Python & OpenCV: How to crop half-formed bounding boxes

Question

I have a script that creates gridlines for a gridless table:

Before script:

After script:

Is there a simple way, using OpenCV, to crop the "after script" image so that it includes only the four-sided bounding boxes? Sample output:

EDIT:

I'm currently working on a solution that finds the first/last all-black pixel line going vertically/horizontally. It'll work, but was wondering whether there's something more elegant.

Answer 1

Here is one way to do that in Python/OpenCV by getting the min and max x and y from all the contours except the largest.

Input:

import cv2
import numpy as np

# read image
img = cv2.imread('test_table.png')
hh, ww = img.shape[:2]

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

# threshold
thresh = cv2.threshold(gray, 128, 255, cv2.THRESH_BINARY)[1]

# crop 1 pixel and add 1 pixel white border to ensure outer white regions not considered small contours
thresh = thresh[1:hh-1, 1:ww-1]
thresh = cv2.copyMakeBorder(thresh, 1,1,1,1, borderType=cv2.BORDER_CONSTANT, value=(255,255,255))

# get contours
contours = cv2.findContours(thresh, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
contours = contours[0] if len(contours) == 2 else contours[1]
big_contour = max(contours, key=cv2.contourArea)

# get min and max x and y from all bounding boxes larger than half the image size
area_thresh = hh * ww / 2
xmin = ww
ymin = hh
xmax = 0
ymax = 0

for cntr in contours:
    area = cv2.contourArea(cntr)
    if area < area_thresh:
        x,y,w,h = cv2.boundingRect(cntr)
        xmin = x if (x < xmin) else xmin
        ymin = y if (y < ymin) else ymin
        xmax = x+w-1 if (x+w-1 > xmax ) else xmax
        ymax = y+h-1 if (y+h-1 > ymax) else ymax


# draw bounding box     
bbox = img.copy()
cv2.rectangle(bbox, (xmin, ymin), (xmax, ymax), (0, 0, 255), 2)

# crop img at bounding box, but add 2 all around to keep the black lines
result = img[ymin-3:ymax+3, xmin-3:xmax+3]

# save results
cv2.imwrite('test_table_bbox.png',bbox)
cv2.imwrite('test_table_trimmed.png',result)

# show results
cv2.imshow("thresh", thresh)
cv2.imshow("bbox", bbox)
cv2.imshow("result", result)
cv2.waitKey(0)
cv2.destroyAllWindows()

Bounding Box of all the bounding boxes on input:

Trimmed Image: