OpenCV：直方图均衡

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步骤:

第 1 步:手动均衡

第 2 步:通过使用 OpenCV 函数

什么是图像直方图?

它是图像强度分布的图形表示。它量化了所考虑的每个强度值的像素数。

第 1 步:手动均衡

%matplotlib inline

from IPython．display import display, Math, Latex

import numpy as np

import matplotlib．pyplot as plt

from PIL import Image

img = Image．open('DATA/einstein．jpg')

plt．imshow(img)

输出:

显示彩涩图像

将图像转换为 numpy 数组,以便 OpenCV 可以使用:

img = np．asanyarray(img)

img．shape

输出:

(2354, 2560, 3)

将 RGB 转换为灰度:

import cv2

img = cv2．cvtColor(img, cv2．COLOR_BGR2GRAY)

img．shape

输出:

(2354, 2560)

显示图像:

plt．imshow(img, cmap='gray')

输出:

我们现在知道如何处理直方图了

img．max()

输出:

255

img．min()

输出:

0

img．shape

输出:

(2354, 2560)

把它展平:

flat = img．flatten()

# 1 row 2354 x 2560 = 6．026．240

flat．shape

输出:

(6026240,)

显示直方图

plt．hist(flat, bins=50)

请注意,灰度值在某个值周围分布很差

什么是直方图均衡?

为了更清楚,从上图中,你可以看到像素似乎聚集在可用强度范围的中间。直方图均衡所做的就是扩大这个范围。

# formula for creating the histogram

display(Math(r'P_x(j) = sum_{i=0}^{j} P_x(i)'))

# create our own histogram function

def get_histogram(image, bins):

# array with size of bins, set to zeros

histogram = np．zeros(bins)

# loop through pixels and sum up counts of pixels

for pixel in image:

histogram[pixel] += 1

# return our final result

return histogram

hist = get_histogram(flat, 256)

plt．plot(hist)

[]

# create our cumulative sum function

def cumsum(a):

a = iter(a)

b = [next(a)]

for i in a:

b．append(b[-1] + i)

return np．array(b)

# execute the fn

cs = cumsum(hist)

# display the result

plt．plot(cs)

[]

# formula to calculate cumulation sum

display(Math(r's_k = sum_{j=0}^{k} {rac{n_j}{N}}'))

# re-normalize cumsum values to be between 0-255

# numerator & denomenator

nj = (cs – cs．min()) * 255

N = cs．max() – cs．min()

# re-normalize the cdf

cs = nj / N

plt．plot(cs)

[]

Casting:

# cast it back to uint8 since we can't use floating point values in imagescs =

cs．astype('uint8')

plt．plot(cs)

输出:

[]

获取 CDF:

# get the value from cumulative sum for every index in flat, and set that as img_new

img_new = cs[flat]

# we see a much more evenly distributed histogram

plt．hist(img_new, bins=50)

它是如何工作的?

均衡意味着将一个分布(给定的直方图)映射到另一个分布(强度值的更广泛和更均匀的分布),因此强度值分布在整个范围内。

# get the value from cumulative sum for every index in flat, and set that as img_new

img_new = cs[flat]

# we see a much more evenly distributed histogram

plt．hist(img_new, bins=50)

# put array back into original shape since we flattened it

img_new = np．reshape(img_new, img．shape)

img_new

输出:

array([[233, 231, 228, ．．．, 216, 216, 215],

[233, 230, 228, ．．．, 215, 215, 214],

[233, 231, 229, ．．．, 213, 213, 212],

．．．,

[115, 107, 96, ．．．, 180, 187, 194],

[111, 103, 93, ．．．, 187, 189, 192],

[111, 103, 93, ．．．, 187, 189, 192]], dtype=uint8)

一探究竟：

# set up side-by-side image display

fig = plt．figure()

fig．set_figheight(15)

fig．set_figwidth(15)

fig．add_subplot(1,2,1)

plt．imshow(img, cmap='gray')

# display the new image

fig．add_subplot(1,2,2)

plt．imshow(img_new, cmap='gray')

plt．show(block=True)

使用 OpenCV equalizeHist(img) 方法

第 2 步:通过使用 OpenCV 函数

# Reading image via OpenCV and Equalize it right away!

img = cv2．imread('DATA/einstein．jpg',0)

equ = cv2．equalizeHist(img)

准备好!这就是你需要做的!

fig = plt．figure()

fig．set_figheight(15)

fig．set_figwidth(15)

fig．add_subplot(1,2,1)

plt．imshow(img, cmap='gray')

# display the Equalized (equ) image

fig．add_subplot(1,2,2)

plt．imshow(equ, cmap='gray')

plt．show(block=True)

print(“That´s it! Thank you once again!I hope will be helpful．”)

输出:

That´s it! Thank you once again!

I hope will be helpful．