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How to perform image rotation in OpenCV using Python?
To perform image rotation by an angle, we first need to get the rotation matrix. To find the rotation matrix, we apply cv2.getRotationMatrix2D() function. The syntax for this function is −
M = cv2.getRotationMatrix2D(cr, degree, scale)
Where cr is the center of rotation, degree is the angle by which image is rotated and scale is scale factor to scale up or scale down the image.
The rotation matrix M is a 2×2 matrix (numpy array). We pass the rotation matrix M to cv2.warpAffine() function as an argument. See the syntax below −
Syntax
cv2.warpAffine(img, M, (width, height))
Here,
img − The input image which is to be rotated.
M − Rotation matrix defined above.
(width, height) − Width and height of the image after rotation.
Steps
To perform an image rotation, you can follow the steps given below −
Import the required library. In all the following Python examples, the required Python library is OpenCV. Make sure you have already installed it.
import cv2
Read the input image using cv2.imread() function. Pass the full path of the input image.
img = cv2.imread('interior1.jpg')
Define a rotation matrix M using cv2.getRotationMatrix2D(cr, degree, scale) function. Pass the center of rotation cr, rotation angle degree, and scale factor scale to the function, for example, cr=(width/2, height/2), degree=30, scale=1.
M = cv2.getRotationMatrix2D(cr,30,1)
Rotate the image using cv2.warpAffine() method.
img = cv2.warpAffine(img,M,(w,h))
Display the rotated image.
cv2.imshow('Image Translation', img)
cv2.waitKey(0)
cv2.destroyAllWindows()
Let's look at some examples for a clear understanding about the question.
Input Image
We use the following image as the input file in the examples below.
Example 1
In this program, we rotate the input image by an angle of 30 degrees in anticlockwise direction. The center of rotation is the midpoint (center) of the image, i.e., (width/2, height/2).
# import required libraries import cv2 # read the input image img = cv2.imread('interior1.jpg') # access height and width of the image height, width, _ = img.shape # define center of rotation cr = (width/2,height/2) # get the rotation matrix M = cv2.getRotationMatrix2D(cr,30,1) # apply warpAffine() method to perform image rotation dst = cv2.warpAffine(img,M,(width,height)) # display the rotated image cv2.imshow('Image',dst) cv2.waitKey(0) cv2.destroyAllWindows()
Output
On execution, it will produce the following output window −
Notice that the mean squared error is a scalar value.
Example 2
In this program, we rotate the input image by an angle of 15 degrees in clockwise direction. The center of rotation is (0,0).
import cv2 import numpy as np img = cv2.imread('interior1.jpg') h,w, _ = img.shape M = cv2.getRotationMatrix2D((0,0),-15,1) dst = cv2.warpAffine(img,M,(w,h)) cv2.imshow('Image',dst) cv2.waitKey(0) cv2.destroyAllWindows()
Output
When you run the above Python program, it will produce the following output window −
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