Robustly crop rotated bounding box on photos

Question

I'm trying to extract the rotated bounding box of contours robustly. I would like to take an image, find the largest contour, get its rotated bounding box, rotate the image to make the bounding box vertical, and crop to size.

For a demonstration, here is an original image linked in the following code. I would like to end up with that shoe rotated to vertical and cropped to size. The following code from this answer seems to work on simple images like opencv lines, etc., but not on photos.

Ends up with this, which is rotated and cropped wrong:

EDIT: After changing the threshold type to cv2.THRESH_BINARY_INV, it now is rotated correctly but cropped wrong:

import cv2
import matplotlib.pyplot as plt
import numpy as np
import urllib.request
plot = lambda x: plt.imshow(x, cmap='gray').figure


url = 'https://i.imgur.com/4E8ILuI.jpg'
img_path = 'shoe.jpg'

urllib.request.urlretrieve(url, img_path)
img = cv2.imread(img_path, 0)
plot(img)


threshold_value, thresholded_img = cv2.threshold(
    img, 250, 255, cv2.THRESH_BINARY)
_, contours, _ = cv2.findContours(thresholded_img, 1, 1)
contours.sort(key=cv2.contourArea, reverse=True)

shoe_contour = contours[0][:, 0, :]
min_area_rect = cv2.minAreaRect(shoe_contour)

def crop_minAreaRect(img, rect):

    # rotate img
    angle = rect[2]
    rows, cols = img.shape[0], img.shape[1]
    M = cv2.getRotationMatrix2D((cols / 2, rows / 2), angle, 1)
    img_rot = cv2.warpAffine(img, M, (cols, rows))

    # rotate bounding box
    rect0 = (rect[0], rect[1], 0.0)
    box = cv2.boxPoints(rect)
    pts = np.int0(cv2.transform(np.array([box]), M))[0]
    pts[pts < 0] = 0

    # crop
    img_crop = img_rot[pts[1][1]:pts[0][1],
                       pts[1][0]:pts[2][0]]

    return img_crop


cropped = crop_minAreaRect(thresholded_img, min_area_rect)
plot(cropped)

How can I get the correct cropping?

---

Answer 1

After some research, this is what I get:

This is how I get it:

• pad the original image on each side (500 pixels in my case)
• find the four corner points of the shoe (the four points should form a polygon enclosing the shoe, but do not need to be exact rectangle)
• employing the code here to crop the shoe:

img = cv2.imread("padded_shoe.jpg")
# four corner points for padded shoe
cnt = np.array([
    [[313, 794]],
    [[727, 384]],
    [[1604, 1022]],
    [[1304, 1444]]
])
print("shape of cnt: {}".format(cnt.shape))
rect = cv2.minAreaRect(cnt)
print("rect: {}".format(rect))

box = cv2.boxPoints(rect)
box = np.int0(box)
width = int(rect[1][0])
height = int(rect[1][1])

src_pts = box.astype("float32")
dst_pts = np.array([[0, height-1],
                    [0, 0],
                    [width-1, 0],
                    [width-1, height-1]], dtype="float32")
M = cv2.getPerspectiveTransform(src_pts, dst_pts)
warped = cv2.warpPerspective(img, M, (width, height))

Cheers, hope it helps.