Data Augmentation

Overview

In a computer vision task, if the data volume is small or the scenario of the samples are simple, the training effect will be affected. In this case, you may preprocess images by performing data augmentation, so as to improve generalization of the model.

MindSpore provides the c_transforms and py_transforms modules for data augmentation. You can also customize functions or operators to augment data.

Module	Implementation	Description
c_transforms	Implemented based on C++.	This module provides high performance.
py_transforms	Implemented based on Python PIL	This module provides multiple image augmentation methods and can convert PIL images to NumPy arrays.

The following table lists the common data augmentation operators supported by MindSpore. For details about more data augmentation operators, see MindSpore API.

Module	Operator	Description
c_transforms	RandomCrop	Crops an image of a specified size at a random position.
	RandomHorizontalFlip	Flips the image horizontally based on the specified probability.
	Resize	Resizes the image to the specified size.
	Invert	Inverts the image.
py_transforms	RandomCrop	Crops an image of a specified size at a random position.
	Resize	Resizes the image to the specified size.
	Invert	Inverts the image.
	Compose	Performs the data augmentation operations in the list in sequence.

c_transforms

The following describes how to use common data augmentation operators of the c_transforms module.

RandomCrop

Crops the input image at a random position.

Parameter description:

• size: size of the cropped image.

• padding: number of padded pixels.

• pad_if_needed: specifies whether the original image needs to be padded when it is smaller than the cropped size.

• fill_value: fill value used in the constant fill mode.

• padding_mode: padding mode.

The following example uses a sequential sampler to load the CIFAR-10 dataset [1], randomly crops the loaded image to 10 in both length and width, outputs the image shapes and labels before and after cropping, and displays the cropped image.

Download CIFAR-10 dataset and decompress it. The directory structure is as follows:

└─cifar-10-batches-bin
    ├── batches.meta.txt
    ├── data_batch_1.bin
    ├── data_batch_2.bin
    ├── data_batch_3.bin
    ├── data_batch_4.bin
    ├── data_batch_5.bin
    ├── readme.html
    └── test_batch.bin

import matplotlib.pyplot as plt
import mindspore.dataset as ds
import mindspore.dataset.vision.c_transforms as c_trans

ds.config.set_seed(5)
ds.config.set_num_parallel_workers(1)

DATA_DIR = "cifar-10-batches-bin/"

sampler = ds.SequentialSampler(num_samples=3)
dataset1 = ds.Cifar10Dataset(DATA_DIR, sampler=sampler)

random_crop = c_trans.RandomCrop([10, 10])
dataset2 = dataset1.map(operations=random_crop, input_columns=["image"])

image_list1, label_list1 = [], []
image_list2, label_list2 = [], []
for data1, data2 in zip(dataset1.create_dict_iterator(), dataset2.create_dict_iterator()):
    image_list1.append(data1['image'])
    label_list1.append(data1['label'])
    print("Source image Shape :", data1['image'].shape, ", Source label :", data1['label'])
    image_list2.append(data2['image'])
    label_list2.append(data2['label'])
    print("Cropped image Shape:", data2['image'].shape, ", Cropped label:", data2['label'])
    print("------")

num_samples = len(image_list1) + len(image_list2)
for i in range(num_samples):
    if i < len(image_list1):
        plt.subplot(2, len(image_list1), i + 1)
        plt.imshow(image_list1[i].asnumpy())
        plt.title(label_list1[i].asnumpy())
    else:
        plt.subplot(2, len(image_list2), i + 1)
        plt.imshow(image_list2[i % len(image_list2)].asnumpy())
        plt.title(label_list2[i % len(image_list2)].asnumpy())
plt.show()

The output is as follows:

Source image Shape : (32, 32, 3) , Source label : 6
Cropped image Shape: (10, 10, 3) , Cropped label: 6
------
Source image Shape : (32, 32, 3) , Source label : 9
Cropped image Shape: (10, 10, 3) , Cropped label: 9
------
Source image Shape : (32, 32, 3) , Source label : 9
Cropped image Shape: (10, 10, 3) , Cropped label: 9
------

The following shows the cropped image.

RandomHorizontalFlip

Randomly flips the input image horizontally.

Parameter description:

• prob: probability of flipping a single image.

The following example uses a random sampler to load the CIFAR-10 dataset [1], randomly flips the loaded image in the horizontal direction with a probability of 0.8, outputs the image shapes and labels before and after the flipping, and displays the flipped image.

