A survey on Image Data Augmentation for Deep Learning

Deep Learning models have made incredible progress in discriminative tasks. This has been fueled by the advancement of deep network architectures, powerful computation, and access to big data. Deep neural networks have been successfully applied to Computer Vision tasks such as image classification, object detection, and image segmentation thanks to the development of convolutional neural networks (CNNs). These neural networks utilize parameterized, sparsely connected kernels which preserve the spatial characteristics of images. Convolutional layers sequentially downsample the spatial resolution of images while expanding the depth of their feature maps. This series of convolutional transformations can create much lower-dimensional and more useful representations of images than what could possibly be hand-crafted. The success of CNNs has spiked interest and optimism in applying Deep Learning to Computer Vision tasks.

There are many branches of study that hope to improve current benchmarks by applying deep convolutional networks to Computer Vision tasks. Improving the generalization ability of these models is one of the most difficult challenges. Generalizability refers to the performance difference of a model when evaluated on previously seen data (training data) versus data it has never seen before (testing data). Models with poor generalizability have overfitted the training data. One way to discover overfitting is to plot the training and validation accuracy at each epoch during training. The graph below depicts what overfitting might look like when visualizing these accuracies over training epochs (Fig. 1).

To build useful Deep Learning models, the validation error must continue to decrease with the training error. Data Augmentation is a very powerful method of achieving this. The augmented data will represent a more comprehensive set of possible data points, thus minimizing the distance between the training and validation set, as well as any future testing sets.

Data Augmentation, the focus of this survey, is not the only technique that has been developed to reduce overfitting. The following few paragraphs will introduce other solutions available to avoid overfitting in Deep Learning models. This listing is intended to give readers a broader understanding of the context of Data Augmentation.

Many other strategies for increasing generalization performance focus on the model’s architecture itself. This has led to a sequence of progressively more complex architectures from AlexNet [1] to VGG-16 [2], ResNet [3], Inception-V3 [4], and DenseNet [5]. Functional solutions such as dropout regularization, batch normalization, transfer learning, and pretraining have been developed to try to extend Deep Learning for application on smaller datasets. A brief description of these overfitting solutions is provided below. A complete survey of regularization methods in Deep Learning has been compiled by Kukacka et al. [6]. Knowledge of these overfitting solutions will inform readers about other existing tools, thus framing the high-level context of Data Augmentation and Deep Learning.

  • Dropout [7] is a regularization technique that zeros out the activation values of randomly chosen neurons during training. This constraint forces the network to learn more robust features rather than relying on the predictive capability of a small subset of neurons in the network. Tompson et al. [8] extended this idea to convolutional networks with Spatial Dropout, which drops out entire feature maps rather than individual neurons.

  • Batch normalization [9] is another regularization technique that normalizes the set of activations in a layer. Normalization works by subtracting the batch mean from each activation and dividing by the batch standard deviation. This normalization technique, along with standardization, is a standard technique in the preprocessing of pixel values.

  • Transfer Learning [10, 11] is another interesting paradigm to prevent overfitting. Transfer Learning works by training a network on a big dataset such as ImageNet [12] and then using those weights as the initial weights in a new classification task. Typically, just the weights in convolutional layers are copied, rather than the entire network including fully-connected layers. This is very effective since many image datasets share low-level spatial characteristics that are better learned with big data. Understanding the relationship between transferred data domains is an ongoing research task [13]. Yosinski et al. [14] find that transferability is negatively affected primarily by the specialization of higher layer neurons and difficulties with splitting co-adapted neurons.

  • Pretraining [15] is conceptually very similar to transfer learning. In Pretraining, the network architecture is defined and then trained on a big dataset such as ImageNet [12]. This differs from Transfer Learning because in Transfer Learning, the network architecture such as VGG-16 [2] or ResNet [3] must be transferred as well as the weights. Pretraining enables the initialization of weights using big datasets, while still enabling flexibility in network architecture design.

  • One-shot and Zero-shot learning [16, 17] algorithms represent another paradigm for building models with extremely limited data. One-shot learning is commonly used in facial recognition applications [18]. An approach to one-shot learning is the use of siamese networks [19] that learn a distance function such that image classification is possible even if the network has only been trained on one or a few instances. Another very popular approach to one-shot learning is the use of memory-augmented networks [20]. Zero-shot learning is a more extreme paradigm in which a network uses input and output vector embeddings such as Word2Vec [21] or GloVe [22] to classify images based on descriptive attributes.

