OpenCV在转灰度图的时候常用到cvtcolor，但是有的时候发现如此转灰度图的方法会导致生成三通道的灰度图？
测试如下：
Mat firstpic = imread("IMG.JPG");
cout << "channnels : " << firstpic.channels() << endl; //结果 channels：3
Mat pic2(firstpic.rows, firstpic.cols, CV_8UC1);//建立单通道图片
cvtColor(firstpic, pic2, CV_BGR2GRAY);//灰度图存储到单通道pic2中
cvtColor(firstpic, firstpic, CV_BGR2GRAY);//将fistpic变成灰度图存回原图
cout << "channnels : " << dep1.channels()<<endl; //结果 channels：1
cout << "channnels : " << firstpic.channels() << endl; //结果 channels：1由此可见：cvtcolor转成灰度图的时候就已经将通道数变成1；但是为何再后续操作过程中再次读取的时候会变成三通道？
测试如下：
Mat depthpic = imread("pic2.bmp"); //读取上例子中保存的灰度图 Mat pic2
cout << "channnels : " << depthpic.channels() << endl; //结果 channels：3其实问题在于imread，OpenCV中imread原型：
Mat imread( const String& filename, int flags = IMREAD_COLOR );其中我们往往忽略了flags，其定义如下：
//! Imread flags
enum ImreadModes {
IMREAD_UNCHANGED = -1, //!< If set, return the loaded image as is (with alpha channel, otherwise it gets cropped).
IMREAD_GRAYSCALE = 0, //!< If set, always convert image to the single channel grayscale image.
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.
};flags默认值为1，所以我们在读取图片的时候默认读入三通道BGB格式，因此可以通过指定读取图片格式的方法来控制通道数：
Mat depthpic = imread("pic2.bmp", -1); //flags=-1 表示保持原图格式不变，flags=0表示读入单通道灰度图。在此均可
cout << "channnels : " << depthpic.channels() << endl; //结果 channels：1