CODEWITHSUNDEEP
pythonopencv
user3806406
user3806406

Reputation: 1

Shape recognition using Hu moments from OpenCV in Python

I have a problem using the Hu moments for shape recognition. The goal is to be able to recognize the two white circles and the two white squares on the left in the picture. https://i.sstatic.net/wVzYa.jpg

I tried using the cv2.approxPolyDP method but it doesn't quite work when there is a rotation. For the white circles I used the cv2.HoughCircles method and it works pretty well. However, I really need to use the Hu moments, because it seems it is a better method. I have this code below:

import cv2
import numpy as np

nomeimg = "coded_target.jpg"
img = cv2.imread(nomeimg)

gray = cv2.imread(nomeimg,0)
ret,thresh = cv2.threshold(gray,127,255,cv2.THRESH_BINARY_INV) 
element = cv2.getStructuringElement(cv2.MORPH_CROSS,(4,4))
imgbnbin = thresh
imgbnbin = cv2.dilate(imgbnbin, element)

#find contour
contours,hierarchy=cv2.findContours(imgbnbin,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

#Elimination small contours
Areacontours = list()
for i in Areacontours:
    area = cv2.contourArea(contours[i])
    if (area > 90 ):
        Areacontours.append(contours[i])
    contours = Areacontours

print('found objects')
print(len(contours))

print("humoments")
mom = cv2.moments(contours[0])
Humoments = cv2.HuMoments(mom)
Humoments2 = -np.sign(Humoments)*np.log10(np.abs(Humoments))
print(Humoments2)

It returns 7 numbers which are the Hu invariants. I tried rotating the picture and I see that only the last two are changing. It also says that it only found 1 object found when there are obviously more than that. Is it normal? I thought of using templates for shape identification purposes but I don't know how to do it: I believe I should exploit the Hu moments of the templates and see where it fits but I'm not sure on how to achieve it. I appreciate the help.

Upvotes: 0

Views: 8568

Answers (1)

GStav
GStav

Reputation: 1166

You can create a template image of the squares and implement a template matching technique in order to detect it on the image.

You can also detect the contour of the template image and use the function cv2.matchshapes . However this function is used in order to compare two images. So, I guess you will have to make a window with the same size with you template and run it through you original image in order to detect which part is the best match (minimum value for the function matchshape).

Upvotes: 2

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