Preprocess Images for Deep Learning

To train a network and make predictions on new data, your images must match the input size of the network. If you need to adjust the size of your images to match the network, then you can rescale or crop your data to the required size.

You can effectively increase the amount of training data by applying randomized augmentation to your data. Augmentation also enables you to train networks to be invariant to distortions in image data. For example, you can add randomized rotations to input images so that a network is invariant to the presence of rotation in input images. An augmentedImageDatastore provides a convenient way to apply a limited set of augmentations to 2-D images for classification problems.

For more advanced preprocessing operations, to preprocess images for regression problems, or to preprocess 3-D volumetric images, you can start with a built-in datastore. You can also preprocess images according to your own pipeline by using the transform and combine functions.

Resize Images Using Rescaling and Cropping

You can store image data as a numeric array, an ImageDatastore object, or a table. An ImageDatastore enables you to import data in batches from image collections that are too large to fit in memory. You can use an augmented image datastore or a resized 4-D array for training, prediction, and classification. You can use a resized 3-D array for prediction and classification only.

There are two ways to resize image data to match the input size of a network.

  • Rescaling multiplies the height and width of the image by a scaling factor. If the scaling factor is no identical in the vertical and horizontal directions, then rescaling changes the spatial extents of the pixels and the aspect ratio.

  • Cropping extracts a subregion of the image and preserves the spatial extent of each pixel. You can crop images from the center or from random positions in the image.

Resizing OptionData FormatResizing FunctionSample Code
Rescaling
  • 3-D array representing a single color or multispectral image

  • 3-D array representing a stack of grayscale images

  • 4-D array representing a stack of images

imresize

im = imresize(I,inputSize);

inputSize specifies the dimensions of the rescaled image.

  • 4-D array representing a stack of images

  • ImageDatastore

  • table

augmentedImageDatastore

auimds = augmentedImageDatastore(inputSize,I);

inputSize specifies the dimensions of the rescaled image.

Cropping
  • 3-D array representing a single color or multispectral image

imcrop

im = imcrop(I,rect);

rect specifies the size and position of the 2-D cropping window.

  • 3-D array representing a stack of grayscale images

  • 4-D array representing a stack of color or multispectral images

imcrop3

im = imcrop3(I,cuboid);

cuboid specifies the size and position of the 3-D cropping window.

  • 4-D array representing a stack of images

  • ImageDatastore

  • table

augmentedImageDatastore

auimds = augmentedImageDatastore(inputSize,I,'OutputSizeMode',m);

Specify m as 'centercrop' to crop from the center of the input image.

Specify m as 'randomcrop' to crop from a random location in the input image.

Augment Images for Training with Random Geometric Transformations

For image classification problems, you can use an augmentedImageDatastore to augment images with a random combination of resizing, rotation, reflection, shear, and translation transformations.

The diagram shows how trainNetwork uses an augmented image datastore to transform training data for each epoch. For an example of the workflow, see Train Network with Augmented Images.

  1. Specify training images.

  2. Configure image transformation options, such as the range of rotation angles and whether to apply reflection at random, by creating an imageDataAugmenter.

    Tip

    To preview the transformations applied to sample images, use the augment function.

  3. Create an augmentedImageDatastore. Specify the training images, the size of output images, and the imageDataAugmenter. The size of output images must be compatible with the size of the imageInputLayer of the network.

  4. Train the network, specifying the augmented image datastore as the data source for trainNetwork. For each iteration of training, the augmented image datastore applies a random combination of transformations to images in the mini-batch of training data.

    When you use an augmented image datastore as a source of training images, the datastore randomly perturbs the training data for each epoch, so that each epoch uses a slightly different data set. The actual number of training images at each epoch does not change. The transformed images are not stored in memory.

Perform Additional Image Processing Operations Using Built-In Datastores

Some datastores perform specific and limited image preprocessing operations when they read a batch of data. These application-specific datastores are listed in the table. You can use these datastores as a source of training, validation, and test data sets for deep learning applications that use Deep Learning Toolbox™. All of these datastores return data in a format supported by trainNetwork.

DatastoreDescription
augmentedImageDatastoreApply random affine geometric transformations, including resizing, rotation, reflection, shear, and translation, for training deep neural networks. For an example, see Transfer Learning Using AlexNet.
pixelLabelImageDatastoreApply identical affine geometric transformations to images and corresponding ground truth labels for training semantic segmentation networks (requires Computer Vision Toolbox™). For an example, see Semantic Segmentation Using Deep Learning.
randomPatchExtractionDatastoreExtract multiple pairs of random patches from images or pixel label images (requires Image Processing Toolbox™). You optionally can apply identical random affine geometric transformations to the pairs of patches. For an example, see Single Image Super-Resolution Using Deep Learning.
denoisingImageDatastoreApply randomly generated Gaussian noise for training denoising networks (requires Image Processing Toolbox).

Apply Custom Image Processing Pipelines Using Combine and Transform

To perform more general and complex image preprocessing operations than offered by the application-specific datastores, you can use the transform and combine functions. For more information, see Datastores for Deep Learning.

Transform Datastores with Image Data

The transform function creates an altered form of a datastore, called an underlying datastore, by transforming the data read by the underlying datastore according to a transformation function that you define.

The custom transformation function must accept data in the format returned by the read function of the underlying datastore. For image data in an ImageDatastore, the format depends on the ReadSize property .

  • When ReadSize is 1, the transformation function must accept an integer array. The size of the array is consistent with the type of images in the ImageDatastore. For example, a grayscale image has dimensions m-by-n, a truecolor image has dimensions m-by-n-by-3, and a multispectral image with c channels has dimensions m-by-n-by-c.

  • When ReadSize is greater than 1, the transformation function must accept a cell array of image data. Each element corresponds to an image in the batch.

The transform function must return data that matches the input size of the network. The transform function does not support one-to-many observation mappings.

Tip

The transform function supports prefetching when the underlying ImageDatastore reads a batch of JPG or PNG image files. For these image types, do not use the readFcn argument of ImageDatastore to apply image preprocessing, as this option is usually significantly slower. If you use a custom read function, then ImageDatastore does not prefetch.

Combine Datastores with Image Data

The combine function concatenates the data read from multiple datastores and maintains parity between the datastores.

  • Concatenate data into a two-column table or two-column cell array for training networks with a single input, such as image-to-image regression networks.

  • Concatenate data to a (numInputs+1)-column cell array for training networks with multiple inputs.

See Also

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