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I'm doing it this way because I need it to behave the same way as any other instance of BackgroundSubtractor.

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?

Thank you, but it still suffers from the "Jurassic Park" problem. After a little while, the moving parts become a part of the background :-(

I've got a project where I have a camera mounted on the ceiling pointing to a map on a table.

Before any pieces are placed on the map, I train a background subtractor to know what's background and what's not, and segment those pieces, so that I can get their shape & colors.

However, whenever the lighting changes (ie. you're playing inside next to a window and it starts getting darker) the whole thing falls apart.

This is the only way I've found to correctly identify moving parts within a "game board", but the lighting changes are basically throwing it in the wind.

Any ideas of what I can do to periodically adapt to the lighting conditions without having my pieces becoming part of the background? Any help would be appreciated.

I'm working on an application that will work with an inside mounted camera on the ceiling. The purpose is for it to keep track of objects on a surface.

I need to remove the background, so that I can get the contours of the "diff" that's there, but using BackgroundSubtractorMOG gets frustrating, as I find that its only application is for video.

What I need is to provide a single image that will be the background, and then calculate on each frame from a stream what has changed.

Here's what I have:

#include <libfreenect/libfreenect_sync.h>
#include <opencv2/opencv.hpp>
#include <iostream>
#include <vector>

const char *kBackgroundWindow = "Background";
const char *kForegroundWindow = "Foreground";
const char *kDiffWindow = "Diff";

const cv::Size kCameraSize(cv::Size(640, 480));

int main(int argc, char **argv) {
  uint8_t *raw_frame = (uint8_t *)malloc(640 * 480 * 3);
  uint32_t timestamp;

  // First, we show the background window. A key press will set the background
  // and move on to object detection.
  cvNamedWindow(kBackgroundWindow);
  cv::Mat background(kCameraSize, CV_8UC3, cv::Scalar(0));
  for(;;) {
    freenect_sync_get_video((void **)&raw_frame, &timestamp, 0, FREENECT_VIDEO_RGB);
    background.data = raw_frame;
    cv::cvtColor(background, background, CV_BGR2RGB);

    cv::imshow(kBackgroundWindow, background);
    if(cv::waitKey(10) > 0)
      break;
  }

  // Create two windows, one to show the current feed and one to show the difference between
  // background and feed.
  cvNamedWindow(kForegroundWindow);


  // Canny threshold values for the track bars
  int cannyThresh1 = 20;
  int cannyThresh2 = 50;
  cvNamedWindow(kDiffWindow);
  cv::createTrackbar("Canny Thresh 1", kDiffWindow, &cannyThresh1, 5000, NULL);
  cv::createTrackbar("Canny THresh 2", kDiffWindow, &cannyThresh2, 5000, NULL);


  // Start capturing frames.
  cv::Mat foreground(kCameraSize, CV_8UC3, cv::Scalar(0));
  cv::Mat diff(kCameraSize, CV_8UC3, cv::Scalar(0));

  cv::BackgroundSubtractorMOG2 bg_subtractor(101, 100.0, false);
  bg_subtractor(background, diff, 1);

  for(;;) {
    freenect_sync_get_video((void **)&raw_frame, &timestamp, 0, FREENECT_VIDEO_RGB);
    foreground.data = raw_frame;
    cv::cvtColor(foreground, foreground, CV_BGR2RGB);
    // Calculate the difference between the background
    // and the foreground into diff.
    bg_subtractor(foreground, diff, 0.01);

    // Run the Canny edge detector in the resulting diff
    cv::Canny(diff, diff, cannyThresh1, cannyThresh2);

    cv::imshow(kForegroundWindow, foreground);
    cv::imshow(kDiffWindow, diff);

    cv::waitKey(10);
  }
}

How can I change this so that it doesn't "learn" about the new background, but just uses the static image stored in background?

Thanks!