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I understood that you want to indicate the areas on frame which have motion. You may use motion history approach or contour approach for this work.

Here is a motion history approach used sample method (optimized through forking the OpenCV sample - update_mhi) written with Java. You may call this method in a loop through passing the retrieved video frame's Mat reference to img parameter of method. // Fields... Mat motion, mhi, orient, mask, segmask;

private void update_mhi(Mat img, Mat dst, int diff_threshold) {
    if (videoSignalOkay) {
        double timestamp = (System.nanoTime() - startTime) / 1e9;
        int idx1 = last, idx2;
        Mat silh = Mat.zeros(size, CvType.CV_32FC1);
        cvtColor(img, buf[last], COLOR_BGR2GRAY);
        double angle, count;

        idx2 = (last + 1) % Constants.fps; // index of (last - (N-1))th frame
        last = idx2;

        silh = buf[idx2];
        if (silh == null || silh.empty()) {
            silh = Mat.zeros(size, CvType.CV_32FC1);
        }
        absdiff(buf[idx1], buf[idx2], silh);

        threshold(silh, silh, diff_threshold, 1, THRESH_BINARY);

        updateMotionHistory(silh, mhi, timestamp, Constants.mhiDuration);

        mhi.convertTo(mask, mask.type(), 255.0 / Constants.mhiDuration, (Constants.mhiDuration - timestamp) * 255.0 / (Constants.mhiDuration));
        dst.setTo(new Scalar(0));
        List<Mat> list = new ArrayList<Mat>(3);
        list.add(mask);
        list.add(Mat.zeros(mask.size(), mask.type()));
        list.add(Mat.zeros(mask.size(), mask.type()));

        merge(list, dst);

        calcMotionGradient(mhi, mask, orient, Constants.maxTimeDelta, Constants.minTimeDelta, 3);

        MatOfRect roi = new MatOfRect();
        segmentMotion(mhi, segmask, roi, timestamp, Constants.maxTimeDelta);

        int total = roi.toArray().length;
        Rect[] rois = roi.toArray();
        Rect comp_rect;
        Scalar color;

        for (int i = -1; i < total; i++) {
            if (i < 0) {
                comp_rect = new Rect(0, 0, videoWidth, videoHeight);
                color = new Scalar(255, 255, 255);
                magnitude = 100;
            } else {
                comp_rect = rois[i];
                if (comp_rect.width >= videoWidth/2 || comp_rect.height >= videoHeight/2 || 
                    comp_rect.width < Constants.recfactorx || comp_rect.height < Constants.recfactory || 
                    comp_rect.width + comp_rect.height < (Constants.recfactorx*Constants.recfactory)) // reject very small things
                    continue;
                color = new Scalar(0, 0, 255);
                magnitude = 30;
            }

            Mat silhROI = silh.submat(comp_rect);
            Mat mhiROI = mhi.submat(comp_rect);
            Mat orientROI = orient.submat(comp_rect);
            Mat maskROI = mask.submat(comp_rect);

            angle = calcGlobalOrientation(orientROI, maskROI, mhiROI, timestamp, Constants.mhiDuration);
            angle = 360.0 - angle;
            count = Core.norm(silhROI, NORM_L1);

            silhROI.release();
            mhiROI.release();
            orientROI.release();
            maskROI.release();
            if (count < comp_rect.height * comp_rect.width * Constants.pixelFactor ||
                comp_rect.width == videoWidth || comp_rect.height == videoHeight) {
                continue;
            } else {
                isAnyTrack = true;
            }
            Point center = new Point((comp_rect.x + comp_rect.width / 2), (comp_rect.y + comp_rect.height / 2));
            // Optimizer part! Compare the tracked thing in previous list to control empty area movement detection...
            /*if (isAnyTrack) {
                TrackingPojo trackPojo = new TrackingPojo(comp_rect, center, null); 
                if (notNeededTracking(trackPojo)) {
                    isAnyTrack = false;
                    continue;
                } else {
                    trackList.add(trackPojo);
                    // Show the warning icon...
                    ControlManager.getTheInstance().toggleAlert(true);
                }
            }*/
            circle(img, center, (int) Math.round(magnitude * 1.2), color, 3, LINE_AA, 0);
            Point thePoint = new Point(
                    Math.round(center.x - magnitude * Math.sin(angle * Math.PI / 180)),
                    Math.round(center.y + magnitude * Math.cos(angle * Math.PI / 180)));
            Point thePoint2 = new Point(thePoint.x, thePoint.y - (comp_rect.height / 2 + 10));
            Point thePoint3 = new Point(thePoint2.x, thePoint2.y - 15);
            Core.putText(img, "(" + center.x + ", " + center.y + ")", thePoint, 16, 0.50, new Scalar(255, 0, 0));
            Core.putText(img, "[W:" + comp_rect.width + ", H:" + comp_rect.height + "]", thePoint2, 16, 0.50, new Scalar(0, 255, 0));
            Core.line(img, center, thePoint, color, 3, LINE_AA, 0);
        }
        if (isAnyTrack) {
            // Also play alert!
            ControlManager.getTheInstance().runAlertSound();
        }
        isAnyTrack = false;
    }
}