SSIM无法获得图像差异

Question

我正在研究射击模拟器项目。我正在使用IP摄像机捕获带有弹孔的屏幕。我面临的问题是仅获得新的弹孔。我尝试使用SSIM方法来实现这一目标：

before = cv2.imread('before.png')
after = cv2.imread('after.png')

# Convert images to grayscale
before_gray = cv2.cvtColor(before, cv2.COLOR_BGR2GRAY)
after_gray = cv2.cvtColor(after, cv2.COLOR_BGR2GRAY)

# Compute SSIM between two images
(score, diff) = compare_ssim(before_gray, after_gray, full=True)
# The diff image contains the actual image differences between the two images
# and is represented as a floating point data type in the range [0,1] 
# so we must convert the array to 8-bit unsigned integers in the range
# [0,255] before we can use it with OpenCV
diff = (diff * 255).astype("uint8")

# Threshold the difference image, followed by finding contours to
# obtain the regions of the two input images that differ
thresh = cv2.threshold(diff, 0, 255, cv2.THRESH_BINARY_INV | cv2.THRESH_OTSU)[1]
contours = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours = contours[0] if len(contours) == 2 else contours[1]

contour_sizes = [(cv2.contourArea(contour), contour) for contour in contours]

# The largest contour should be the new detected difference
if len(contour_sizes) > 0:
    largest_contour = max(contour_sizes, key=lambda x: x[0])[1]
    x,y,w,h = cv2.boundingRect(largest_contour)
    cv2.rectangle(before, (x, y), (x + w, y + h), (36,255,12), 2)
    cv2.rectangle(after, (x, y), (x + w, y + h), (36,255,12), 2)

cv2.imshow('before', before)
cv2.imshow('after', after)
cv2.imshow('diff',diff)
cv2.waitKey(0)

我得到这个结果。 我的前后图像相同（由于屏幕上有空气，所以运动很少），但是SSIM方法始终可以检测到相同图像中的变化。

我的第一张图片是：

第二张图片是：

处理后得到的结果是

还有其他获得新差异的方法吗？我尝试在图像上应用Canny方法，但无法正常工作。