Get 1 contour per sign through find_contour and retrieve its Humoments in cv2


It is possible obtain only 5 objects (one per sign) by applying find_contour (opencv module) in this image: ? Now I obtain 64 objects

After that I want to retrieve Humoments and make a comparison with other images. For now i'd try only with the same image a little bit translated, for testing it returns they are the same.

My question I how can I obtain only 5 objects for applying humoments or if there are other solutions to calculate humoments fot the image?

import cv2
im = cv2.imread('Sassatelli 1984 n. 165 mod1.jpg')
imgray = cv2.cvtColor(im,cv2.COLOR_BGR2GRAY)
blur = cv2.GaussianBlur(imgray, (0,0), 5) 
cv2.imshow('Blur', blur)
th = 20
edges = cv2.Canny(blur, th, th*3)
contours, hierarchy = cv2.findContours(edges, cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
print('objects found')
cnt = contours[0]
cv2.imshow('draw contours',blur)
moments = cv2.moments(cnt)
By : postgres

Case 1: Problem with saving image in jpg format When you save a black-and-white-only (ie pixel values 0 and 255 only) image in jpg format, there is lossy compression, which changes the pixel values. If you want to see it, create such an image, save it in jpg, open the saved image and zoom to black-white edge. You can see a pixel value change.

So when you find contours, you expect there is only white objects, but in reality, there is some mid-values also, which is also considered as contours. It increases number of contours.

So to avoid this problem,

  • Better save images in png or any other lossless format etc.
  • Apply a threshold, (with a values of 127 or as you like) to make image real binary one before finding contours.

This is much more explained here : What does result of 'list(contour)' denote?

Case 2: Problem with white background

OpenCV findcontours() is designed to find white objects in black background. So if your background is white, it is also treated as one object. So invert the image before finding contours.

Case 3 : Problem with holes in objects

If you have holes in your object, it is also considered as an object. So if you want only external boundary of the objects, use cv2.RETR_EXTERNAL flag for findcontours() function.

Sample Code:

import cv2
import numpy as np
img = cv2.imread('sof.jpg')
gray = cv2.imread('sof.jpg',0)
ret,thresh = cv2.threshold(gray,127,255,cv2.THRESH_BINARY_INV)

thresholded and inverted image :

enter image description here

Now find the contours, draw it, check the number of contours:


Result :

enter image description here


Here, I have taken only external contours. If you want to remove internal holes from these objects, you will need to use cv2.RETR_TREE or cv2.RETR_CCOMP flags, and check their hierarchy, and remove them. It is explained in this link : Contours 5 : Hierarchy

This video can help you solving your question :)
By: admin