Perspective Transform OMR Sheet - OpenCV Python

Question

I am trying to make a perspective transformation with OpenCV in python. I want to align the image and find the coordinates of top left top right and bottom left and bottom right contours. here is my code so far where I can identify all the contours.

gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
blur = cv2.GaussianBlur(gray,(3,3),0)
edges = cv2.Canny(blur,50,100)
contours, hierarchy = cv2.findContours(edges,cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE) #find contours
cv2.drawContours(img,contours,-1,(0,255,0),2)
cv2.imshow('Contours',img)
cv2.waitKey(0)

Here is an image of what I get.

I'd be very thankful if you could try to help me solve this problem!

Original Image
Demo Inputs:
Demo Inputs Demo Input 1:
Demo Input 1
Demo Input 2:
Demo Input 2
Demo Input 3:
Demo Input 3
Demo Input 4:
Demo Input 4
Desired Output:
Desired Output

Answer 1

Here is one way to do that in Python/OpenCV.

• Read the input
• Read the template (for its dimension)
• Convert the input to gray and threshold
• Pad the threshold (to preserve the corners when apply morphology)
• Apply morphology close
• Remove the padding
• Get the largest external contour
• Get its perimeter and approximate to 4 corners as the input corners for the warping
• Get the output corners for the warping from the dimensions of the template
• Get the perspective transformation matrix
• Warp the input to match the template
• Save the results

Input:

Template:

import cv2
import numpy as np

# read image
img = cv2.imread("omr_test.jpg")
hh, ww = img.shape[:2]

# read template
template = cv2.imread("omr_template.jpg")
ht, wd = template.shape[:2]

# convert img to grayscale
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

# do otsu threshold on gray image
thresh = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY+cv2.THRESH_OTSU)[1]

# pad thresh with black to preserve corners when apply morphology
pad = cv2.copyMakeBorder(thresh, 20, 20, 20, 20, borderType=cv2.BORDER_CONSTANT, value=0)

# apply morphology
kernel = cv2.getStructuringElement(cv2.MORPH_ELLIPSE, (15,15))
morph = cv2.morphologyEx(pad, cv2.MORPH_CLOSE, kernel)

# remove padding
morph = morph[20:hh+20, 20:ww+20]

# get largest external contour
contours = cv2.findContours(morph, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours = contours[0] if len(contours) == 2 else contours[1]
big_contour = max(contours, key=cv2.contourArea)

# get perimeter and approximate a polygon
peri = cv2.arcLength(big_contour, True)
corners = cv2.approxPolyDP(big_contour, 0.04 * peri, True)

# draw polygon on input image from detected corners
polygon = img.copy()
cv2.polylines(polygon, [corners], True, (0,255,0), 2, cv2.LINE_AA)
# Alternate: cv2.drawContours(page,[corners],0,(0,0,255),1)

# print the number of found corners and the corner coordinates
# They seem to be listed counter-clockwise from the top most corner
print(len(corners))
print(corners)

# reformat input corners to x,y list
icorners = []
for corner in corners:
    pt = [ corner[0][0],corner[0][1] ]
    icorners.append(pt)
icorners = np.float32(icorners)

# get corresponding output corners form width and height
ocorners = [ [0,0], [0,ht], [wd,ht], [wd,0] ]
ocorners = np.float32(ocorners)

# get perspective tranformation matrix
M = cv2.getPerspectiveTransform(icorners, ocorners)

# do perspective 
warped = cv2.warpPerspective(img, M, (wd, ht))

# write results
cv2.imwrite("omr_test_thresh.jpg", thresh)
cv2.imwrite("omr_test_morph.jpg", morph)
cv2.imwrite("omr_test_polygon.jpg", polygon)
cv2.imwrite("omr_test_warped.jpg", warped)

# display it
cv2.imshow("thresh", thresh)
cv2.imshow("pad", pad)
cv2.imshow("morph", morph)
cv2.imshow("polygon", polygon)
cv2.imshow("warped", warped)
cv2.waitKey(0)

Threshold Image:

Morphology Image:

Polygon Image:

Warped Input: