Image Stretching in Python without using libraries

image_stretch.py

import numpy as np
from PIL import Image

import matplotlib.pyplot as plt 

gray_img = Image.open('Lenna.png').convert("LA")#밝기와 알파값을 이용해서 Grayscale로 변환
gray_img.show()#grayscale로 변환된 흑백 이미지를 출력
row = gray_img.size[0]
col = gray_img.size[1]
stretch_img = Image.new("L", (row, col))#새 흑백이미지를 생성.
high = 0
low = 255

for x in range(1 , row):
    for y in range(1, col):
        if high < gray_img.getpixel((x,y))[0] :
            high = gray_img.getpixel((x,y))[0]
        if low > gray_img.getpixel((x,y))[0]:
            low = gray_img.getpixel((x,y))[0]
for x in range(1 , row):
    for y in range(1, col):
        stretch_img.putpixel((x,y), int((gray_img.getpixel((x,y))[0]-low)*255/(high-low)))
stretch_img.show()#스트레칭된 이미지 출력

y = gray_img.histogram()
y = y[0:256]
x = np.arange(len(y))
plt.title("original hist")
plt.bar(x, y)
plt.show()

y = stretch_img.histogram()
x = np.arange(len(y))
plt.title("stretch hist")
plt.bar(x, y)
plt.show()