always convert image to the 3 channel BGR color image
enum ImreadModes {
IMREAD_UNCHANGED = -1, //!< If set, return the loaded image as is (with alpha channel, otherwise it gets cropped).//原始图像有alpha通道的话,就保存该通道数据,否则,别的flags会直接将alpha通道丢弃。
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_