How do I calculate the percentage of difference between two images using Python and OpenCV?

Question

I am trying to write a program in Python (with OpenCV) that compares 2 images, shows the difference between them, and then informs the user of the percentage of difference between the images. I have already made it so it generates a .jpg showing the difference, but I can't figure out how to make it calculate a percentage. Does anyone know how to do this?

Thanks in advance.

Answer 1

Here is a simple idea you can adapt. But always ensure the images being compared are of the same shape.

Code:

img1 = cv2.imread('dog.jpg', 0)
img2 = cv2.imread('cat.jpg', 0)

#--- take the absolute difference of the images ---
res = cv2.absdiff(img1, img2)

#--- convert the result to integer type ---
res = res.astype(np.uint8)

#--- find percentage difference based on number of pixels that are not zero ---
percentage = (numpy.count_nonzero(res) * 100)/ res.size

• If img1 and img2 are similar most of the pixels in res would be 0 resulting in a lower percentage.
• If img1 and img2 are different this percentage would be higher.

Note: I have shown for a single channel image and the same can be extended for multi-channel images.

Answer 2

You will need to calculate this on your own. You will need the count of diferent pixels and the size of your original image then a simple math: (diferentPixelsCount / (mainImage.width * mainImage.height))*100