Mask for BackgroundSubtractorMOG2

Question

How can I tell BackgroundSubtractorMOG2 which pixels to update to the background model and which pixels shouldn't.

I am facing problem when there's an object entered the scene and stopped for a few ten seconds, the object will be absorbed into the background model.

I wanted to decrease the learning rate or stop the learning around the particular stopped object but how can I do that? Does BackgroundSubtractorMOG2 support using mask in its update function?

I am using OpenCV 2.4.1.

Answer 1

BackgroundSubtractorMOG2 does not support masking the input. But, if you know which pixels you want to mask you can mask the output: say you've called subtractor(input, fg, learningRate); and you somehow know where the object is now (may be you've been tracking it using mean shift or pattern recognition) just do fg |= mask; where mask is where, as you know from some different source, the object is.

Answer 2

you can replace masked parts with background image:

BackgroundSubtractorMOG2 mog_bgs;
.
.
void my_apply(const cv::Mat& img, cv::Mat& fg, const cv::Mat& mask){
  cv::Mat last_bg;
  cv::Mat masked_img = img.clone();
  mog_bgs.getBackgroundImage(last_bg);
  last_bg.copyTo(masked_img, mask);
  mog_bgs.apply(masked_img, fg);
}

or weight masked parts:

BackgroundSubtractorMOG2 mog_bgs;
.
.
void my_apply(const cv::Mat& img, cv::Mat& fg, const cv::Mat& mask){
  cv::Mat last_bg;
  cv::Mat masked_img = img.clone();
  mog_bgs.getBackgroundImage(last_bg);
  masked_img.forEach<Vec3b>([&](Vec3b& p, const int* position) -> void
    {
        if (mask.at<uchar>(position[0], position[1]) > 0) {
            auto b = last_bg.at<Vec3b>(position[0], position[1]);
            p[0] = p[0]*0.2 + b[0]*0.8;
            p[1] = p[1]*0.2 + b[1]*0.8;
            p[2] = p[2]*0.2 + b[2]*0.8;
        }
    });
  mog_bgs.apply(masked_img, fg);
}

Answer 3

You can accomplish this by setting the learning rate down really low

ie:

mog(input, output, 0.00000001);