Template MatchingΒΆ

We use template matching to identify the occurrence of an image patch (in this case, a sub-image centered on a single coin). Here, we return a single match (the exact same coin), so the maximum value in the match_template result corresponds to the coin location. The other coins look similar, and thus have local maxima; if you expect multiple matches, you should use a proper peak-finding function.

The match_template function uses fast, normalized cross-correlation [1] to find instances of the template in the image. Note that the peaks in the output of match_template correspond to the origin (i.e. top-left corner) of the template.

[1]J. P. Lewis, “Fast Normalized Cross-Correlation”, Industrial Light and Magic.
import numpy as np
import matplotlib.pyplot as plt

from skimage import data
from skimage.feature import match_template

image = data.coins()
coin = image[170:220, 75:130]

result = match_template(image, coin)
ij = np.unravel_index(np.argmax(result), result.shape)
x, y = ij[::-1]

fig = plt.figure(figsize=(8, 3))
ax1 = plt.subplot(1, 3, 1)
ax2 = plt.subplot(1, 3, 2, adjustable='box-forced')
ax3 = plt.subplot(1, 3, 3, sharex=ax2, sharey=ax2, adjustable='box-forced')

ax1.imshow(coin, cmap=plt.cm.gray)

ax2.imshow(image, cmap=plt.cm.gray)
# highlight matched region
hcoin, wcoin = coin.shape
rect = plt.Rectangle((x, y), wcoin, hcoin, edgecolor='r', facecolor='none')

# highlight matched region
ax3.plot(x, y, 'o', markeredgecolor='r', markerfacecolor='none', markersize=10)


Total running time of the script: ( 0 minutes 0.229 seconds)

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