match_template didn't match the correct region

[felixgao]

Description

Way to reproduce

import cv2
import numpy as np
from PIL import Image, ImageFile, ImageOps
from skimage.feature import match_template
from skimage.color import rgb2gray

from pathlib import Path


def skimage_template_matching(template, img_gray, draw_match=True):
    w, h = template.shape[::-1]
    result = match_template(img_gray, template, constant_values=img_gray.mean())
    ij = np.unravel_index(np.argmax(result), result.shape)
    x, y = ij[::-1]
    top_left = (x, y)
    bottom_right = (x+w, y+h)
    if draw_match:
        cv2.rectangle(img_gray, top_left, bottom_right, (0,0,255), 2)
    return top_left, bottom_right, img_gray


if __name__ == '__main__':
    im = Image.open(Path(W-2.png"))
    im = ImageOps.grayscale(im)
    template = np.array(im)
    
    img_file = Image.open(Path("w2 part.jpeg"))
    img_file = ImageOps.grayscale(img_file)
    gray_image = np.array(img_file)
    
    top_left, bottom_right, overlaid = skimage_template_matching(template, gray_image)
    
    print(top_left)
    print(bottom_right)
    
    cv2.imwrite("overlaid.jpeg", overlaid)

the images for the template, image, and overlaid is

Version information

# Paste the output of the following python commands
from __future__ import print_function
import sys; print(sys.version)
import platform; print(platform.platform())
import skimage; print(f'scikit-image version: {skimage.__version__}')
import numpy; print(f'numpy version: {numpy.__version__}')

3.8.7 (default, Apr 28 2021, 16:07:46)
[Clang 12.0.0 (clang-1200.0.32.29)]
macOS-10.16-x86_64-i386-64bit
scikit-image version: 0.18.3
numpy version: 1.21.1

I am expecting the region to be at least closer to the button left corner of the original image rather than the middle of the image.

[rfezzani]

@felixgao, Thank you for your interest in skimage. match_template is not scale invariant! I mean that the template and target image must be at same scale.
It seams in your example that your template resolution is twice your target image image resolution. After rescaling the template I am able to match the correct location:

import numpy as np
import imageio
from skimage import color
from skimage.feature import match_template
from skimage.transform import rescale
from matplotlib import pyplot as plt
from matplotlib.patches import Rectangle

img = color.rgb2gray(imageio.imread("/tmp/target.jpeg"))
template = rescale(
    color.rgb2gray(
        color.rgba2rgb(imageio.imread("/tmp/template.png"))), 0.5)

nl, nc = template.shape

result = match_template(img, template)

i, j = np.unravel_index(np.argmax(result), result.shape)

print(i, j)

_, (ax0, ax1) = plt.subplots(1, 2)

ax0.imshow(result)
ax1.imshow(img, cmap="gray")
ax1.add_artist(Rectangle((j, i), nc, nl, ec="red", fc="None", lw=2))

plt.show()

And obtained

If you don't mind, I will convert this issue to Discussion 😉.