Follow the steps above to download the CIFAR-10 dataset and store it as required.

import matplotlib.pyplot as plt
import mindspore.dataset as ds
import mindspore.dataset.vision.c_transforms as c_trans

ds.config.set_seed(6)
ds.config.set_num_parallel_workers(1)

DATA_DIR = "cifar-10-batches-bin/"

sampler = ds.RandomSampler(num_samples=4)
dataset1 = ds.Cifar10Dataset(DATA_DIR, sampler=sampler)

random_horizontal_flip = c_trans.RandomHorizontalFlip(prob=0.8)
dataset2 = dataset1.map(operations=random_horizontal_flip, input_columns=["image"])

image_list1, label_list1 = [], []
image_list2, label_list2 = [], []
for data1, data2 in zip(dataset1.create_dict_iterator(), dataset2.create_dict_iterator()):
    image_list1.append(data1['image'])
    label_list1.append(data1['label'])
    print("Source image Shape :", data1['image'].shape, ", Source label :", data1['label'])
    image_list2.append(data2['image'])
    label_list2.append(data2['label'])
    print("Flipped image Shape:", data2['image'].shape, ", Flipped label:", data2['label'])
    print("------")

num_samples = len(image_list1) + len(image_list2)
for i in range(num_samples):
    if i < len(image_list1):
        plt.subplot(2, len(image_list1), i + 1)
        plt.imshow(image_list1[i].asnumpy())
        plt.title(label_list1[i].asnumpy())
    else:
        plt.subplot(2, len(image_list2), i + 1)
        plt.imshow(image_list2[i % len(image_list2)].asnumpy())
        plt.title(label_list2[i % len(image_list2)].asnumpy())
plt.show()

The output is as follows:

Source image Shape : (32, 32, 3) , Source label : 3
Flipped image Shape: (32, 32, 3) , Flipped label: 3
------
Source image Shape : (32, 32, 3) , Source label : 6
Flipped image Shape: (32, 32, 3) , Flipped label: 6
------
Source image Shape : (32, 32, 3) , Source label : 6
Flipped image Shape: (32, 32, 3) , Flipped label: 6
------
Source image Shape : (32, 32, 3) , Source label : 9
Flipped image Shape: (32, 32, 3) , Flipped label: 9
------

The following shows the flipped image.

Resize

Resizes the input image.

Parameter description:

• self: target size of the image.

• interpolation: interpolation mode used during resizing.

The following example loads the MNIST dataset [2], resizes the loaded image to (101, 101), outputs the image shapes and labels before and after the resizing, and displays the resized image.

Download the MNIST dataset training image and labels, decompress them and store them in the ./MNIST path. The directory structure is as follows:

└─MNIST
    ├─train-images.idx3-ubyte
    └─train-labels.idx1-ubyte

import matplotlib.pyplot as plt
import mindspore.dataset as ds
import mindspore.dataset.vision.c_transforms as c_trans

DATA_DIR = "MNIST/"

dataset1 = ds.MnistDataset(DATA_DIR, num_samples=4, shuffle=False)

resize = c_trans.Resize(size=[101, 101])
dataset2 = dataset1.map(operations=resize, input_columns=["image"])

image_list1, label_list1 = [], []
image_list2, label_list2 = [], []
for data1, data2 in zip(dataset1.create_dict_iterator(), dataset2.create_dict_iterator()):
    image_list1.append(data1['image'])
    label_list1.append(data1['label'])
    print("Source image Shape :", data1['image'].shape, ", Source label :", data1['label'])
    image_list2.append(data2['image'])
    label_list2.append(data2['label'])
    print("Flipped image Shape:", data2['image'].shape, ", Flipped label:", data2['label'])
    print("------")

num_samples = len(image_list1) + len(image_list2)
for i in range(num_samples):
    if i < len(image_list1):
        plt.subplot(2, len(image_list1), i + 1)
        plt.imshow(image_list1[i].asnumpy().squeeze(), cmap=plt.cm.gray)
        plt.title(label_list1[i].asnumpy())
    else:
        plt.subplot(2, len(image_list2), i + 1)
        plt.imshow(image_list2[i % len(image_list2)].asnumpy().squeeze(), cmap=plt.cm.gray)
        plt.title(label_list2[i % len(image_list2)].asnumpy())
plt.show()

The output is as follows:

Source image Shape : (28, 28, 1) , Source label : 5
Flipped image Shape: (101, 101, 1) , Flipped label: 5
------
Source image Shape : (28, 28, 1) , Source label : 0
Flipped image Shape: (101, 101, 1) , Flipped label: 0
------
Source image Shape : (28, 28, 1) , Source label : 4
Flipped image Shape: (101, 101, 1) , Flipped label: 4
------
Source image Shape : (28, 28, 1) , Source label : 1
Flipped image Shape: (101, 101, 1) , Flipped label: 1
------

The following shows the resized image.

Invert

Inverts the input image.

The following example loads the CIFAR-10 dataset [1], defines and performs the resizing and inverting operations on the loaded image, outputs the image shapes and labels before and after the resizing and inverting operations, and displays the inverted image.

