When a fixed camera observes a scene, the background remains mostly unchanged. In this case, the interesting elements are the moving objects that evolve inside this scene. In order to extract these foreground objects, we need to build a model of the background, and then compare this model with a current frame in order to detect any foreground objects. This is what we will do in this recipe. Foreground extraction is a fundamental step in intelligent surveillance applications.

If we had an image of the background of the scene (that is, a frame that contains no foreground objects) at our disposal, then it would be easy to extract the foreground of a current frame through a simple image difference:

  // compute difference between current image and background
  cv::absdiff(backgroundImage,currentImage,foreground);

Each pixel for which this difference is high enough would then be declared as a foreground pixel. However, most of the time, this background...