Tutorial: How to Scale and Rotate Contours in OpenCV using Python
By: Nvs Abhilash
Tags: Opencv, Opencv Python, Python

Learning by example

Throughout this short tutorial, I would be using a simple image to demonstrate my points, below is some code for imports and loading the image, and generating the contours. You know, that standard stuff!

import cv2

import numpy as np



import matplotlib.pyplot as plt



# Reading the image

im = cv2.imread('sample_img.png')



# Converting image to grayscale

imgray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)



# Thresholding and getting contours from the image

ret, thresh = cv2.threshold(imgray, 127, 255, 0)

contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE) im_copy = im.copy() cv2.drawContours(im_copy, contours, 0, (255, 0, 0), 3) plt.imshow(im_copy)

plt.axis("off");

Sample image with detected contour

How to resize contours

Well, the idea is pretty simple, and if you have some understanding of high-school math, below steps would seem very sensible:

Step 1: Translate the contour to the origin

Step 2: Scale each point of the contour

Step 3: Translate back the contour to it’s original place.

Below I explain and show code equivalents of each step.

Translate the contour to the origin

To translate the contour to the origin, we just have to subtract the coordinates of the centroid of the contour with all the points. And the centroid of the contour can be found using the cv2.moments function.

Getting the centroid of the contour

M = cv2.moments(cnt)

cx = int(M['m10']/M['m00'])

cy = int(M['m01']/M['m00'])

Translating the contour by subtracting the center with all the points

cnt_norm = cnt - [cx, cy]

Scale each point of the contour

To scale each point of the contour, we just have to multiply the scale with the contour points.

cnt_scaled = cnt_norm * scale

Translate back the contour to it’s original place.

This step can be achieved by simply adding back the centroid co-ordinates to the contour points and that’s it!

cnt_scaled = cnt_scaled + [cx, cy]

cnt_scaled = cnt_scaled.astype(np.int32)

Combining all the steps into a function for scaling

Hence we can write a simple function based on the above steps as below:

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

And we can call this function to test that it works fine!

RED BOUNDRY : Original contour

: Original contour GREEN BOUNDRY: Scaled contour

cnt_scaled = scale_contour(contours[0], 0.3)



im_copy = im.copy()

cv2.drawContours(im_copy, contours, 0, (255, 0, 0), 3)

cv2.drawContours(im_copy, [cnt_scaled], 0, (0, 255, 0), 3)



plt.imshow(im_copy)

plt.axis("off");

Original and scaled contour

How to rotate contours

Rotating contours are also simple, and again would just take some high school math to understand the steps.

Step 1: Translate the contour to the origin

Step 2: Rotate each point of the contour

Step 3: Translate back the contour to it’s original place.

Translate the contour to the origin

Getting the center

M = cv2.moments(cnt)

cx = int(M[‘m10’]/M[‘m00’])

cy = int(M[‘m01’]/M[‘m00’])

Translating the contour by subtracting the center with all the points

cnt_norm = cnt — [cx, cy]

Rotating each point of the contour

Once you have put the contour to the origin, we have to rotate each point. And for me understanding rotation is much easier in polar co-ordinates as compared to Cartesian co-ordinates (I hope for you too 😉). So, that’s what we’ll do! Convert the points to polar co-ordinates, add the rotation, and convert it back to Cartesian co-ordinates.

These are some helper functions which I borrowed from here to convert back and forth between Polar and Cartesian co-ordinates

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

Converting the x-y (cartesian) co-ordinates to theta-rho (polar)

coordinates = cnt_norm[:, 0, :]

xs, ys = coordinates[:, 0], coordinates[:, 1]

thetas, rhos = cart2pol(xs, ys)

Adding the angle to the thetas .

thetas_deg = np.rad2deg(thetas)

thetas_new_deg = (thetas_deg + angle) % 360

thetas_new = np.deg2rad(thetas_new_deg)

Note: I converted the radians to degrees because I chose to give angle in degrees form (and it makes modulo operation much cleaner). Then I again convert it back to radians.

Convert the new polar coordinates to cartesian co-ordinates:

xs, ys = pol2cart(thetas, rhos)

cnt_norm[:, 0, 0] = xs

cnt_norm[:, 0, 1] = ys

Translate back the contour to it’s original place.

Add center coordinates to the contours’ coordinates

cnt_rotated = cnt_norm + [cx, cy]

cnt_rotated = cnt_rotated.astype(np.int32)

Combining all the steps into a function for rotation

Hence we can write a simple function based on the above steps as below:

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

And we can call this function to test that it works fine!

RED BOUNDRY : Original contour

: Original contour GREEN BOUNDRY: Scaled contour

cnt_rotated = rotate_contour(contours[0], 60) im_copy = im.copy()

cv2.drawContours(im_copy, contours, 0, (255, 0, 0), 3)

cv2.drawContours(im_copy, [cnt_rotated], 0, (0, 255, 0), 3) plt.imshow(im_copy)

plt.axis("off");

Original (red) and rotated (green) contour

Having fun with random rotation, scale, and translation

Here is a simple code to generated random rotation, scaling and translation (not covered in the tutorial but it’s just a simple addition of co-ordinates).

im_copy = im.copy()

cv2.drawContours(im_copy, contours, 0, (255, 0, 0), 3)



for i in range(20):

cnt_rotated = rotate_contour(contours[0], np.random.random() * 90)

cnt_scaled = scale_contour(cnt_rotated, np.random.random() * 0.5)



cnt_translated = cnt_scaled + np.random.randint(low=-200, high=200, size=(2,))



r, g, b = [int(np.random.choice(range(255))) for _ in range(3)]

cv2.drawContours(im_copy, [cnt_translated], 0, (r, g, b), -1)



plt.imshow(im_copy)

plt.axis("off");

Random rotation + scaling + translation

In-case someone wants to dissect this code, these are the key things I did: