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CutMix, MixUp, and RandAugment image augmentation with KerasCV

Author: lukewood
Date created: 2022/04/08
Last modified: 2022/04/08
Description: Use KerasCV to augment images with CutMix, MixUp, RandAugment, and more.

View in Colab GitHub source


Overview

KerasCV makes it easy to assemble state-of-the-art, industry-grade data augmentation pipelines for image classification and object detection tasks. KerasCV offers a wide suite of preprocessing layers implementing common data augmentation techniques.

Perhaps three of the most useful layers are keras_cv.layers.CutMix, keras_cv.layers.MixUp, and keras_cv.layers.RandAugment. These layers are used in nearly all state-of-the-art image classification pipelines.

This guide will show you how to compose these layers into your own data augmentation pipeline for image classification tasks. This guide will also walk you through the process of customizing a KerasCV data augmentation pipeline.


Imports & setup

This tutorial requires you to have KerasCV installed:

pip install keras-cv

We begin by importing all required packages:

import keras_cv
import matplotlib.pyplot as plt
import tensorflow as tf
import tensorflow_datasets as tfds
from tensorflow import keras
from tensorflow.keras import applications
from tensorflow.keras import losses
from tensorflow.keras import optimizers

Data loading

This guide uses the 102 Category Flower Dataset for demonstration purposes.

To get started, we first load the dataset:

BATCH_SIZE = 32
AUTOTUNE = tf.data.AUTOTUNE
tfds.disable_progress_bar()
data, dataset_info = tfds.load("oxford_flowers102", with_info=True, as_supervised=True)
train_steps_per_epoch = dataset_info.splits["train"].num_examples // BATCH_SIZE
val_steps_per_epoch = dataset_info.splits["test"].num_examples // BATCH_SIZE

Next, we resize the images to a constant size, (224, 224), and one-hot encode the labels. Please note that keras_cv.layers.CutMix and keras_cv.layers.MixUp expect targets to be one-hot encoded. This is because they modify the values of the targets in a way that is not possible with a sparse label representation.

IMAGE_SIZE = (224, 224)
num_classes = dataset_info.features["label"].num_classes


def to_dict(image, label):
    image = tf.image.resize(image, IMAGE_SIZE)
    image = tf.cast(image, tf.float32)
    label = tf.one_hot(label, num_classes)
    return {"images": image, "labels": label}


def prepare_dataset(dataset, split):
    if split == "train":
        return (
            dataset.shuffle(10 * BATCH_SIZE)
            .map(to_dict, num_parallel_calls=AUTOTUNE)
            .batch(BATCH_SIZE)
        )
    if split == "test":
        return dataset.map(to_dict, num_parallel_calls=AUTOTUNE).batch(BATCH_SIZE)


def load_dataset(split="train"):
    dataset = data[split]
    return prepare_dataset(dataset, split)


train_dataset = load_dataset()

Let's inspect some samples from our dataset:

def visualize_dataset(dataset, title):
    plt.figure(figsize=(6, 6)).suptitle(title, fontsize=18)
    for i, samples in enumerate(iter(dataset.take(9))):
        images = samples["images"]
        plt.subplot(3, 3, i + 1)
        plt.imshow(images[0].numpy().astype("uint8"))
        plt.axis("off")
    plt.show()


visualize_dataset(train_dataset, title="Before Augmentation")

png

Great! Now we can move onto the augmentation step.


RandAugment

RandAugment has been shown to provide improved image classification results across numerous datasets. It performs a standard set of augmentations on an image.

To use RandAugment in KerasCV, you need to provide a few values:

  • value_range describes the range of values covered in your images
  • magnitude is a value between 0 and 1, describing the strength of the perturbations applied
  • augmentations_per_image is an integer telling the layer how many augmentations to apply to each individual image
  • (Optional) magnitude_stddev allows magnitude to be randomly sampled from a distribution with a standard deviation of magnitude_stddev
  • (Optional) rate indicates the probability to apply the augmentation applied at each layer.

You can read more about these parameters in the RandAugment API documentation.

Let's use KerasCV's RandAugment implementation.

rand_augment = keras_cv.layers.RandAugment(
    value_range=(0, 255),
    augmentations_per_image=3,
    magnitude=0.3,
    magnitude_stddev=0.2,
    rate=0.5,
)


def apply_rand_augment(inputs):
    inputs["images"] = rand_augment(inputs["images"])
    return inputs


train_dataset = load_dataset().map(apply_rand_augment, num_parallel_calls=AUTOTUNE)

Finally, let's inspect some of the results:

visualize_dataset(train_dataset, title="After RandAugment")

png

Try tweaking the magnitude settings to see a wider variety of results.


CutMix and MixUp: generate high-quality inter-class examples

CutMix and MixUp allow us to produce inter-class examples. CutMix randomly cuts out portions of one image and places them over another, and MixUp interpolates the pixel values between two images. Both of these prevent the model from overfitting the training distribution and improve the likelihood that the model can generalize to out of distribution examples. Additionally, CutMix prevents your model from over-relying on any particular feature to perform its classifications. You can read more about these techniques in their respective papers:

In this example, we will use CutMix and MixUp independently in a manually created preprocessing pipeline. In most state of the art pipelines images are randomly augmented by either CutMix, MixUp, or neither. The function below implements both.

cut_mix = keras_cv.layers.CutMix()
mix_up = keras_cv.layers.MixUp()


def cut_mix_and_mix_up(samples):
    samples = cut_mix(samples, training=True)
    samples = mix_up(samples, training=True)
    return samples


train_dataset = load_dataset().map(cut_mix_and_mix_up, num_parallel_calls=AUTOTUNE)

visualize_dataset(train_dataset, title="After CutMix and MixUp")

png

Great! Looks like we have successfully added CutMix and MixUp to our preprocessing pipeline.


