System information (version)

• OpenCV => 4.5
• Operating System / Platform => any
• Compiler => any

Detailed description

ImgCodecs support reading and writing of Bitmaps (*.bmp) with imread and imwrite. BMP format has a wide variety of supported image types, like 8-bit RGB or others. The current implementation supports many of them (modules/imgcodecs/src/grfmt_bmp.cpp
), but unfortunately it does not support 16-bit grayscale format.
16-Bit grayscale format is only supported by other formats like PNG as the documentation says.

As BMP writing is fast, as there is no further image processing involved, I would like to see support for 16-bit grayscale Bitmaps.
Currently all 16-bit grayscale images are converted to 8-bit, which is not correct. (at least a warning "not supported" should appear).

Summary:

• imwrite .bmp 16-bit grayscale missing
• imread .bmp 16-bit grayscale missing

Thanks for helping

Steps to reproduce

#include <iostream>
#include <opencv2/imgcodecs.hpp>

int main(int argc, char *argv[])
{
    cv::Mat imOut = cv::Mat::ones(cv::Size(1024, 768), CV_16UC1);
    cv::imwrite("test.bmp", imOut);
    std::cout << imOut.depth() << std::endl;  // 2 = CV_16U

    // saved image test.bmp has only 8-bit instead of 16-bit
    cv::Mat imIn = cv::imread("test.bmp", cv::IMREAD_ANYDEPTH);
    std::cout << imIn.depth() << std::endl;  // 0 = CV_8U  --> this is should be also 2 (CV_16U)
    return 0;
}

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  answers.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc