opencv读入灰度图片却显示三通道

opencv中的imread函数用法：

C++：Mat imread(const string& filename, intflags=1 );

Python：cv2.imread(filename, flags=None)

其中第一个参数是载入图片名字，第二个参数是int类型的flags，为载入标识，它指定一个加载图像的颜色类型。可以看到它自带缺省值1。其中：
flags >0返回一个3通道的彩色图像。
flags =0返回灰度图像。
flags <0返回包含Alpha通道的加载的图像。
你用函数的时候没有设置第二个参数，就会默认值为1。尽管你读入的是灰度图，还是自动转化成三通道图像。可以将第二个参数置为0得到灰度图像

附：cv2.imread()

imread(filename, flags=None)
filename
imread(filename[, flags]) -> retval . @brief Loads an image from a file. . . @anchor imread . . The function imread loads an image from the specified file and returns it. If the image cannot be . read (because of missing file, improper permissions, unsupported or invalid format), the function . returns an empty matrix ( Mat::data==NULL ). . . Currently, the following file formats are supported: . . - Windows bitmaps - *.bmp, *.dib (always supported) . - JPEG files - *.jpeg, *.jpg, *.jpe (see the Note section) . - JPEG 2000 files - *.jp2 (see the Note section) . - Portable Network Graphics - *.png (see the Note section) . - WebP - *.webp (see the Note section) . - Portable image format - *.pbm, *.pgm, *.ppm *.pxm, *.pnm (always supported) . - PFM files - *.pfm (see the Note section) . - Sun rasters - *.sr, *.ras (always supported) . - TIFF files - .tiff, .tif (see the Note section) . - OpenEXR Image files - .exr (see the Note section) . - Radiance HDR - .hdr, .pic (always supported) . - Raster and Vector geospatial data supported by GDAL (see the Note section) . . @note . - The function determines the type of an image by the content, not by the file extension. . - In the case of color images, the decoded images will have the channels stored in B G R order. . - When using IMREAD_GRAYSCALE, the codec’s internal grayscale conversion will be used, if available. . Results may differ to the output of cvtColor() . - On Microsoft Windows OS and MacOSX, the codecs shipped with an OpenCV image (libjpeg, . libpng, libtiff, and libjasper) are used by default. So, OpenCV can always read JPEGs, PNGs, . and TIFFs. On MacOSX, there is also an option to use native MacOSX image readers. But beware . that currently these native image loaders give images with different pixel values because of . the color management embedded into MacOSX. . - On Linux, BSD flavors and other Unix-like open-source operating systems, OpenCV looks for . codecs supplied with an OS image. Install the relevant packages (do not forget the development . files, for example, “libjpeg-dev”, in Debian and Ubuntu) to get the codec support or turn . on the OPENCV_BUILD_3RDPARTY_LIBS flag in CMake. . - In the case you set WITH_GDAL flag to true in CMake and @ref IMREAD_LOAD_GDAL to load the image, . then the GDAL driver will be used in order to decode the image, supporting . the following formats: Raster, . Vector. . - If EXIF information are embedded in the image file, the EXIF orientation will be taken into account . and thus the image will be rotated accordingly except if the flag @ref IMREAD_IGNORE_ORIENTATION is passed. . - Use the IMREAD_UNCHANGED flag to keep the floating point values from PFM image. . - By default number of pixels must be less than 2^30. Limit can be set using system . variable OPENCV_IO_MAX_IMAGE_PIXELS . . @param filename Name of file to be loaded. . @param flags Flag that can take values of cv::ImreadModes