OpenCV: How to detect nested geometrical shapes?

Question

Apologies, I'm new to OpenCV. How to detect nested geometrical shapes in OpenCV? I got this answer about outer shapes, but I need something like a triangle in a square kind of thing. Also, is there a way to make it work with rounded corners? Example:

Answer 1

Try this code for finding contours

import cv2
img = cv2.imread('shapes.png', 0)
thresh = cv2.threshold(img, 60, 255, cv2.THRESH_BINARY_INV)[1]
cnts, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

In the code above the image is read in gray format. Then while thresholding the format used is binary_INV because we want the background as black and the foreground as white before finding contours. Your displayed test image has the opposite. Now while finding contours you will need to use RETR_TREE and not RETR_EXTERNAL because the latter only finds the external contours where as the former will find all contours. Now you use any of the links provided for finding sides.