OpenCV图像读取(imread) 显示(imshow) 保存(imwrite)的冷知识点

OpenCV图像读取(imread) 显示(imshow) 保存(imwrite)的冷知识点，虽然很基础，但也有用。

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：

//! 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：

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

//! 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：

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：

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？