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I have this code that basically does a "dumb" background subtraction on two frames.

void FrameDifferenceBGS::operator()(cv::InputArray _image, cv::OutputArray _fgmask, double learningRate)
{
  cv::Mat img_input = _image.getMat();

  if(img_input.empty())
    return;

  _fgmask.create(img_input.size(), CV_8U);
  cv::Mat img_foreground = _fgmask.getMat();

  if(img_input_prev.empty())
  {
    img_input.copyTo(img_input_prev);
    return;
  }

  cv::absdiff(img_input_prev, img_input, img_foreground);

  if(img_foreground.channels() == 3)
    cv::cvtColor(img_foreground, img_foreground, CV_BGR2GRAY);

  if(enableThreshold)
    cv::threshold(img_foreground, img_foreground, threshold, 255, cv::THRESH_BINARY);

  if(showOutput)
    cv::imshow("Frame Difference", img_foreground);

  img_input.copyTo(img_input_prev);
  img_foreground.copyTo(_fgmask);
  firstTime = false;
}

If I don't add img_foreground.copyTo(_fgmask) in the end, the output array isn't updated with the result of img_foreground, resulting on a black image when this is called.

What am I doing wrong / should be doing here?