python - How to find the distance between two concentric contours, for different angles?

Question

I have an image with two contours, where one contour is always 'inside' another. I want to find the distance between the two contours for 90 different angles (meaning, distance at every 4 degrees). How do I go about doing it?

Here's an example image:

Thank you!

Answer 1

In the following code, I have just given you the example for the vertical line, the rest can be obtained by rotating the line. Result looks like this, instead of drawing you can use the coordinates for distance calculation.

import shapely.geometry as shapgeo
import numpy as np
import cv2


img = cv2.imread('image.jpg', 0)
ret, img =cv2.threshold(img, 128, 255, cv2.THRESH_BINARY)

#Fit the ellipses
_, contours0, hierarchy = cv2.findContours( img.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
outer_ellipse = [cv2.approxPolyDP(contours0[0], 0.1, True)]
inner_ellipse = [cv2.approxPolyDP(contours0[2], 0.1, True)]

h, w = img.shape[:2]
vis = np.zeros((h, w, 3), np.uint8)
cv2.drawContours( vis, outer_ellipse, -1, (255,0,0), 1)
cv2.drawContours( vis, inner_ellipse, -1, (0,0,255), 1)

##Extract contour of ellipses
cnt_outer = np.vstack(outer_ellipse).squeeze()
cnt_inner = np.vstack(inner_ellipse).squeeze()

#Determine centroid
M = cv2.moments(cnt_inner)
cx = int(M['m10']/M['m00'])
cy = int(M['m01']/M['m00'])
print cx, cy

#Draw full segment lines 
cv2.line(vis,(cx,0),(cx,w),(150,0,0),1)

# Calculate intersections using Shapely
# http://toblerity.org/shapely/manual.html
PolygonEllipse_outer= shapgeo.asLineString(cnt_outer)
PolygonEllipse_inner= shapgeo.asLineString(cnt_inner)
PolygonVerticalLine=shapgeo.LineString([(cx,0),(cx,w)])


insecouter= np.array(PolygonEllipse_outer.intersection(PolygonVerticalLine)).astype(np.int)
insecinner= np.array(PolygonEllipse_inner.intersection(PolygonVerticalLine)).astype(np.int)
cv2.line(vis,(insecouter[0,0], insecinner[1,1]),(insecouter[1,0], insecouter[1,1]),(0,255,0),2)
cv2.line(vis,(insecouter[0,0], insecinner[0,1]),(insecouter[1,0], insecouter[0,1]),(0,255,0),2)

cv2.imshow('contours', vis)

0xFF & cv2.waitKey()
cv2.destroyAllWindows()