如果 absdiff() 相同图像（只是稍微垂直移动）差异 > 则正确15%？

Question

如果我有 2 个几乎完全相同的图像（除了其中一个图像的所有像素向上移动 2 个像素），并且我使用 cv2.absdiff() 比较它们，则差异 > > 15%。这对我来说听起来不太合适？

我本以为差距会小很多。这是正确的吗？还是我的数学不正确？

以下是我的代码：

def calculate_difference_measure():
    # Load in same image twice
    img1 = cv2.imread('./i1.png')
    img2 = img1.copy()

    # Translate one image slightly up
    img2 = imutils.translate(img2, 0, 2)
    
    # Difference the 2 images and get all values below 15
    diff = cv2.absdiff(img1, img2)
    gray_diff = cv2.cvtColor(diff, cv2.COLOR_BGR2GRAY)
    thresh_diff = cv2.threshold(gray_diff, 15, 255, cv2.THRESH_BINARY)[1]
    
    # Calculate the difference between the 2 images
    total_pixels = img1.shape[0] * img1.shape[1] * 1.0
    diff_on_pixels = cv2.countNonZero(thresh_diff) * 1.0
    difference_measure = diff_on_pixels / total_pixels
    # Outputs 'difference_measure: 0.1768186919702071'
    print('difference_measure: {}'.format(difference_measure))

我的代码用于比较的测试图像。
：

Answer 1

是的，据我从 cv2.absdiff 的文档中了解到，它计算像素的差异。为了形象化，我创建了一个棋盘并移动了 1 行 - 这导致了 100% 的差异。

import cv2
import imutils
import numpy as np


def calculate_difference_measure(img1: np.array, img2: np.array) -> float:
    diff = cv2.absdiff(img1, img2)
    thresh_diff = cv2.threshold(diff, 15, 255, cv2.THRESH_BINARY)[1]

    # Calculate the difference between the 2 images
    total_pixels = img1.shape[0] * img1.shape[1] * 1.0
    diff_on_pixels = cv2.countNonZero(thresh_diff) * 1.0
    difference_measure = diff_on_pixels / total_pixels
    print('difference_measure: {}'.format(difference_measure))
    return difference_measure


def main():
    chess1 = np.uint8(([150, 50] * 4 + [50, 150] * 4) * 4).reshape((8, 8))
    chess2 = imutils.translate(chess1.copy(), 0, 1)

    calculate_difference_measure(img1=chess1, img2=chess1)  # 0 diff
    calculate_difference_measure(img1=chess1, img2=chess2)  # 1 diff (100% different)

    chess1 = cv2.resize(chess1, (400, 400), interpolation=cv2.INTER_AREA)
    chess2 = cv2.resize(chess2, (400, 400), interpolation=cv2.INTER_AREA)
    cv2.imshow('original', chess1)
    cv2.imshow('translated', chess2)
    cv2.waitKey(0)
    return


if __name__ == '__main__':
    main()