Python-openCV: Extracting (x,y) coordinates of point features on an image

edit

i'd go another way for this.

rather use findContours(), filter down to the largest, then use cv2.minAreaRect(), and cv2.boxPoints(), to find the 4 corner points:

import cv2
import numpy as np

im = cv2.imread("plank.jpg")
gray = cv2.cvtColor(im,cv2.COLOR_BGR2GRAY);
gray = cv2.GaussianBlur(gray, (5, 5), 0)
_, bin = cv2.threshold(gray,120,255,1) # inverted threshold (light obj on dark bg)
bin = cv2.dilate(bin, None)  # fill some holes
bin = cv2.dilate(bin, None)
bin = cv2.erode(bin, None)   # dilate made our shape larger, revert that
bin = cv2.erode(bin, None)
bin, contours, hierarchy = cv2.findContours(bin, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)

rc = cv2.minAreaRect(contours[0])
box = cv2.boxPoints(rc)
for p in box:
    pt = (p[0],p[1])
    print pt
    cv2.circle(im,pt,5,(200,0,0),2)
cv2.imshow("plank", im)
cv2.waitKey()

output:

(182.5, 736.50006)
(53.0, 607.00006)
(618.99994, 41.000122)
(748.49994, 170.50012)

The code above wasn't buiding with Python3. Just in case if this is useful, see the code below with minor corrections.

import cv2 as cv
import numpy as np

im = cv.imread("plank.jpg")
gray = cv.cvtColor(im, cv.COLOR_BGR2GRAY)
gray = cv.GaussianBlur(gray, (5, 5), 0)
_, bin = cv.threshold(gray,120,255,1) # inverted threshold (light obj on dark bg)
bin = cv.dilate(bin, None)  # fill some holes
bin = cv.dilate(bin, None)
bin = cv.erode(bin, None)   # dilate made our shape larger, revert that
bin = cv.erode(bin, None)
contours, hierarchy = cv.findContours(bin, cv.RETR_LIST, cv.CHAIN_APPROX_SIMPLE)

rc = cv.minAreaRect(contours[0])
box = cv.boxPoints(rc)
for p in box:
    pt = (p[0],p[1])
    print(pt)
    cv.circle(im,pt,5,(200,0,0),2)
cv.imshow("plank", im)
cv.waitKey()