OpenCV Python : rotate image without cropping sides

image-processing, opencv, python, rotation

Solution

This is by far the best solution i have found for rotating images while avoiding cropping the image.

Rotate an image without cropping in OpenCV in C++

import cv2

def rotate_image(mat, angle):
    """
    Rotates an image (angle in degrees) and expands image to avoid cropping
    """
    
    height, width = mat.shape[:2] # image shape has 3 dimensions
    image_center = (width/2, height/2) # getRotationMatrix2D needs coordinates in reverse order (width, height) compared to shape
    
    rotation_mat = cv2.getRotationMatrix2D(image_center, angle, 1.)
    
    # rotation calculates the cos and sin, taking absolutes of those.
    abs_cos = abs(rotation_mat[0,0]) 
    abs_sin = abs(rotation_mat[0,1])
    
    # find the new width and height bounds
    bound_w = int(height * abs_sin + width * abs_cos)
    bound_h = int(height * abs_cos + width * abs_sin)
    
    # subtract old image center (bringing image back to origo) and adding the new image center coordinates
    rotation_mat[0, 2] += bound_w/2 - image_center[0]
    rotation_mat[1, 2] += bound_h/2 - image_center[1]
    
    # rotate image with the new bounds and translated rotation matrix
    rotated_mat = cv2.warpAffine(mat, rotation_mat, (bound_w, bound_h))
    return rotated_mat

You can add checks to avoid some calculations when the angles are 90*n, but this function will work for any angle as is.

Taking advantage of `numpy` vectorization

def rotate_image(img, angle):
    size_reverse = np.array(img.shape[1::-1]) # swap x with y
    M = cv2.getRotationMatrix2D(tuple(size_reverse / 2.), angle, 1.)
    MM = np.absolute(M[:,:2])
    size_new = MM @ size_reverse
    M[:,-1] += (size_new - size_reverse) / 2.
    return cv2.warpAffine(img, M, tuple(size_new.astype(int)))

Problem

Original source

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• Rotate an image without cropping in OpenCV in C++