Removing black background/black stray straight lines from a captcha in python

Question

I am trying read text from this image] using Python with OpenCV.

However, black background in corners if this pic is messing with the text output and is giving wrong text.

I tried to used Adaptive Gaussian Thresholding in OpenCV using code:

import cv2 as cv
import numpy as np
from matplotlib import pyplot as plt
img=cv.imread(file_path,0)

img = cv.medianBlur(img,5)
ret,th1 = cv.threshold(img,127,255,cv.THRESH_BINARY)

th2 =cv.adaptiveThreshold(img,255,cv.ADAPTIVE_THRESH_MEAN_C,
        cv.THRESH_BINARY,11,2)

**th3 = cv.adaptiveThreshold(img,255,cv.ADAPTIVE_THRESH_GAUSSIAN_C,
        cv.THRESH_BINARY,11,2)**

titles = ['Original Image', 'Global Thresholding (v = 127)',
        'Adaptive Mean Thresholding', 'Adaptive Gaussian Thresholding']

images = [img, th1, th2, th3]

for i in range(4):
    plt.subplot(2,2,i+1),plt.imshow(images[i],'gray')
    plt.title(titles[i])
    plt.xticks([]),plt.yticks([])

plt.show()

The output of this code as AGT_result

How to extract the words only?

Asked By: Optimus

||

Source

Answer 1

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

Read the input
Map the corner black to white with a threshold on black that is slightly darker than your text to mitigate the antialiased white-black transition
Use morphology close to clean up dotted border
Save the result

Input:

import cv2
import numpy as np

img = cv2.imread('text_black_corners.png')


# map the black corners to white   
img2 = img.copy()
img2[np.where((img2 <= [150,150,150]).all(axis=2))] = [255,255,255]

# apply morphology close
kernel = cv2.getStructuringElement(cv2.MORPH_RECT , (3,3))
result = cv2.morphologyEx(img2, cv2.MORPH_CLOSE, kernel)

# write result to disk
cv2.imwrite("text_black_corners_removed.png", result)

# display it
cv2.imshow("img2", img2)
cv2.imshow("result", result)
cv2.waitKey(0)

Result:

Answered By: fmw42

Answer 2

As an ad-hoc solution, we may use cv2.floodFill 4 times – one at each corner:

img = cv.imread(file_path, 0)

rows, cols = img.shape

cv.floodFill(img, None, seedPoint=(0, 0), newVal=255, loDiff=1, upDiff=1)  # Fill the top left corner.
cv.floodFill(img, None, seedPoint=(cols-1, 0), newVal=255, loDiff=1, upDiff=1)  # Fill the top right corner.
cv.floodFill(img, None, seedPoint=(0, rows-1), newVal=255, loDiff=1, upDiff=1)  # Fill the bottop left corner.
cv.floodFill(img, None, seedPoint=(cols-1, rows-1), newVal=255, loDiff=1, upDiff=1)  # Fill the bottom right corner.

Result after cv.medianBlur:

Answered By: Rotem

Removing black background/black stray straight lines from a captcha in python

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