cv::Mat grayimage = cv::imread(buf,flag)
buf:arry type or vector of bytes
flag: IMREAD_GRAYSCALE (No matter which type is passed in, it will be saved as a grayscale image)
IMREAD_UNCHANGED (what type of picture is passed in, it will be saved as what type)
IMREAD_COLOR (no matter which type is passed in, it will be saved as a color map)
In addition: cv::cvtColor(source,dest,cv::COLOR_BGR2GRAY) converts grayscale image to color image