How to find approximately triangles in image by using opencv

Question

I have this image:

And I want to find isosceles right triangles like this:

How can I do it? Thanks for your help.

Answer 1

You could try to blur your image, convert in to gray scale and apply differnet threshold to your image. Next step is to find and arrange your contours and maybe filter them out with limiting sizes.

import cv2
import numpy as np

img = cv2.imread('triangle.png')
blur = cv2.GaussianBlur(img,(5,5),0)
values = [30, 40, 50, 60, 70, 80, 90]
gray_image = cv2.cvtColor(blur, cv2.COLOR_BGR2GRAY)
for i in values:
    ret, threshold = cv2.threshold(gray_image,i,255,cv2.THRESH_BINARY)
    im, contours, hierarchy = cv2.findContours(threshold,cv2.RETR_TREE,cv2.CHAIN_APPROX_NONE)
    area = sorted(contours, key=cv2.contourArea, reverse=True)
    for j in range(1, len(area)):
        contour = area[j]
        size = cv2.contourArea(contour)
        if 10 < float(size) < 140000:
            cv2.drawContours(img, [contour], -1, (0,255,0), 2)
cv2.imshow('img', img)

Result:

Note:

Transformation to different color space and applying different threshold could improve the resut.