Follow the steps above to download the CIFAR-10 data set and store it as required.

import matplotlib.pyplot as plt
import mindspore.dataset as ds
import mindspore.dataset.vision.c_transforms as c_trans

ds.config.set_seed(8)

DATA_DIR = "cifar-10-batches-bin/"

dataset1 = ds.Cifar10Dataset(DATA_DIR, num_samples=4, shuffle=True)

resize = c_trans.Resize(size=[101, 101])
invert = c_trans.Invert()
dataset2 = dataset1.map(operations=[resize, invert], input_columns=["image"])

image_list1, label_list1 = [], []
image_list2, label_list2 = [], []
for data1, data2 in zip(dataset1.create_dict_iterator(), dataset2.create_dict_iterator()):
    image_list1.append(data1['image'])
    label_list1.append(data1['label'])
    print("Source image Shape :", data1['image'].shape, ", Source label :", data1['label'])
    image_list2.append(data2['image'])
    label_list2.append(data2['label'])
    print("Flipped image Shape:", data2['image'].shape, ", Flipped label:", data2['label'])
    print("------")

num_samples = len(image_list1) + len(image_list2)
for i in range(num_samples):
    if i < len(image_list1):
        plt.subplot(2, len(image_list1), i + 1)
        plt.imshow(image_list1[i].asnumpy().squeeze(), cmap=plt.cm.gray)
        plt.title(label_list1[i].asnumpy())
    else:
        plt.subplot(2, len(image_list2), i + 1)
        plt.imshow(image_list2[i % len(image_list2)].asnumpy().squeeze(), cmap=plt.cm.gray)
        plt.title(label_list2[i % len(image_list2)].asnumpy())
plt.show()

The output is as follows:

Source image Shape : (32, 32, 3) , Source label : 4
Flipped image Shape: (32, 32, 3) , Flipped label: 4
------
Source image Shape : (32, 32, 3) , Source label : 9
Flipped image Shape: (32, 32, 3) , Flipped label: 9
------
Source image Shape : (32, 32, 3) , Source label : 6
Flipped image Shape: (32, 32, 3) , Flipped label: 6
------
Source image Shape : (32, 32, 3) , Source label : 5
Flipped image Shape: (32, 32, 3) , Flipped label: 5
------

The following shows the inverted image.

py_transforms

The following describes how to use common data augmentation operators of the py_transforms module.

Compose

Receives a transforms list and applies the data augmentation operations in the list to dataset images in sequence.

The following example loads the CIFAR-10 dataset [1], defines the decoding, resizing, and data type conversion operations, applies the operations to the loaded image, outputs the image shapes and labels before and after the processing, and displays the processed image.

Follow the steps above to download the CIFAR-10 dataset and store it as required.

import matplotlib.pyplot as plt
import mindspore.dataset as ds
import mindspore.dataset.vision.py_transforms as py_trans
from mindspore.dataset.transforms.py_transforms import Compose
from PIL import Image

ds.config.set_seed(8)

DATA_DIR = "cifar-10-batches-bin/"

dataset1 = ds.Cifar10Dataset(DATA_DIR, num_samples=5, shuffle=True)
def decode(image):
    return Image.fromarray(image)

transforms_list = [
  decode,
  py_trans.Resize(size=(200,200)),
  py_trans.ToTensor()
]
compose_trans = Compose(transforms_list)
dataset2 = dataset1.map(operations=compose_trans, input_columns=["image"])

image_list, label_list = [], []
for data in dataset2.create_dict_iterator():
    image_list.append(data['image'])
    label_list.append(data['label'])
    print("Transformed image Shape:", data['image'].shape, ", Transformed label:", data['label'])

num_samples = len(image_list)
for i in range(num_samples):
    plt.subplot(1, len(image_list), i + 1)
    plt.imshow(image_list[i].asnumpy().transpose(1, 2, 0))
    plt.title(label_list[i].asnumpy())
plt.show()

The output is as follows:

Transformed image Shape: (3, 200, 200) , Transformed label: 4
Transformed image Shape: (3, 200, 200) , Transformed label: 9
Transformed image Shape: (3, 200, 200) , Transformed label: 6
Transformed image Shape: (3, 200, 200) , Transformed label: 5
Transformed image Shape: (3, 200, 200) , Transformed label: 7

The following shows the processed image.

Usage Instructions

Do not use c_transforms and py_transforms together because they apply to images in different ways and using them together will reduce the processing performance.

Using both C++ and Python will cause the cost of switching between them. You are advised not to use operators of the two modules together. However, it is acceptable to use a proper number of operators together.

Recommended usage:

• Use py_transform or c_transform separately.

  

• Use py_transform and then c_transform.

  

• Use c_transform and then py_transform.

  

Not recommended:

• Frequent switching between c_transforms and py_transforms.

  

References

[1] Alex Krizhevsky. Learning Multiple Layers of Features from Tiny Images.

[2] Y. LeCun, L. Bottou, Y. Bengio, and P. Haffner. Gradient-based learning applied to document recognition.