Customizing your augmentation pipeline

Perhaps you want to exclude an augmentation from RandAugment, or perhaps you want to include the keras_cv.layers.GridMask as an option alongside the default RandAugment augmentations.

KerasCV allows you to construct production grade custom data augmentation pipelines using the keras_cv.layers.RandomAugmentationPipeline layer. This class operates similarly to RandAugment; selecting a random layer to apply to each image augmentations_per_image times. RandAugment can be thought of as a specific case of RandomAugmentationPipeline. In fact, our RandAugment implementation inherits from RandomAugmentationPipeline internally.

In this example, we will create a custom RandomAugmentationPipeline by removing RandomRotation layers from the standard RandAugment policy, and substitutex a GridMask layer in its place.

As a first step, let's use the helper method RandAugment.get_standard_policy() to create a base pipeline.

layers = keras_cv.layers.RandAugment.get_standard_policy(
    value_range=(0, 255), magnitude=0.75, magnitude_stddev=0.3
)

First, let's filter out RandomRotation layers

layers = [
    layer for layer in layers if not isinstance(layer, keras_cv.layers.RandomRotation)
]

Next, let's add keras_cv.layers.GridMask to our layers:

layers = layers + [keras_cv.layers.GridMask()]

Finally, we can put together our pipeline

pipeline = keras_cv.layers.RandomAugmentationPipeline(
    layers=layers, augmentations_per_image=3
)


def apply_pipeline(inputs):
    inputs["images"] = pipeline(inputs["images"])
    return inputs

Let's check out the results!

train_dataset = load_dataset().map(apply_pipeline, num_parallel_calls=AUTOTUNE)
visualize_dataset(train_dataset, title="After custom pipeline")

png

Awesome! As you can see, no images were randomly rotated. You can customize the pipeline however you like:

pipeline = keras_cv.layers.RandomAugmentationPipeline(
    layers=[keras_cv.layers.GridMask(), keras_cv.layers.Grayscale(output_channels=3)],
    augmentations_per_image=1,
)

This pipeline will either apply GrayScale or GridMask:

train_dataset = load_dataset().map(apply_pipeline, num_parallel_calls=AUTOTUNE)
visualize_dataset(train_dataset, title="After custom pipeline")

png

Looks great! You can use RandomAugmentationPipeline however you want.


Training a CNN

As a final exercise, let's take some of these layers for a spin. In this section, we will use CutMix, MixUp, and RandAugment to train a state of the art ResNet50 image classifier on the Oxford flowers dataset.

def preprocess_for_model(inputs):
    images, labels = inputs["images"], inputs["labels"]
    images = tf.cast(images, tf.float32)
    return images, labels


train_dataset = (
    load_dataset()
    .map(apply_rand_augment, num_parallel_calls=AUTOTUNE)
    .map(cut_mix_and_mix_up, num_parallel_calls=AUTOTUNE)
)

visualize_dataset(train_dataset, "CutMix, MixUp and RandAugment")

train_dataset = train_dataset.map(preprocess_for_model, num_parallel_calls=AUTOTUNE)

test_dataset = load_dataset(split="test")
test_dataset = test_dataset.map(preprocess_for_model, num_parallel_calls=AUTOTUNE)

train_dataset = train_dataset.prefetch(AUTOTUNE)
test_dataset = test_dataset.prefetch(AUTOTUNE)

train_dataset = train_dataset
test_dataset = test_dataset

png

Next we should create a the model itself. Notice that we use label_smoothing=0.1 in the loss function. When using MixUp, label smoothing is highly recommended.

input_shape = IMAGE_SIZE + (3,)


def get_model():
    model = keras_cv.models.DenseNet121(
        include_rescaling=True, include_top=True, classes=num_classes
    )
    model.compile(
        loss=losses.CategoricalCrossentropy(label_smoothing=0.1),
        optimizer=optimizers.SGD(momentum=0.9),
        metrics=["accuracy"],
    )
    return model

Finally we train the model:

strategy = tf.distribute.MirroredStrategy()
with strategy.scope():
    model = get_model()
    model.fit(
        train_dataset,
        epochs=1,
        validation_data=test_dataset,
    )
32/32 [==============================] - 769s 24s/step - loss: 4.7812 - accuracy: 0.0108 - val_loss: 4.6148 - val_accuracy: 0.0241

Conclusion & next steps

That's all it takes to assemble state of the art image augmentation pipeliens with KerasCV!

As an additional exercise for readers, you can:

  • Perform a hyper parameter search over the RandAugment parameters to improve the classifier accuracy
  • Substitute the Oxford Flowers dataset with your own dataset
  • Experiment with custom RandomAugmentationPipeline objects.

Currently, between Keras core and KerasCV there are 28 image augmentation layers! Each of these can be used independently, or in a pipeline. Check them out, and if you find an augmentation techniques you need is missing please file a GitHub issue on KerasCV.