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In contrast to the techniques mentioned above, Data Augmentation approaches overfitting from the root of the problem, the training dataset. This is done under the assumption that more information can be extracted from the original dataset through augmentations. These augmentations artificially inflate the training dataset size by either data warping or oversampling. Data warping augmentations transform existing images such that their label is preserved. This encompasses augmentations such as geometric and color transformations, random erasing, adversarial training, and neural style transfer. Oversampling augmentations create synthetic instances and add them to the training set. This includes mixing images, feature space augmentations, and generative adversarial networks (GANs). Oversampling and Data Warping augmentations do not form a mutually exclusive dichotomy. For example, GAN samples can be stacked with random cropping to further inflate the dataset. Decisions around final dataset size, test-time augmentation, curriculum learning, and the impact of resolution are covered in this survey under the “Design considerations for image Data Augmentation” section. Descriptions of individual augmentation techniques will be enumerated in the “Image Data Augmentation techniques” section. A quick taxonomy of the Data Augmentations is depicted below in Fig. 2.

Before discussing image augmentation techniques, it is useful to frame the context of the problem and consider what makes image recognition such a difficult task in the first place. In classic discriminative examples such as cat versus dog, the image recognition software must overcome issues of viewpoint, lighting, occlusion, background, scale, and more. The task of Data Augmentation is to bake these translational invariances into the dataset such that the resulting models will perform well despite these challenges.

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It is a generally accepted notion that bigger datasets result in better Deep Learning models [23, 24]. However, assembling enormous datasets can be a very daunting task due to the manual effort of collecting and labeling data. Limited datasets is an especially prevalent challenge in medical image analysis. Given big data, deep convolutional networks have been shown to be very powerful for medical image analysis tasks such as skin lesion classification as demonstrated by Esteva et al. [25]. This has inspired the use of CNNs on medical image analysis tasks [26] such as liver lesion classification, brain scan analysis, continued research in skin lesion classification, and more. Many of the images studied are derived from computerized tomography (CT) and magnetic resonance imaging (MRI) scans, both of which are expensive and labor-intensive to collect. It is especially difficult to build big medical image datasets due to the rarity of diseases, patient privacy, the requirement of medical experts for labeling, and the expense and manual effort needed to conduct medical imaging processes. These obstacles have led to many studies on image Data Augmentation, especially GAN-based oversampling, from the application perspective of medical image classification.

Many studies on the effectiveness of Data Augmentation utilize popular academic image datasets to benchmark results. These datasets include MNIST hand written digit recognition, CIFAR-10/100, ImageNet, tiny-imagenet-200, SVHN (street view house numbers), Caltech-101/256, MIT places, MIT-Adobe 5K dataset, Pascal VOC, and Stanford Cars. The datasets most frequently discussed are CIFAR-10, CIFAR-100, and ImageNet. The expansion of open-source datasets has given researchers a wide variety of cases to compare performance results of Data Augmentation techniques. Most of these datasets such as ImageNet would be classified as big data. Many experiments constrain themselves to a subset of the dataset to simulate limited data problems.

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In addition to our focus on limited datasets, we will also consider the problem of class imbalance and how Data Augmentation can be a useful oversampling solution. Class imbalance describes a dataset with a skewed ratio of majority to minority samples. Leevy et al. [27] describe many of the existing solutions to high-class imbalance across data types. Our survey will show how class-balancing oversampling in image data can be done with Data Augmentation.

Many aspects of Deep Learning and neural network models draw comparisons with human intelligence. For example, a human intelligence anecdote of transfer learning is illustrated in learning music. If two people are trying to learn how to play the guitar, and one already knows how to play the piano, it seems likely that the piano-player will learn to play the guitar faster. Analogous to learning music, a model that can classify ImageNet images will likely perform better on CIFAR-10 images than a model with random weights.

Data Augmentation is similar to imagination or dreaming. Humans imagine different scenarios based on experience. Imagination helps us gain a better understanding of our world. Data Augmentation methods such as GANs and Neural Style Transfer can ‘imagine’ alterations to images such that they have a better understanding of them. The remainder of the paper is organized as follows: A brief “Background” is provided to give readers a historical context of Data Augmentation and Deep Learning. “Image Data Augmentation techniques” discusses each image augmentation technique in detail along with experimental results. “Design considerations for image Data Augmentation” discusses additional characteristics of augmentation such as test-time augmentation and the impact of image resolution. The paper concludes with a “Discussion” of the presented material, areas of “Future work”, and “Conclusion”.