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Open In Colab This notebook is authored by F. Chollet and contains code samples from Chapter 5, Section 2 of Deep Learning with Python.

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Training a ConvNet from Scratch on a Small Dataset

Having to train an image classification model using only very little data is a common situation in computer vision. As a practical example, we focus on classifying images as “dogs” or “cats”, using 4000 pictures (2000 cats, 2000 dogs). We cover three strategies:
  1. Training from scratch - baseline accuracy of ~71%
  2. Data augmentation - improves to ~82% accuracy
  3. Transfer learning - achieves up to 95% accuracy

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The Relevance of Deep Learning for Small-Data Problems

Deep learning models are highly repurposable. Pre-trained models (usually trained on ImageNet) can bootstrap powerful vision models from very little data.

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Building the Network

from keras import layers
from keras import models

model = models.Sequential()
model.add(layers.Conv2D(32, (3, 3), activation='relu', input_shape=(150, 150, 3)))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Conv2D(64, (3, 3), activation='relu'))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Conv2D(128, (3, 3), activation='relu'))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Conv2D(128, (3, 3), activation='relu'))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Flatten())
model.add(layers.Dense(512, activation='relu'))
model.add(layers.Dense(1, activation='sigmoid'))

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Data Augmentation

Data augmentation generates more training data from existing samples via random transformations: 
from keras.preprocessing.image import ImageDataGenerator

datagen = ImageDataGenerator(
    rotation_range=40,
    width_shift_range=0.2,
    height_shift_range=0.2,
    shear_range=0.2,
    zoom_range=0.2,
    horizontal_flip=True,
    fill_mode='nearest')

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Model with Dropout

Adding dropout to fight overfitting: 
model = models.Sequential()
model.add(layers.Conv2D(32, (3, 3), activation='relu', input_shape=(150, 150, 3)))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Conv2D(64, (3, 3), activation='relu'))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Conv2D(128, (3, 3), activation='relu'))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Conv2D(128, (3, 3), activation='relu'))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Flatten())
model.add(layers.Dropout(0.5))
model.add(layers.Dense(512, activation='relu'))
model.add(layers.Dense(1, activation='sigmoid'))
With data augmentation and dropout, we achieve ~82% accuracy - a 15% relative improvement over the non-regularized model.
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