How to remove unclosed curves in a binary image?

Question

After some processing, I have this binary image:

I want to remove the unclosed curves i.e. the top left and the bottom right curves. Can you suggest me the algorithm for doing this? Thanks.

Answer 1

As @John Zwinck mentions this can be done using floodfill, but I figure your problem is you want to return to the original black background, and retain the contours of the closed shapes. While you could use contours to figure this out, here is a fairly simple approach that will remove all non-closed and unenclosed line segments from an image, even if they are attached to a closed shape, But retain the edges of the closed curves.

1. floodfill image with white - this removes your problem non-closed lines, but also the borders of your wanted objects.
2. erode the image, then invert it
3. AND the image with the original image - thus restoring the borders.

Output:

The code is in python, but should easily translate to the usual C++ cv2 usage.

import cv2
import numpy as np

im = cv2.imread('I7qZP.png',cv2.CV_LOAD_IMAGE_GRAYSCALE)
im2 = im.copy()
mask = np.zeros((np.array(im.shape)+2), np.uint8)
cv2.floodFill(im, mask, (0,0), (255))
im = cv2.erode(im, np.ones((3,3)))
im = cv2.bitwise_not(im)
im = cv2.bitwise_and(im,im2)
cv2.imshow('show', im)
cv2.imwrite('fin.png',im)
cv2.waitKey()

Answer 2

You're looking for Flood Fill: http://en.wikipedia.org/wiki/Flood_fill