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Transforms

Usually, the directly-loaded raw data cannot be directly fed into the neural network for training, and we need to preprocess the data at this time. MindSpore provides different kinds of data transforms that can be used with the Data Processing Pipeline for data preprocessing. All Transforms can be passed in via the map method to process the specified data columns.

mindspore.dataset provides Transforms for different data types such as image, text and audio, and also supports using Lambda functions. The descriptions are as follows.

import numpy as np
from PIL import Image
from download import download
from mindspore.dataset import transforms, vision, text
from mindspore.dataset import GeneratorDataset, MnistDataset

Common Transforms

The mindspore.dataset.transforms module supports a set of common Transforms. Here we take Compose as an example to introduce its usage.

Compose

Compose takes a sequence of data enhancement operations and then combines them into a single data enhancement operation. We still present the application effect of Transforms based on the Mnist dataset.

# Download data from open datasets

url = "https://mindspore-website.obs.cn-north-4.myhuaweicloud.com/" \
      "notebook/datasets/MNIST_Data.zip"
path = download(url, "./", kind="zip", replace=True)

train_dataset = MnistDataset('MNIST_Data/train')

Downloading data from https://mindspore-website.obs.cn-north-4.myhuaweicloud.com/notebook/datasets/MNIST_Data.zip (10.3 MB)

file_sizes: 100%|██████████████████████████| 10.8M/10.8M [00:01<00:00, 9.01MB/s]
Extracting zip file...
Successfully downloaded / unzipped to ./

image, label = next(train_dataset.create_tuple_iterator())
print(image.shape)

(28, 28, 1)

composed = transforms.Compose(
    [
        vision.Rescale(1.0 / 255.0, 0),
        vision.Normalize(mean=(0.1307,), std=(0.3081,)),
        vision.HWC2CHW()
    ]
)

train_dataset = train_dataset.map(composed, 'image')
image, label = next(train_dataset.create_tuple_iterator())
print(image.shape)

(1, 28, 28)

For more common Transforms, see mindspore.dataset.transforms.

Vision Transforms

The mindspore.dataset.vision module provides a series of Transforms for image data. Rescale, Normalize and HWC2CHW transforms are used in the Mnist data processing. The descriptions are as follows.

Rescale

The Rescale transform is used to resize the image pixel values and consists of two parameters:

  • rescale: scaling factor.

  • shift: shift factor.

Each pixel of the image will be adjusted according to these two parameters and the output pixel value will be \(output_{i} = input_{i} * rescale + shift\).

Here we first use numpy to generate a random image with pixel values in [0, 255] and scale its pixel values.

random_np = np.random.randint(0, 255, (48, 48), np.uint8)
random_image = Image.fromarray(random_np)
print(random_np)

[[170  10 218 ...  81 128  96]
 [  2 107 146 ... 239 178 165]
 [232 137 235 ... 222 109 216]
 ...
 [193 140  60 ...  72 133 144]
 [232 175  58 ...  55 110  94]
 [152 241 105 ... 187  45  43]]

To present a more visual comparison of the data before and after Transform, we use Eager mode demo of Transforms. First instantiate the Transform object, and then call the object for data processing.

rescale = vision.Rescale(1.0 / 255.0, 0)
rescaled_image = rescale(random_image)
print(rescaled_image)

[[0.6666667  0.03921569 0.854902   ... 0.31764707 0.5019608  0.37647063]
 [0.00784314 0.41960788 0.57254905 ... 0.93725497 0.69803923 0.64705884]
 [0.909804   0.5372549  0.9215687  ... 0.8705883  0.427451   0.8470589 ]
 ...
 [0.7568628  0.54901963 0.23529413 ... 0.28235295 0.52156866 0.5647059 ]
 [0.909804   0.6862745  0.227451   ... 0.21568629 0.43137258 0.36862746]
 [0.59607846 0.9450981  0.41176474 ... 0.73333335 0.1764706  0.16862746]]

It can be seen that each pixel value is scaled after using Rescale.

Normalize

The Normalize transform is used for normalization of the input image and consists of three parameters:

  • mean: the mean value of each channel in the image.

  • std: the standard deviation of each channel in the image.

  • is_hwc: whether the format of input image is (height, width, channel) or (channel, height, width).

