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opencv threshold、adaptiveThreshold、Otsu opencv函数threshold、adaptiveThreshold、Otsu二值化的实现

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threshold:固定阈值二值化,

ret, dst = cv2.threshold(src, thresh, maxval, type)

官方文档的示例代码:

import cv2
import numpy as np
from matplotlib import pyplot as plt
img = cv2.imread('gradient.png',0)
ret,thresh1 = cv2.threshold(img,127,255,cv2.THRESH_BINARY)
ret,thresh2 = cv2.threshold(img,127,255,cv2.THRESH_BINARY_INV)
ret,thresh3 = cv2.threshold(img,127,255,cv2.THRESH_TRUNC)
ret,thresh4 = cv2.threshold(img,127,255,cv2.THRESH_TOZERO)
ret,thresh5 = cv2.threshold(img,127,255,cv2.THRESH_TOZERO_INV)
titles = ['Original Image','BINARY','BINARY_INV','TRUNC','TOZERO','TOZERO_INV']
images = [img, thresh1, thresh2, thresh3, thresh4, thresh5]
for i in xrange(6):
  plt.subplot(2,3,i+1),plt.imshow(images[i],'gray')
  plt.title(titles[i])
  plt.xticks([]),plt.yticks([])
plt.show()

结果为:

 

adaptiveThreshold:自适应阈值二值化

自适应阈值二值化函数根据图片一小块区域的值来计算对应区域的阈值,从而得到也许更为合适的图片。

dst = cv2.adaptiveThreshold(src, maxval, thresh_type, type, Block Size, C)

官方文档的示例代码:

import cv2
import numpy as np
from matplotlib import pyplot as plt
img = cv2.imread('sudoku.png',0)
img = cv2.medianBlur(img,5)
ret,th1 = cv2.threshold(img,127,255,cv2.THRESH_BINARY)
th2 = cv2.adaptiveThreshold(img,255,cv2.ADAPTIVE_THRESH_MEAN_C,\
      cv2.THRESH_BINARY,11,2)
th3 = cv2.adaptiveThreshold(img,255,cv2.ADAPTIVE_THRESH_GAUSSIAN_C,\
      cv2.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 xrange(4):
  plt.subplot(2,2,i+1),plt.imshow(images[i],'gray')
  plt.title(titles[i])
  plt.xticks([]),plt.yticks([])
plt.show()

结果为:

 

Otsu's Binarization: 基于直方图的二值化

Otsu's Binarization是一种基于直方图的二值化方法,它需要和threshold函数配合使用。

Otsu过程:
1. 计算图像直方图;
2. 设定一阈值,把直方图强度大于阈值的像素分成一组,把小于阈值的像素分成另外一组;
3. 分别计算两组内的偏移数,并把偏移数相加;
4. 把0~255依照顺序多为阈值,重复1-3的步骤,直到得到最小偏移数,其所对应的值即为结果阈值。

官方文档的示例代码:

import cv2
import numpy as np
from matplotlib import pyplot as plt
img = cv2.imread('noisy2.png',0)
# global thresholding
ret1,th1 = cv2.threshold(img,127,255,cv2.THRESH_BINARY)
# Otsu's thresholding
ret2,th2 = cv2.threshold(img,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU)
# Otsu's thresholding after Gaussian filtering
blur = cv2.GaussianBlur(img,(5,5),0)
ret3,th3 = cv2.threshold(blur,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU)
# plot all the images and their histograms
images = [img, 0, th1,
     img, 0, th2,
     blur, 0, th3]
titles = ['Original Noisy Image','Histogram','Global Thresholding (v=127)',
     'Original Noisy Image','Histogram',"Otsu's Thresholding",
     'Gaussian filtered Image','Histogram',"Otsu's Thresholding"]
for i in xrange(3):
  plt.subplot(3,3,i*3+1),plt.imshow(images[i*3],'gray')
  plt.title(titles[i*3]), plt.xticks([]), plt.yticks([])
  plt.subplot(3,3,i*3+2),plt.hist(images[i*3].ravel(),256)
  plt.title(titles[i*3+1]), plt.xticks([]), plt.yticks([])
  plt.subplot(3,3,i*3+3),plt.imshow(images[i*3+2],'gray')
  plt.title(titles[i*3+2]), plt.xticks([]), plt.yticks([])
plt.show()

结果为:

 

参考文献:http://docs.opencv.org/3.2.0/d7/d4d/tutorial_py_thresholding.html

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