OpenCV - Adjusting photo with skew angle (tilt)

Question

I have a camera pointing at a Zen Garden from above. However, the camera is fixed on the side rather than directly above the plate. As a result, the image looks like this (note the skewed shape of the rectangle):

Is there a way to process the image so that the sand area can look more or less like a perfect square?

cap = cv2.VideoCapture(0)
while True:
    ret, img = cap.read()
    img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY) 
    img = cv2.flip(img,0)
    cv2.imshow('Cropping', img)
    if cv2.waitKey(500) & 0xff == 27:
        cv2.destroyAllWindows()
        break

Many thanks.

Answer 1

You can do perspective transformation on your image. Here's a first shot that gets you in the ballpark:

import cv2
import numpy as np

img = cv2.imread('zen.jpg')
rows, cols, ch = img.shape    
pts1 = np.float32(
    [[cols*.25, rows*.95],
     [cols*.90, rows*.95],
     [cols*.10, 0],
     [cols,     0]]
)
pts2 = np.float32(
    [[cols*0.1, rows],
     [cols,     rows],
     [0,        0],
     [cols,     0]]
)    
M = cv2.getPerspectiveTransform(pts1,pts2)
dst = cv2.warpPerspective(img, M, (cols, rows))
cv2.imshow('My Zen Garden', dst)
cv2.imwrite('zen.jpg', dst)
cv2.waitKey()

You can fiddle more with the numbers, but you get the idea.

Here's some online examples. The first link has a useful plot of peg points that correlate to the transformation matrix:

• Geometric Transformations of Images
• 4 Point OpenCV getPerspective Transform Example
• Learn OpenCV by Examples: Perspective Transform