cv2.approxPolyDP() , cv2.arcLength() How these works

Question

How do these function works? I am using Python3.7 and OpenCv 4.2.0. Thanks in Advance.

approx = cv2.approxPolyDP(cnt, 0.01*cv2.arcLength(cnt, True), True)

Trenna

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Thanks sir. @MihailDuchev If you don't mind please recommend some source of learning. – Trenna

Answer 1

If you are looking for a example snippet, below is one:

import cv2
import imutils

# edged is the edge detected image
cnts = cv2.findContours(edged, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
cnts = imutils.grab_contours(cnts)
cnts = sorted(cnts, key = cv2.contourArea, reverse = True)[:5]
# loop over the contours
for c in cnts:
    # approximate the contour
    peri = cv2.arcLength(c, True)
    approx = cv2.approxPolyDP(c, 0.02 * peri, True)
    # if our approximated contour has four points, then we
    # can assume that we have found our screen
    if len(approx) == 4:
        screenCnt = approx
        break

In the above snippet, first it finds the contours from a edge detected image, then it sorts the contours to find the five largest contours. Finally it loop over the contours and used cv2.approxPolyDP function to smooth and approximate the quadrilateral. cv2.approxPolyDP works for the cases where there are sharp edges in the contours like a document boundary.

Papilloma

Thank You. and how cv2.arcLength() works. I know definition.Wanted to know why it is being used and how it works(I saw on internet but didn't get it). – Trenna

cv2.arcLength() is used to calculate the perimeter of the contour. If the second argument is True then it considers the contour to be closed. Then this perimeter is used to calculate the epsilon value for cv2.approxPolyDP() function with a precision factor for approximating a shape. You can look into the link: opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/… where it is explained. There is also a link to the wiki for Douglas-Peucker algorithm behind cv2.approxPolyDP() method. – Papilloma