一、明暗检测

图像亮度通俗理解便是图像的明暗程度，它是图片质量的评价参数之一。计算整张图片的亮度有多种方法。

# coding=utf-8
import cv2 as cv
from PIL import Image, ImageStat

def image_brightness1(rgb_image):
    '''
    检测图像亮度(亮度平均值方法)
    '''
    w, h = rgb_image.size
    # 转换为HSV格式
    hsv_image = cv.cvtColor(np.array(rgb_image, 'f'), cv.COLOR_RGB2HSV)
    # 累计总亮度值
    sum_brightness = np.sum(hsv_image[:,:,2])
    area = w*h  # 总像素数
    # 平均亮度
    avg = sum_brightness/area
    return avg

def image_brightness2(rgb_image):
    '''
    检测图像亮度(灰度平均值方法)
    '''
    gray_image = rgb_image.convert('L')
    stat = ImageStat.Stat(gray_image)   
    return stat.mean[0]

def image_brightness3(rgb_image):
    '''
    检测图像亮度(基于经验公式)
    '''
    stat = ImageStat.Stat(rgb_image)   
    r, g, b = stat.mean   
    return math.sqrt(0.241*(r**2) + 0.691*(g**2) + 0.068*(b**2))

def image_brightness4(rgb_image):
    '''
    检测图像亮度(基于RMS)
    '''
    stat = ImageStat.Stat(rgb_image)   
    r, g, b = stat.rms   
    return math.sqrt(0.241*(r**2) + 0.691*(g**2) + 0.068*(b**2))

pil_im = Image.open('test.jpg')
print(image_brightness1(pil_im))
print(image_brightness2(pil_im))
print(image_brightness3(pil_im))
print(image_brightness4(pil_im))

二、明暗调节

有了图像亮度后如何进行调整呢？我在这里采用了空域法中的gamma变换，通过如下方法计算gamma值

def calc_gamma(brightness):
    '''
    计算gamma值
    '''
    return brightness/127.0

然后用得到的gamma值进行亮度变换

import numpy as np
from skimage import exposure
import matplotlib.pyplot as plt

def array_to_image(image_arr):
    '''
    数组还原为图片
    INPUT  -> 数组
    OUTPUT -> 单张图文件
    '''
    if len(image_arr.shape) == 3:  # 格式为(height(rows), weight(colums), 3)
        r = Image.fromarray(np.uint8(image_arr[:,:,0]))
        g = Image.fromarray(np.uint8(image_arr[:,:,1]))
        b = Image.fromarray(np.uint8(image_arr[:,:,2]))
        image = Image.merge("RGB", (r, g, b))
        return image        
    elif len(image_arr.shape) == 2:  # 格式为(height(rows), weight(colums))
        return Image.fromarray(np.uint8(image_arr))

def image_gamma_transform(pil_im, gamma):
    '''
    伽马校正
    INPUT  -> 单张图文件
    OUTPUT -> 处理后的图文件
    '''
    image_arr = np.array(pil_im)
    image_arr2 = exposure.adjust_gamma(image_arr, gamma)
    return array_to_image(image_arr2)

pil_im = Image.open('test.jpg')
brightness = image_brightness4(pil_im)
newimage = image_gamma_transform(pil_im, calc_gamma(brightness))

result = np.hstack((np.array(pil_im),np.array(newimage)))
plt.imshow(result)
plt.show()