.. DO NOT EDIT. .. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY. .. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE: .. "auto_examples/features_detection/plot_windowed_histogram.py" .. LINE NUMBERS ARE GIVEN BELOW. .. only:: html .. note:: :class: sphx-glr-download-link-note :ref:`Go to the end ` to download the full example code or to run this example in your browser via Binder .. rst-class:: sphx-glr-example-title .. _sphx_glr_auto_examples_features_detection_plot_windowed_histogram.py: ======================== Sliding window histogram ======================== Histogram matching can be used for object detection in images [1]_. This example extracts a single coin from the ``skimage.data.coins`` image and uses histogram matching to attempt to locate it within the original image. First, a box-shaped region of the image containing the target coin is extracted and a histogram of its grayscale values is computed. Next, for each pixel in the test image, a histogram of the grayscale values in a region of the image surrounding the pixel is computed. ``skimage.filters.rank.windowed_histogram`` is used for this task, as it employs an efficient sliding window based algorithm that is able to compute these histograms quickly [2]_. The local histogram for the region surrounding each pixel in the image is compared to that of the single coin, with a similarity measure being computed and displayed. The histogram of the single coin is computed using ``numpy.histogram`` on a box shaped region surrounding the coin, while the sliding window histograms are computed using a disc shaped structural element of a slightly different size. This is done in aid of demonstrating that the technique still finds similarity in spite of these differences. To demonstrate the rotational invariance of the technique, the same test is performed on a version of the coins image rotated by 45 degrees. References ---------- .. [1] Porikli, F. "Integral Histogram: A Fast Way to Extract Histograms in Cartesian Spaces" CVPR, 2005. Vol. 1. IEEE, 2005 .. [2] S.Perreault and P.Hebert. Median filtering in constant time. Trans. Image Processing, 16(9):2389-2394, 2007. .. GENERATED FROM PYTHON SOURCE LINES 37-136 .. image-sg:: /auto_examples/features_detection/images/sphx_glr_plot_windowed_histogram_001.png :alt: Quantized image, Coin from 2nd row, 4th column, Original image with overlaid similarity, Rotated image with overlaid similarity :srcset: /auto_examples/features_detection/images/sphx_glr_plot_windowed_histogram_001.png :class: sphx-glr-single-img .. code-block:: Python import numpy as np import matplotlib import matplotlib.pyplot as plt from skimage import data, transform from skimage.util import img_as_ubyte from skimage.morphology import disk from skimage.filters import rank matplotlib.rcParams['font.size'] = 9 def windowed_histogram_similarity(image, footprint, reference_hist, n_bins): # Compute normalized windowed histogram feature vector for each pixel px_histograms = rank.windowed_histogram(image, footprint, n_bins=n_bins) # Reshape coin histogram to (1,1,N) for broadcast when we want to use it in # arithmetic operations with the windowed histograms from the image reference_hist = reference_hist.reshape((1, 1) + reference_hist.shape) # Compute Chi squared distance metric: sum((X-Y)^2 / (X+Y)); # a measure of distance between histograms X = px_histograms Y = reference_hist num = (X - Y) ** 2 denom = X + Y denom[denom == 0] = np.inf frac = num / denom chi_sqr = 0.5 * np.sum(frac, axis=2) # Generate a similarity measure. It needs to be low when distance is high # and high when distance is low; taking the reciprocal will do this. # Chi squared will always be >= 0, add small value to prevent divide by 0. similarity = 1 / (chi_sqr + 1.0e-4) return similarity # Load the `skimage.data.coins` image img = img_as_ubyte(data.coins()) # Quantize to 16 levels of grayscale; this way the output image will have a # 16-dimensional feature vector per pixel quantized_img = img // 16 # Select the coin from the 4th column, second row. # Coordinate ordering: [x1,y1,x2,y2] coin_coords = [184, 100, 228, 148] # 44 x 44 region coin = quantized_img[coin_coords[1] : coin_coords[3], coin_coords[0] : coin_coords[2]] # Compute coin histogram and normalize coin_hist, _ = np.histogram(coin.flatten(), bins=16, range=(0, 16)) coin_hist = coin_hist.astype(float) / np.sum(coin_hist) # Compute a disk shaped mask that will define the shape of our sliding window # Example coin is ~44px across, so make a disk 61px wide (2 * rad + 1) to be # big enough for other coins too. footprint = disk(30) # Compute the similarity across the complete image similarity = windowed_histogram_similarity( quantized_img, footprint, coin_hist, coin_hist.shape[0] ) # Now try a rotated image rotated_img = img_as_ubyte(transform.rotate(img, 45.0, resize=True)) # Quantize to 16 levels as before quantized_rotated_image = rotated_img // 16 # Similarity on rotated image rotated_similarity = windowed_histogram_similarity( quantized_rotated_image, footprint, coin_hist, coin_hist.shape[0] ) fig, axes = plt.subplots(nrows=2, ncols=2, figsize=(10, 10)) axes[0, 0].imshow(quantized_img, cmap='gray') axes[0, 0].set_title('Quantized image') axes[0, 0].axis('off') axes[0, 1].imshow(coin, cmap='gray') axes[0, 1].set_title('Coin from 2nd row, 4th column') axes[0, 1].axis('off') axes[1, 0].imshow(img, cmap='gray') axes[1, 0].imshow(similarity, cmap='hot', alpha=0.5) axes[1, 0].set_title('Original image with overlaid similarity') axes[1, 0].axis('off') axes[1, 1].imshow(rotated_img, cmap='gray') axes[1, 1].imshow(rotated_similarity, cmap='hot', alpha=0.5) axes[1, 1].set_title('Rotated image with overlaid similarity') axes[1, 1].axis('off') plt.tight_layout() plt.show() .. rst-class:: sphx-glr-timing **Total running time of the script:** (0 minutes 1.741 seconds) .. _sphx_glr_download_auto_examples_features_detection_plot_windowed_histogram.py: .. only:: html .. container:: sphx-glr-footer sphx-glr-footer-example .. container:: binder-badge .. image:: images/binder_badge_logo.svg :target: https://mybinder.org/v2/gh/scikit-image/scikit-image/v0.23.2?filepath=notebooks/auto_examples/features_detection/plot_windowed_histogram.ipynb :alt: Launch binder :width: 150 px .. container:: sphx-glr-download sphx-glr-download-jupyter :download:`Download Jupyter notebook: plot_windowed_histogram.ipynb ` .. container:: sphx-glr-download sphx-glr-download-python :download:`Download Python source code: plot_windowed_histogram.py ` .. only:: html .. rst-class:: sphx-glr-signature `Gallery generated by Sphinx-Gallery `_