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150 lines
5.1 KiB
150 lines
5.1 KiB
from __future__ import print_function
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from builtins import zip
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from builtins import range
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from past.builtins import xrange
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import matplotlib
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import numpy as np
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from scipy.ndimage import uniform_filter
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def extract_features(imgs, feature_fns, verbose=False):
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"""
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Given pixel data for images and several feature functions that can operate on
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single images, apply all feature functions to all images, concatenating the
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feature vectors for each image and storing the features for all images in
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a single matrix.
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Inputs:
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- imgs: N x H X W X C array of pixel data for N images.
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- feature_fns: List of k feature functions. The ith feature function should
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take as input an H x W x D array and return a (one-dimensional) array of
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length F_i.
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- verbose: Boolean; if true, print progress.
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Returns:
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An array of shape (N, F_1 + ... + F_k) where each column is the concatenation
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of all features for a single image.
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"""
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num_images = imgs.shape[0]
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if num_images == 0:
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return np.array([])
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# Use the first image to determine feature dimensions
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feature_dims = []
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first_image_features = []
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for feature_fn in feature_fns:
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feats = feature_fn(imgs[0].squeeze())
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assert len(feats.shape) == 1, 'Feature functions must be one-dimensional'
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feature_dims.append(feats.size)
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first_image_features.append(feats)
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# Now that we know the dimensions of the features, we can allocate a single
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# big array to store all features as columns.
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total_feature_dim = sum(feature_dims)
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imgs_features = np.zeros((num_images, total_feature_dim))
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imgs_features[0] = np.hstack(first_image_features).T
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# Extract features for the rest of the images.
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for i in range(1, num_images):
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idx = 0
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for feature_fn, feature_dim in zip(feature_fns, feature_dims):
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next_idx = idx + feature_dim
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imgs_features[i, idx:next_idx] = feature_fn(imgs[i].squeeze())
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idx = next_idx
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if verbose and i % 1000 == 999:
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print('Done extracting features for %d / %d images' % (i+1, num_images))
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return imgs_features
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def rgb2gray(rgb):
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"""Convert RGB image to grayscale
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Parameters:
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rgb : RGB image
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Returns:
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gray : grayscale image
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"""
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return np.dot(rgb[...,:3], [0.299, 0.587, 0.144])
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def hog_feature(im):
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"""Compute Histogram of Gradient (HOG) feature for an image
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Modified from skimage.feature.hog
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http://pydoc.net/Python/scikits-image/0.4.2/skimage.feature.hog
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Reference:
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Histograms of Oriented Gradients for Human Detection
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Navneet Dalal and Bill Triggs, CVPR 2005
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Parameters:
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im : an input grayscale or rgb image
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Returns:
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feat: Histogram of Gradient (HOG) feature
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"""
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# convert rgb to grayscale if needed
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if im.ndim == 3:
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image = rgb2gray(im)
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else:
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image = np.at_least_2d(im)
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sx, sy = image.shape # image size
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orientations = 9 # number of gradient bins
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cx, cy = (8, 8) # pixels per cell
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gx = np.zeros(image.shape)
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gy = np.zeros(image.shape)
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gx[:, :-1] = np.diff(image, n=1, axis=1) # compute gradient on x-direction
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gy[:-1, :] = np.diff(image, n=1, axis=0) # compute gradient on y-direction
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grad_mag = np.sqrt(gx ** 2 + gy ** 2) # gradient magnitude
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grad_ori = np.arctan2(gy, (gx + 1e-15)) * (180 / np.pi) + 90 # gradient orientation
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n_cellsx = int(np.floor(sx / cx)) # number of cells in x
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n_cellsy = int(np.floor(sy / cy)) # number of cells in y
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# compute orientations integral images
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orientation_histogram = np.zeros((n_cellsx, n_cellsy, orientations))
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for i in range(orientations):
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# create new integral image for this orientation
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# isolate orientations in this range
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temp_ori = np.where(grad_ori < 180 / orientations * (i + 1),
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grad_ori, 0)
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temp_ori = np.where(grad_ori >= 180 / orientations * i,
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temp_ori, 0)
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# select magnitudes for those orientations
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cond2 = temp_ori > 0
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temp_mag = np.where(cond2, grad_mag, 0)
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orientation_histogram[:,:,i] = uniform_filter(temp_mag, size=(cx, cy))[round(cx/2)::cx, round(cy/2)::cy].T
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return orientation_histogram.ravel()
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def color_histogram_hsv(im, nbin=10, xmin=0, xmax=255, normalized=True):
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"""
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Compute color histogram for an image using hue.
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Inputs:
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- im: H x W x C array of pixel data for an RGB image.
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- nbin: Number of histogram bins. (default: 10)
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- xmin: Minimum pixel value (default: 0)
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- xmax: Maximum pixel value (default: 255)
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- normalized: Whether to normalize the histogram (default: True)
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Returns:
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1D vector of length nbin giving the color histogram over the hue of the
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input image.
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"""
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ndim = im.ndim
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bins = np.linspace(xmin, xmax, nbin+1)
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hsv = matplotlib.colors.rgb_to_hsv(im/xmax) * xmax
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imhist, bin_edges = np.histogram(hsv[:,:,0], bins=bins, density=normalized)
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imhist = imhist * np.diff(bin_edges)
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# return histogram
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return imhist
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