Detect Polygons in Image using OpenCV Python

OpenCV Python Server Side Programming Programming

We first detect all the object contours in the image to detect a polygon. Then Loop over all contours. Find the approximate contour for each of the contours. If the number of vertex points in the approximate contour is 5 or more then draw the contour and set it as a triangle. See the below pseudocode.

for cnt in contours:
   approx = cv2.approxPolyDP()
   if len(approx) >= 5:
      cv2.drawContours()
      cv2.putText("Polygon")

Steps

We could use the following steps to detect polygons in an image ?

Import the required library. In all the following examples, the required Python library is OpenCV. Make sure you have already installed it.
Read the input image using cv2.imread() and convert it to grayscale.
Apply thresholding cv2.threshold() on the grayscale image to create a binary image. Adjust the second parameter to get a better contour detection.
Find the contours in the image using cv2.findContours() function.
Select a contour (say first contour) cnt from the lists of contours. Or Loop over all the detected contours.
Compute the approximate contour points (approx) for each contour cnt using cv2.approxPolyDP() function.
If the total number of vertex points in the approximate contour approx is 5 or more, then draw the approximate contours on the image and set it a polygon.
Display the image with drawn contours and approximate contours on it.

Let's have a look at the example below for a better understanding.

Example

In this Python program, we detect the polygons in the input image. We also draw the contours of the detected polygons.


# import required libraries
import cv2

# read the input image
img = cv2.imread('polygons.png')

# convert the image to grayscale
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

# apply thresholding to convert the grayscale image to a binary image
ret,thresh = cv2.threshold(gray,50,255,0)

# find the contours
contours,hierarchy = cv2.findContours(thresh, cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
print("Number of contours detected:",len(contours))
for cnt in contours:
   approx = cv2.approxPolyDP(cnt, 0.01*cv2.arcLength(cnt, True), True)
   (x,y)=cnt[0,0]

   if len(approx) >= 5:
      img = cv2.drawContours(img, [approx], -1, (0,255,255), 3)
      cv2.putText(img, 'Polygon', (x, y),
cv2.FONT_HERSHEY_SIMPLEX, 0.6, (255, 255, 0), 2)
cv2.imshow("Polygon", img)
cv2.waitKey(0)
cv2.destroyAllWindows()