Each channel of the image will be adjusted according to mean and std, and the formula is \(output_{c} = \frac{input_{c} - mean_{c}}{std_{c}}\), where \(c\) represents the channel index.

normalize = vision.Normalize(mean=(0.1307,), std=(0.3081,))
normalized_image = normalize(rescaled_image)
print(normalized_image)

[[ 1.7395868  -0.29693064  2.3505423  ...  0.60677403  1.2050011
   0.7976976 ]
 [-0.3987565   0.9377082   1.4341093  ...  2.617835    1.8414128
   1.6759458 ]
 [ 2.5287375   1.3195552   2.5669222  ...  2.4014552   0.9631647
   2.3250859 ]
 ...
 [ 2.0323365   1.3577399   0.33948112 ...  0.49221992  1.2686423
   1.4086528 ]
 [ 2.5287375   1.803228    0.31402466 ...  0.27583995  0.9758929
   0.77224106]
 [ 1.5104787   2.6432917   0.9122518  ...  1.9559668   0.14855757
   0.12310111]]

HWC2CHW

The HWC2CHW transform is used to convert the image format. The two different formats (height, width, channel) or (channel, height, width) may be targeted and optimized in different hardware devices. MindSpore sets HWC as the default image format and uses this transform for processing when CHW format is required.

Here we first process the normalized_image in the previous section to HWC format, and then convert it. You can see the change of the shape before and after the conversion.

hwc_image = np.expand_dims(normalized_image, -1)
hwc2chw = vision.HWC2CHW()
chw_image = hwc2chw(hwc_image)
print(hwc_image.shape, chw_image.shape)

(48, 48, 1) (1, 48, 48)

For more Vision Transforms, see mindspore.dataset.vision.

Text Transforms

The mindspore.dataset.text module provides a series of Transforms for text data. Unlike image data, text data requires operations such as Tokenize, building word lists, and Token to Index. Here is a brief description of its usage.

First we define three pieces of text as the data to be processed and load them by using GeneratorDataset.

texts = ['Welcome to Beijing']

test_dataset = GeneratorDataset(texts, 'text')

PythonTokenizer

Tokenize is a basic transformation to process text data. MindSpore provides many different Tokenizers. Take PythonTokenizer as example, it allows users to customize the token strategy. Then we can perform tokenization on the input text based on the map operation.

def my_tokenizer(content):
    return content.split()

test_dataset = test_dataset.map(text.PythonTokenizer(my_tokenizer))
print(next(test_dataset.create_tuple_iterator()))

[Tensor(shape=[3], dtype=String, value= ['Welcome', 'to', 'Beijing'])]

Lookup

Lookup is a vocabulary mapping transformation used to convert Token to Index. Before using Lookup, you need to construct a vocabulary, either by loading an existing vocabulary or by using Vocab to generate a vocabulary. Here we choose to use Vocab.from_dataset method to generate a vocabulary from a dataset.

vocab = text.Vocab.from_dataset(test_dataset)

After obtaining the vocabulary, we can use the vocab method to view the vocabulary.

print(vocab.vocab())

{'to': 2, 'Beijing': 0, 'Welcome': 1}

After generating the vocabulary, you can perform the vocabulary mapping transformation with the map method to convert Token to Index.

test_dataset = test_dataset.map(text.Lookup(vocab))
print(next(test_dataset.create_tuple_iterator()))

[Tensor(shape=[3], dtype=Int32, value= [1, 2, 0])]

For more Text Transforms, see mindspore.dataset.text.

Lambda Transforms

Lambda functions are anonymous functions that do not require a name and consist of a single expression that is evaluated when called. Lambda Transforms can load arbitrarily-defined Lambda functions, providing enough flexibility. Here, we start with a simple Lambda function that multiplies the input data by 2:

test_dataset = GeneratorDataset([1, 2, 3], 'data', shuffle=False)
test_dataset = test_dataset.map(lambda x: x * 2)
print(list(test_dataset.create_tuple_iterator()))

[[Tensor(shape=[], dtype=Int64, value= 2)], [Tensor(shape=[], dtype=Int64, value= 4)], [Tensor(shape=[], dtype=Int64, value= 6)]]

You can see that after map is passed into the Lambda function, the data is iteratively obtained for the multiply-2 operation.

We can also define more complex functions that work with the Lambda function to achieve complex data processing:

def func(x):
    return x * x + 2

test_dataset = test_dataset.map(lambda x: func(x))

print(list(test_dataset.create_tuple_iterator()))

[[Tensor(shape=[], dtype=Int64, value= 6)], [Tensor(shape=[], dtype=Int64, value= 18)], [Tensor(shape=[], dtype=Int64, value= 38)]]