如何在不损坏文本的情况下去除图像中的背景噪声？

Question

I am processing images for tesseract's ocr. I need help to get rid of the background noise without damaging the text.

Example input image

I have tried median blurring and removing small connected components ( How do I remove the dots / noise without damaging the text? ). The problem with connected components is that the noise can have larger connections and I cannot get rid of it without also removing the minus sign. Any suggestion how to move forward?

Answer 1

Since your image is only black/white, you can do simple thresholding and morphological transformations to filter the image. If your image input was not black and white, you could do blurring techniques such as cv2.medianBlur() or cv2.GaussianBlur() to smooth the image as a preprocessing step. Then you could perform morphological operations with various kernel sizes or construct custom kernels with cv2.getStructuringElement() . Generally, a larger kernel size ( 7x7 or 9x9 ) will remove more noise but also remove the desired details as opposed to a smaller kernel ( 3x3 or 5x5 ). There is a trade off depending on how much noise you want to remove while balancing the amount of details to preserve. Take a look at this answer for colored captchas.

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Threshold

Morph close

Invert image for Tesseract

Result

-63 164

import cv2
import pytesseract

pytesseract.pytesseract.tesseract_cmd = r"C:\Program Files\Tesseract-OCR\tesseract.exe"

image = cv2.imread('1.png',0)
thresh = cv2.threshold(image, 150, 255, cv2.THRESH_BINARY_INV)[1]

kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (5,5))
opening = cv2.morphologyEx(thresh, cv2.MORPH_OPEN, kernel)

result = 255 - opening
cv2.imshow('thresh', thresh)
cv2.imshow('opening', opening)
cv2.imshow('result', result)

print(pytesseract.image_to_string(result))
cv2.waitKey()