maxwoe/image-rotation-angle-estimation

1"""Rotation utilities that preserve image content without artificial borders."""
2
3import cv2
4import numpy as np
5import math
6
7
8def rotate_image(img, angle, rotation_center=None, expand=False, border_mode=cv2.BORDER_CONSTANT, border_value=0):
9    """Rotates an image (angle in degrees) and optionally expands to avoid cropping."""
10    h, w = img.shape[:2]
11    if rotation_center is None:
12        rotation_center = (w/2, h/2)
13
14    M = cv2.getRotationMatrix2D(rotation_center, angle, 1.0)
15
16    if expand:
17        abs_cos = abs(M[0, 0])
18        abs_sin = abs(M[0, 1])
19        wn = int(h * abs_sin + w * abs_cos)
20        hn = int(h * abs_cos + w * abs_sin)
21        M[0, 2] += wn/2 - rotation_center[0]
22        M[1, 2] += hn/2 - rotation_center[1]
23    else:
24        wn, hn = w, h
25
26    rotated = cv2.warpAffine(
27        img, M, (wn, hn), borderMode=border_mode, borderValue=border_value)
28
29    return rotated, M
30
31
32def largest_rotated_rect(w, h, angle):
33    """Compute the largest axis-aligned rectangle within a rotated rectangle."""
34    if w <= 0 or h <= 0:
35        return 0, 0
36
37    width_is_longer = w >= h
38    side_long, side_short = (w, h) if width_is_longer else (h, w)
39
40    sin_a, cos_a = abs(math.sin(angle)), abs(math.cos(angle))
41    if side_short <= 2.*sin_a*cos_a*side_long or abs(sin_a-cos_a) < 1e-10:
42        x = 0.5*side_short
43        wr, hr = (x/sin_a, x/cos_a) if width_is_longer else (x/cos_a, x/sin_a)
44    else:
45        cos_2a = cos_a*cos_a - sin_a*sin_a
46        wr, hr = (w*cos_a - h*sin_a)/cos_2a, (h*cos_a - w*sin_a)/cos_2a
47
48    return wr, hr
49
50
51def rotate_image_crop_max_area(image, angle):
52    """Rotate image and crop to the largest inscribed rectangle (no borders).
53
54    Args:
55        image: numpy array (OpenCV image)
56        angle: Rotation angle in degrees
57
58    Returns:
59        Rotated and cropped numpy array
60    """
61    h, w = image.shape[:2]
62    rotated, _ = rotate_image(image, angle, expand=True)
63    wr, hr = largest_rotated_rect(w, h, math.radians(angle))
64
65    h_rot, w_rot = rotated.shape[:2]
66    y1 = h_rot//2 - int(hr/2)
67    y2 = y1 + int(hr)
68    x1 = w_rot//2 - int(wr/2)
69    x2 = x1 + int(wr)
70
71    return rotated[y1:y2, x1:x2]
72