Helper function to resize and rotate contours using OpenCV

rotate_contour.py

import cv2
import numpy as np


def cart2pol(x, y):
    theta = np.arctan2(y, x)
    rho = np.hypot(x, y)
    return theta, rho


def pol2cart(theta, rho):
    x = rho * np.cos(theta)
    y = rho * np.sin(theta)
    return x, y


def rotate_contour(cnt, angle):
    M = cv2.moments(cnt)
    cx = int(M['m10']/M['m00'])
    cy = int(M['m01']/M['m00'])

    cnt_norm = cnt - [cx, cy]
    
    coordinates = cnt_norm[:, 0, :]
    xs, ys = coordinates[:, 0], coordinates[:, 1]
    thetas, rhos = cart2pol(xs, ys)
    
    thetas = np.rad2deg(thetas)
    thetas = (thetas + angle) % 360
    thetas = np.deg2rad(thetas)
    
    xs, ys = pol2cart(thetas, rhos)
    
    cnt_norm[:, 0, 0] = xs
    cnt_norm[:, 0, 1] = ys

    cnt_rotated = cnt_norm + [cx, cy]
    cnt_rotated = cnt_rotated.astype(np.int32)

    return cnt_rotated

scale_contour.py

import cv2
import numpy as np


def scale_contour(cnt, scale):
    M = cv2.moments(cnt)
    cx = int(M['m10']/M['m00'])
    cy = int(M['m01']/M['m00'])

    cnt_norm = cnt - [cx, cy]
    cnt_scaled = cnt_norm * scale
    cnt_scaled = cnt_scaled + [cx, cy]
    cnt_scaled = cnt_scaled.astype(np.int32)

    return cnt_scaled

Thank you for all good guide for rotating countours with openCV. Want to ask you about subtracting ( normalizing ) countour pixel coordinates. I did not get why exactly you did that, can you explain ?