Image data preprocessing

2023-03-05 11:58| 来源: 网络整理| 查看: 265

Image data preprocessing image_dataset_from_directory function tf.keras.preprocessing.image_dataset_from_directory( directory, labels="inferred", label_mode="int", class_names=None, color_mode="rgb", batch_size=32, image_size=(256, 256), shuffle=True, seed=None, validation_split=None, subset=None, interpolation="bilinear", follow_links=False, crop_to_aspect_ratio=False, **kwargs )

Generates a tf.data.Dataset from image files in a directory.

If your directory structure is:

main_directory/ ...class_a/ ......a_image_1.jpg ......a_image_2.jpg ...class_b/ ......b_image_1.jpg ......b_image_2.jpg

Then calling image_dataset_from_directory(main_directory, labels='inferred') will return a tf.data.Dataset that yields batches of images from the subdirectories class_a and class_b, together with labels 0 and 1 (0 corresponding to class_a and 1 corresponding to class_b).

Supported image formats: jpeg, png, bmp, gif. Animated gifs are truncated to the first frame.

Arguments

directory: Directory where the data is located. If labels is "inferred", it should contain subdirectories, each containing images for a class. Otherwise, the directory structure is ignored. labels: Either "inferred" (labels are generated from the directory structure), None (no labels), or a list/tuple of integer labels of the same size as the number of image files found in the directory. Labels should be sorted according to the alphanumeric order of the image file paths (obtained via os.walk(directory) in Python). label_mode: - 'int': means that the labels are encoded as integers (e.g. for sparse_categorical_crossentropy loss). - 'categorical' means that the labels are encoded as a categorical vector (e.g. for categorical_crossentropy loss). - 'binary' means that the labels (there can be only 2) are encoded as float32 scalars with values 0 or 1 (e.g. for binary_crossentropy). - None (no labels). class_names: Only valid if "labels" is "inferred". This is the explict list of class names (must match names of subdirectories). Used to control the order of the classes (otherwise alphanumerical order is used). color_mode: One of "grayscale", "rgb", "rgba". Default: "rgb". Whether the images will be converted to have 1, 3, or 4 channels. batch_size: Size of the batches of data. Default: 32. image_size: Size to resize images to after they are read from disk. Defaults to (256, 256). Since the pipeline processes batches of images that must all have the same size, this must be provided. shuffle: Whether to shuffle the data. Default: True. If set to False, sorts the data in alphanumeric order. seed: Optional random seed for shuffling and transformations. validation_split: Optional float between 0 and 1, fraction of data to reserve for validation. subset: One of "training" or "validation". Only used if validation_split is set. interpolation: String, the interpolation method used when resizing images. Defaults to bilinear. Supports bilinear, nearest, bicubic, area, lanczos3, lanczos5, gaussian, mitchellcubic. follow_links: Whether to visits subdirectories pointed to by symlinks. Defaults to False. crop_to_aspect_ratio: If True, resize the images without aspect ratio distortion. When the original aspect ratio differs from the target aspect ratio, the output image will be cropped so as to return the largest possible window in the image (of size image_size) that matches the target aspect ratio. By default (crop_to_aspect_ratio=False), aspect ratio may not be preserved. **kwargs: Legacy keyword arguments.

Returns

A tf.data.Dataset object. - If label_mode is None, it yields float32 tensors of shape (batch_size, image_size[0], image_size[1], num_channels), encoding images (see below for rules regarding num_channels). - Otherwise, it yields a tuple (images, labels), where images has shape (batch_size, image_size[0], image_size[1], num_channels), and labels follows the format described below.

Rules regarding labels format: - if label_mode is int, the labels are an int32 tensor of shape (batch_size,). - if label_mode is binary, the labels are a float32 tensor of 1s and 0s of shape (batch_size, 1). - if label_mode is categorial, the labels are a float32 tensor of shape (batch_size, num_classes), representing a one-hot encoding of the class index.

Rules regarding number of channels in the yielded images: - if color_mode is grayscale, there's 1 channel in the image tensors. - if color_mode is rgb, there are 3 channel in the image tensors. - if color_mode is rgba, there are 4 channel in the image tensors.

load_img function tf.keras.preprocessing.image.load_img( path, grayscale=False, color_mode="rgb", target_size=None, interpolation="nearest" )

Loads an image into PIL format.

Usage:

image = tf.keras.preprocessing.image.load_img(image_path) input_arr = tf.keras.preprocessing.image.img_to_array(image) input_arr = np.array([input_arr]) # Convert single image to a batch. predictions = model.predict(input_arr)

Arguments

path: Path to image file. grayscale: DEPRECATED use color_mode="grayscale". color_mode: One of "grayscale", "rgb", "rgba". Default: "rgb". The desired image format. target_size: Either None (default to original size) or tuple of ints (img_height, img_width). interpolation: Interpolation method used to resample the image if the target size is different from that of the loaded image. Supported methods are "nearest", "bilinear", and "bicubic". If PIL version 1.1.3 or newer is installed, "lanczos" is also supported. If PIL version 3.4.0 or newer is installed, "box" and "hamming" are also supported. By default, "nearest" is used.

Returns

A PIL Image instance.

Raises

ImportError: if PIL is not available. ValueError: if interpolation method is not supported. img_to_array function tf.keras.preprocessing.image.img_to_array(img, data_format=None, dtype=None)

Converts a PIL Image instance to a Numpy array.

Usage:

from PIL import Image img_data = np.random.random(size=(100, 100, 3)) img = tf.keras.preprocessing.image.array_to_img(img_data) array = tf.keras.preprocessing.image.img_to_array(img)

Arguments

img: Input PIL Image instance. data_format: Image data format, can be either "channels_first" or "channels_last". Defaults to None, in which case the global setting tf.keras.backend.image_data_format() is used (unless you changed it, it defaults to "channels_last"). dtype: Dtype to use. Default to None, in which case the global setting tf.keras.backend.floatx() is used (unless you changed it, it defaults to "float32")

Returns

A 3D Numpy array.

Raises

ValueError: if invalid img or data_format is passed.

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