Вы можете сделать это, найдя выпуклую оболочку областей и затем подгоняя к ней эллипс в Python / OpenCV.
Ввод:
import cv2
import numpy as np
# read image
img = cv2.imread('half_ellipses.jpg')
hh, ww = img.shape[:2]
print(hh,ww)
# convert to grayscale
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
# threshold to binary and invert
thresh = cv2.threshold(gray, 128, 255, cv2.THRESH_BINARY)[1]
thresh = 255 - thresh
# get convex hull of white pixels
points = np.column_stack(np.where(thresh.transpose() > 0))
hull = cv2.convexHull(points)
((centx,centy), (width,height), angle) = cv2.fitEllipse(hull)
print ((centx,centy), (width,height), angle)
# draw polygon
result = img.copy()
cv2.ellipse(result, (int(centx),int(centy)), (int(width/2),int(height/2)), angle, 0, 360, (0,0,255), 2)
cv2.imshow('image', img)
cv2.imshow('result', result)
cv2.waitKey(0)
cv2.destroyAllWindows()
# save results
cv2.imwrite('half_ellipses_hull_ellipse.png', result)
Результат: