    x, y = vects
    return K.sqrt(K.sum(K.square(x - y), axis=1, keepdims=True))


def eucl_dist_output_shape(shapes):
    shape1, shape2 = shapes
    return (shape1[0], 1)


def contrastive_loss(y_true, y_pred):
    '''Contrastive loss from Hadsell-et-al.'06
    http://yann.lecun.com/exdb/publis/pdf/hadsell-chopra-lecun-06.pdf
    '''
    margin = 1
    return K.mean(y_true * K.square(y_pred) + (1 - y_true) * K.square(K.maximum(margin - y_pred, 0)))


def create_base_network(input_dim):
    '''Base network to be shared (eq. to feature extraction).
    '''
    seq = Sequential()
    seq.add(Dense(128, input_shape=(input_dim,), activation='relu'))
    seq.add(Dropout(0.1))
    seq.add(Dense(128, activation='relu'))
    seq.add(Dropout(0.1))
    seq.add(Dense(128, activation='relu'))
    return seq


def compute_accuracy(predictions, labels):
    '''Compute classification accuracy with a fixed threshold on distances.
    '''
    return labels[predictions.ravel() < 0.5].mean()


input_dim = 784
nb_epoch = 20
batch_size=128

datagen = DataGenerator(batch_size)

# network definition
base_network = create_base_network(input_dim)

input_a = Input(shape=(input_dim,))
input_b = Input(shape=(input_dim,))

# because we re-use the same instance `base_network`,
# the weights of the network
# will be shared across the two branches
processed_a = base_network(input_a)
processed_b = base_network(input_b)

distance = Lambda(euclidean_distance, output_shape=eucl_dist_output_shape)([processed_a, processed_b])

model = Model(input=[input_a, input_b], output=distance)

# train
rms = RMSprop()
model.compile(loss=contrastive_loss, optimizer=rms)
model.fit_generator(generator=datagen.next_train(), samples_per_epoch=datagen.samples_per_train, nb_epoch=nb_epoch, validation_data=datagen.next_val(), nb_val_samples=datagen.samples_per_val)