How to get how close an image is to another by image subtraction?

I have a reference image, which I use to compare the camera feed to. (For example, data/reference.jpg.) From the subtraction result, is there a way to tell how much your camera feed’s current frame resembles the reference image?

Code example:

# OpenCV library.
import cv2 

# Import Pi camera.
from picamera.array import PiRGBArray  # Generates a 3D RGB array
from picamera import PiCamera  # Gives usage to Pi Camera

# Resolution of the camera capture & display.
res_w = 640
res_h = 480
resolution = (res_w, res_h)

# Initialize the camera.
camera = PiCamera()

# Set the camera resolution.
camera.resolution = resolution

# Set the number of frames per second.
camera.framerate = 30

# Generates a 3D RGB array and stores it in rawCapture.
raw_capture = PiRGBArray(camera, size=resolution)

# Continues capture of frames.
cap = camera.capture_continuous(raw_capture, format="bgr", use_video_port=True)

# Capture frames continuously from the camera.
for frame in cap:

    # Grab the raw NumPy array representing the image.
    image = frame.array
    # Path to the reference image.
    reference_path = "data/reference.jpg"

    # If the reference doesn't exist, take a picture.
    if not path.isfile(reference_path):
        # Write the initial frame to a file.
        cv2.imwrite(reference_path, image)

    # Reference image for detection.
    reference = cv2.imread(reference_path)

    # Subtract the two images from each other.
    subtraction = reference - image

    # Show the image (with lines).
    cv2.imshow("Output", subtraction)

    # Wait for keyPress for 1 millisecond.
    key = cv2.waitKey(1) & 0xFF

    # Clear the stream in preparation for the next frame.

    # If the `q` key was pressed, break from the loop.
    if key == ord("q"):

I’m using picamera so excuse me about the unusual capture method. Also, as you can see subtraction is the variable I want to get this data from, but any other solution to the problem will be appreciated.


cv2.absdiff would be a start. it takes care that subtraction of unsigned integers doesn’t wrap around and give you a nonsensical value.

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