OpenCV中函数imread的参数flags的枚举值(可设置值)及其具体意义

imread函数的原型如下：

C++:
Mat cv::imread(const String & filename,int flags = IMREAD_COLOR)

Python:
retval=cv.imread(filename[, flags])

第二个参数flags是一个可选参数，默认值为“ IMREAD_COLOR”其枚举值(可设置值)的具体意义如下：

上面这张截图来自网页：
https://docs.opencv.org/4.1.2/d4/da8/group__imgcodecs.html#ga61d9b0126a3e57d9277ac48327799c80

具体的枚举值如下：

enum  	cv::ImreadModes {
  cv::IMREAD_UNCHANGED = -1,
  cv::IMREAD_GRAYSCALE = 0,
  cv::IMREAD_COLOR = 1,
  cv::IMREAD_ANYDEPTH = 2,
  cv::IMREAD_ANYCOLOR = 4,
  cv::IMREAD_LOAD_GDAL = 8,
  cv::IMREAD_REDUCED_GRAYSCALE_2 = 16,
  cv::IMREAD_REDUCED_COLOR_2 = 17,
  cv::IMREAD_REDUCED_GRAYSCALE_4 = 32,
  cv::IMREAD_REDUCED_COLOR_4 = 33,
  cv::IMREAD_REDUCED_GRAYSCALE_8 = 64,
  cv::IMREAD_REDUCED_COLOR_8 = 65,
  cv::IMREAD_IGNORE_ORIENTATION = 128
}

下面一个参数一个参数的说。
IMREAD_UNCHANGED
Python: cv.IMREAD_UNCHANGED
If set, return the loaded image as is (with alpha channel, otherwise it gets cropped).
设置这个参数表示原样读取图像，即使图像有alpha通道（alpha通道用于控制图像的透明度），读取时也会保留，否则不会保留alpha通道

IMREAD_GRAYSCALE
Python: cv.IMREAD_GRAYSCALE
If set, always convert image to the single channel grayscale image (codec internal conversion).
设置这个参数表示把图像转换成灰度图像后读取。

IMREAD_COLOR
Python: cv.IMREAD_COLOR
If set, always convert image to the 3 channel BGR color image.
设置这个参数表示将图像转换成3通的BGR彩色图像。

IMREAD_ANYDEPTH
Python: cv.IMREAD_ANYDEPTH
If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.
设置这个参数表示16位或32位深度的图像在读取时也按16位或32位深度的图像读取，如果没有选择这个参数，则会转换为8位深度的图像。

IMREAD_ANYCOLOR
Python: cv.IMREAD_ANYCOLOR
If set, the image is read in any possible color format.
设置这个参数，那么图像将以任意的彩色格式被读取。

IMREAD_LOAD_GDAL
Python: cv.IMREAD_LOAD_GDAL
If set, use the gdal driver for loading the image.
设置这个参数，则在读取图像时会使用GDAL驱动加速图像的读取。
PS:GDAL-Geospatial Data Abstraction Library 是使用C/C++语言编写的用于读写空间数据的一套跨平台开源库。简单地说，GDAL是一个操作各种栅格地理数据格式的库。包括读取、写入、转换、处理。

IMREAD_REDUCED_GRAYSCALE_2
Python: cv.IMREAD_REDUCED_GRAYSCALE_2
If set, always convert image to the single channel grayscale image and the image size reduced 1/2.
设置这个参数，读取图像时会将图像转换成灰度图并将图像尺寸缩小为原来的1/2

IMREAD_REDUCED_COLOR_2
Python: cv.IMREAD_REDUCED_COLOR_2
If set, always convert image to the 3 channel BGR color image and the image size reduced 1/2.
设置这个参数，读取图像时会将图像转换成BGR三通道图像，并且尺寸缩小为原来的1/2

IMREAD_REDUCED_GRAYSCALE_4
Python: cv.IMREAD_REDUCED_GRAYSCALE_4
If set, always convert image to the single channel grayscale image and the image size reduced 1/4.
设置这个参数，读取图像时会将图像转换成灰度图并将图像尺寸缩小为原来的1/4

IMREAD_REDUCED_COLOR_4
Python: cv.IMREAD_REDUCED_COLOR_4
If set, always convert image to the 3 channel BGR color image and the image size reduced 1/4.
设置这个参数，读取图像时会将图像转换成BGR三通道图像，并且尺寸缩小为原来的1/4

IMREAD_REDUCED_GRAYSCALE_8
Python: cv.IMREAD_REDUCED_GRAYSCALE_8
设置这个参数，读取图像时会将图像转换成灰度图并将图像尺寸缩小为原来的1/8

IMREAD_REDUCED_COLOR_8
Python: cv.IMREAD_REDUCED_COLOR_8
设置这个参数，读取图像时会将图像转换成BGR三通道图像，并且尺寸缩小为原来的1/8

IMREAD_IGNORE_ORIENTATION
Python: cv.IMREAD_IGNORE_ORIENTATION
If set, do not rotate the image according to EXIF’s orientation flag.
设置这个参数，不会根据EXIF格式图像的方向设置旋转图像。

Python示例代码如下：

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# 图像处理开发需求、图像处理接私活挣零花钱，请加微信/QQ 2487872782
# 图像处理开发资料、图像处理技术交流请加QQ群，群号 271891601

import cv2 as cv

img01 = cv.imread('./image/food-01.jpg')  # 不设置第二个参数，即取默认值“IMREAD_COLOR”
img02 = cv.imread('./image/food-01.jpg', cv.IMREAD_UNCHANGED)
img03 = cv.imread('./image/food-01.jpg', cv.IMREAD_GRAYSCALE)
img04 = cv.imread('./image/food-01.jpg', cv.IMREAD_COLOR)
img05 = cv.imread('./image/food-01.jpg', cv.IMREAD_ANYDEPTH)
img06 = cv.imread('./image/food-01.jpg', cv.IMREAD_ANYCOLOR)
img07 = cv.imread('./image/food-01.jpg', cv.IMREAD_LOAD_GDAL)
img08 = cv.imread('./image/food-01.jpg', cv.IMREAD_REDUCED_GRAYSCALE_2)
img09 = cv.imread('./image/food-01.jpg', cv.IMREAD_REDUCED_COLOR_2)
img10 = cv.imread('./image/food-01.jpg', cv.IMREAD_REDUCED_GRAYSCALE_4)
img11 = cv.imread('./image/food-01.jpg', cv.IMREAD_REDUCED_COLOR_4)
img12 = cv.imread('./image/food-01.jpg', cv.IMREAD_REDUCED_GRAYSCALE_8)
img13 = cv.imread('./image/food-01.jpg', cv.IMREAD_REDUCED_COLOR_8)
img13 = cv.imread('./image/food-01.jpg', cv.IMREAD_IGNORE_ORIENTATION)