我应该如何从 OpenCV 中的阈值图像中去除噪声？

Question

我想删除不属于图像中字母和数字的任何内容。输入图像是这样的：

我试过用canny边缘检测，但是容易受噪声影响，噪声轮廓很大。由于这个原因，形态学操作也没有成功。我试过 cv2.MORPH_CLOSE 但噪音区域变大了。

我的代码在这里，但它目前在消除噪音方面完全没用：

import imutils

input=cv2.imread("n4.jpg")
resized = imutils.resize(input, width=700)
cv2.imshow("resized",resized)

blur = cv2.GaussianBlur(resized,(7,7),0)
cv2.imshow("blur",blur)

gray = cv2.cvtColor(resized, cv2.COLOR_BGR2GRAY)
threshINV  = cv2.threshold(gray, 140, 255, cv2.THRESH_BINARY_INV)[1]
cv2.imshow("thresh",threshINV)

e = cv2.Canny(threshINV,20,50)
cv2.imshow("e",e)

kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (4,4))
close = cv2.morphologyEx(threshINV, cv2.MORPH_CLOSE, kernel)
cv2.imshow("close",close)


edged = cv2.Canny(gray, 20, 50)
dilat = cv2.dilate(edged, None, iterations=1)
cv2.imshow("test",dilat)
cv2.waitKey(0)
cv2.destroyAllWindows()

我看过这个 example and this other example，但是由于噪声的大小以及我想保留的轮廓没有可定义的形状，它们无法工作。

我也看过这个 method，但我还是认为它行不通，因为没有要平滑的整体轮廓。

Answer 1

您发布的图片很有挑战性。
我发布的解决方案对于您发布的图片来说太具体了。
我尽量保持它的通用性，但我不希望它在其他图像上效果很好。
您可以使用它来获取有关消除噪音的更多选项的想法。

解决方案主要基于找到连接的组件并移除较小的组件 - 被认为是噪声。

我使用 pytesseract OCR 检查结果对于 OCR 是否足够清晰。

代码如下（请阅读评论）：

import numpy as np
import scipy.signal
import cv2
import pytesseract

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

# Read input image
input = cv2.imread("n4.jpg")

# Convert to Grayscale.
gray = cv2.cvtColor(input, cv2.COLOR_BGR2GRAY)

# Convert to binary and invert polarity
ret, thresh = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY_INV + cv2.THRESH_OTSU)

# Find connected components (clusters)
nlabel, labels, stats, centroids = cv2.connectedComponentsWithStats(thresh, connectivity=8)


# Remove small clusters: With both width<=10 and height<=10 (clean small size noise).
for i in range(nlabel):
    if (stats[i, cv2.CC_STAT_WIDTH] <= 10) and (stats[i, cv2.CC_STAT_HEIGHT] <= 10):
        thresh[labels == i] = 0

#Use closing with very large horizontal kernel
mask = cv2.morphologyEx(thresh, cv2.MORPH_CLOSE, np.ones((1, 150)))

# Find connected components (clusters) on mask
nlabel, labels, stats, centroids = cv2.connectedComponentsWithStats(mask, connectivity=8)

# Find label with maximum area
# 
largest_label = 1 + np.argmax(stats[1:, cv2.CC_STAT_AREA])

# Set to zero all clusters that are not the largest cluster.
thresh[labels != largest_label] = 0

# Use closing with horizontal kernel of 15 (connecting components of digits)
mask = cv2.morphologyEx(thresh, cv2.MORPH_CLOSE, np.ones((1, 15)))

# Find connected components (clusters) on mask again
nlabel, labels, stats, centroids = cv2.connectedComponentsWithStats(mask, connectivity=8)

# Remove small clusters: With both width<=30 and height<=30
for i in range(nlabel):
    if (stats[i, cv2.CC_STAT_WIDTH] <= 30) and (stats[i, cv2.CC_STAT_HEIGHT] <= 30):
        thresh[labels == i] = 0

# Use closing with horizontal kernel of 15, this time on thresh
thresh = cv2.morphologyEx(thresh, cv2.MORPH_CLOSE, np.ones((1, 15)))

# Use median filter with 3x5 mask (using OpenCV medianBlur with k=5 is removes important details).
thresh = scipy.signal.medfilt(thresh, (3,5))

# Inverse polarity
thresh = 255 - thresh

# Apply OCR
data = pytesseract.image_to_string(thresh, config="-c tessedit"
                                                  "_char_whitelist=ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890-/"
                                                  " --psm 6"
                                                  " ")

print(data)

# Show image for testing
cv2.imshow('thresh', thresh)
cv2.waitKey(0)
cv2.destroyAllWindows()

thresh（干净的图像）：

OCR 结果：EXPO22016/01-2019

我应该如何从 OpenCV 中的阈值图像中去除噪声？

How should I remove noise from this thresholded image in OpenCV?

python

ocr

opencv

image-processing