c++ - cvtColor ignores dstCn argument OpenCV

Question

I am trying to create a program which imports an RGB image and converts it to grayscale. I would like the output image to consist of 3 channels. To achieve that I use cv::cvtColor function with dstCn parameter set to 3:

cv::Mat mat = cv::imread("lena.bmp");
std::cout << CV_MAT_CN(mat.type()) << "
"; // prints "3", OK
cv::cvtColor(mat, mat, cv::COLOR_BGR2GRAY, 3);
std::cout << CV_MAT_CN(mat.type()) << "
"; // prints "1" regardless of dstCn

but it looks like dstCn isn't taken into account, and the output array has only 1 channel.

The OpenCV documentation says:

dstCn - number of channels in the destination image; if the parameter is 0, the number of the channels is derived automatically from src and code.

It's a very basic case and I am aware there are plenty of workarounds, but I would like to know whether it is a bug or my incomprehension.

question from:https://stackoverflow.com/questions/65672169/cvtcolor-ignores-dstcn-argument-opencv

Answer 1

The answer can be found in the OpenCV source code. Let's have a look at cvtColor function in imageproc/src/color.cpp file. There is a very long switch-case, so I only post here the most interesting part:

void cvtColor( InputArray _src, OutputArray _dst, int code, int dcn )
{
    ...
    switch( code )
    {
    ...
    case COLOR_BGR2GRAY: case COLOR_BGRA2GRAY:
    case COLOR_RGB2GRAY: case COLOR_RGBA2GRAY:
        cvtColorBGR2Gray(_src, _dst, swapBlue(code));
        break;
    }
}

The code from my question uses COLOR_BGR2GRAY. Nothing special is done before the switch statement. Invoking swapBlue does not do anything interesting too. We can see that this case completely ignores dcn (aka dstCn). So it seems to be fully intentional and my idea was wrong from the start.

I have also found a similar post on OpenCV forum where Doomb0t pointed that:

the concept of greyscale is that you have one channel describing the intensity on a gradual scale between black and white. So, it is not clear why would you need a 3 channels greyscale image (...)