CImg: How to save a grayscale?

When I use CImg to load a .BMP, how can I know whether it is a gray-scale or color image? I have tried as follows, but failed:

cimg_library::CImg<unsigned char> img("lena_gray.bmp");

const int spectrum = img.spect开发者_JAVA百科rum();

img.save("lenaNew.bmp");

To my expectation, no matter what kind of .BMP I have loaded, spectrum will always be 3. As a result, when I load a gray-scale and save it, the result size will be 3 times bigger than it is.

I just want to save a same image as it is loaded. How do I save as gray-scale?

I guess the BMP format always store images as RGB-coded data, so reading a BMP will always result in a color image. If you know your image is scalar, all channels will be the same, so you can discard two of them (here keeping the first one).

img.channel(0);

If you want to check that it is a scalar image, you can test the equality between channels, as

const CImg<unsigned char> R = img.get_shared_channel(0),
                          G = img.get_shared_channel(1),
                          B = img.get_shared_channel(2);
if (R==G && R==B) {
    .. Your image is scalar !
} else {
    .. Your image is in color.
}