Plot digitizer for single datapoint Python

I need to analyse 900 of these frames, that is giving the angle of the little cardboard piece with respect to the polar. Is there any library that I can use to automate the process in python? I would like to feed the program with the folder directory and that it would give back a list with all the angles in each frame.

Solution

OpenCV is a powerful tool for image manipulation.

I've tried to come up with a solution for you problem, but I do not know how stable this solution is across the 900 images. In particular, the 0° - 180° axis must not be significantly tilted, as I am not correcting for that.

import cv2 as cv
import numpy as np

#load image
img = cv.imread(r'path_to_your_image.jpg', cv.IMREAD_GRAYSCALE)

#detect circle
circles = cv.HoughCircles(img, cv.HOUGH_GRADIENT, 1, 100,
                               param1=100, param2=30,
                               minRadius=50, maxRadius=500)
# get circle center (raise Exception if circle was not detected)
try:
    center = (int(circles[0,0,0]), int(circles[0,0,1]))
except IndexError:
    raise Exception("Unable to identify center.")
# plot center of circle
cv.circle(img, center, 1, (0, 100, 100), 3)

# dilate and threshold image to only see the rectangle as small dot
kernel = np.ones((35, 35), np.uint8)
img_dilate = cv.dilate(img, kernel, iterations=1)
_, img_dilate_thres = cv.threshold(img_dilate,120,255,cv.THRESH_BINARY)
# get center position of remaining dot
rect_x = np.argmin(img_dilate_thres.mean(axis=0))
rect_y = np.argmin(img_dilate_thres.mean(axis=1))
cv.circle(img, (rect_x, rect_y), 1, (0, 100, 100), 3)

# get angle between circle center and patch
angle = np.arctan2(center[1] - rect_y, rect_x - center[0])
degree = angle / (2*np.pi) * 360

# display angle, wait for user to close image
font = cv.FONT_HERSHEY_SIMPLEX
cv.putText(img, f'{degree:.1f}', (rect_x, rect_y), font, 2, (0, 255, 0), 2, cv.LINE_AA)
cv.imshow("detected circles", img)
cv.waitKey(0)