一、读取图像：imread() 与imreadmulti()
1. imread()函数第二个参数flags有很多选择，如下：
代码语言：javascript复制
//! Imread flags
enum ImreadModes {
IMREAD_UNCHANGED = -1, //!< If set, return the loaded image as is (with alpha channel, otherwise it gets cropped). Ignore EXIF orientation.
IMREAD_GRAYSCALE = 0, //!< If set, always convert image to the single channel grayscale image (codec internal conversion).
IMREAD_COLOR = 1, //!< If set, always convert image to the 3 channel BGR color image.
IMREAD_ANYDEPTH = 2, //!< If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.
IMREAD_ANYCOLOR = 4, //!< If set, the image is read in any possible color format.
IMREAD_LOAD_GDAL = 8, //!< If set, use the gdal driver for loading the image.
IMREAD_REDUCED_GRAYSCALE_2 = 16, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/2.
IMREAD_REDUCED_COLOR_2 = 17, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/2.
IMREAD_REDUCED_GRAYSCALE_4 = 32, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/4.
IMREAD_REDUCED_COLOR_4 = 33, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/4.
IMREAD_REDUCED_GRAYSCALE_8 = 64, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/8.
IMREAD_REDUCED_COLOR_8 = 65, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/8.
IMREAD_IGNORE_ORIENTATION = 128 //!< If set, do not rotate the image according to EXIF's orientation flag.
};
默认是IMREAD_COLOR 模式读取，会将图片转为3通道BGR彩图，读入进来type变成了CV_8UC3，如果你想以原本类型读取，那就选择IMREAD_UNCHANGED，那么图像原本是什么类型，读进来还是什么类型，这个参数主要在我们读取一些16位或者32为浮点型图像时就比较有用，因为一些计算需要这样的类型，有时候也为了计算提高精度。
2. imreadmulti()函数是用来一次性读取多张图片的，看下定义：
代码语言：javascript复制CV_EXPORTS_W bool imreadmulti(const String& filename, CV_OUT std::vector<Mat>& mats, int flags = IMREAD_ANYCOLOR);一个string类型的文件路径，输出是Mat类型的vector，也就是多张图像，比如这里我有一张tif格式的图片，它本来是2张图片组成的，那么我就可以一次性读进来，然后对vector进行处理，避免我对图片路径进行字符串的格式化。
二、显示图像：namedWindow() 和 imshow()
1. imshow()函数默认显示窗口模式是WINDOW_AUTOSIZE，它的好处是可以根据图像的大小自动调整大小显示，缺点是不能调整窗口大小，如果你想调整窗口大小，那么就要先使用namedWindow()函数，并将第二个参数设置为WINDOW_NORMAL
代码语言：javascript复制
//! Flags for cv::namedWindow
enum WindowFlags {
WINDOW_NORMAL = 0x00000000, //!< the user can resize the window (no constraint) / also use to switch a fullscreen window to a normal size.
WINDOW_AUTOSIZE = 0x00000001, //!< the user cannot resize the window, the size is constrainted by the image displayed.
WINDOW_OPENGL = 0x00001000, //!< window with opengl support.
WINDOW_FULLSCREEN = 1, //!< change the window to fullscreen.
WINDOW_FREERATIO = 0x00000100, //!< the image expends as much as it can (no ratio constraint).
WINDOW_KEEPRATIO = 0x00000000, //!< the ratio of the image is respected.
WINDOW_GUI_EXPANDED=0x00000000, //!< status bar and tool bar
WINDOW_GUI_NORMAL = 0x00000010, //!< old fashious way
};
2. 另外一个冷门知识点是imshow显示的时候，我们可以选中窗口，进行图片的复制(Ctrl C)和保存(Ctrl S)
int main()
{
Mat img = imread("./lena.jpg", IMREAD_UNCHANGED);
if (img.empty())
{
cout << "Image is empty, please check again!" << endl;
return -1;
}
namedWindow("img", WINDOW_NORMAL);
imshow("img", img);
while (1)
{
if (waitKey(0) == 27)
break;
}
return 0;
}可以用上一段代码尝试，先选中窗口，然后按下Ctrl C，打开画图工具或者PPT进行粘贴，或者Ctrl S会弹出对话框保存本地，很方便调，避免去加imwrite().
三、保存图像：imwrite()
1. 大家比较熟悉的应该是用imwrite()来保存单张图片，我们也可以用它来一次性保存多张图片到一个文件中，看函数说明：
下面是代码演示：
代码语言：javascript复制vector<Mat>imgs;
Mat img1 = imread("./1.jpg", IMREAD_UNCHANGED);
Mat img2 = imread("./2.jpg", IMREAD_UNCHANGED);
imgs.push_back(img1);
imgs.push_back(img2);
imwrite("result.tif", imgs);保存之后看不出来？那就用imreadmulti()函数来读取，代码如下：
代码语言：javascript复制vector<Mat>imgs;
imreadmulti("./result.tif", imgs, IMREAD_UNCHANGED);
cout << imgs.size() << endl;
//多张图片可以使用for循环来读取
imshow("1", imgs.at(0));
imshow("2", imgs.at(1));
waitKey(0);
.tif这个格式如果你用过halcon就会经常看到它的身影, 为什么老是用它？
