How to implement Optical Flow tracker?

Question

I'm using the OpenCV wrapper - Emgu CV, and I'm trying to implement a motion tracker using Optical Flow, but I can't figure out a way to combine the horizontal and vertical information retrieved from the OF algorithm:

flowx = new Image<Gray, float>(size);
flowy = new Image<Gray, float>(size);

OpticalFlow.LK(currImg, prevImg, new Size(15, 15), flowx, flowy);

My problem is not knowing how to combine the info of vertical and horizontal movement in order to build the tracker of moving objects? A new image?

By the way, there is a easy way to display the flow info on the current frame?

Thanks in advance.

Answer 1

Here is the function i have defined in my youtube head movement tracker video tutorial. You can find the full source code attached to the video

void ComputeDenseOpticalFlow()
    {
        // Compute dense optical flow using Horn and Schunk algo
        velx = new Image<Gray, float>(faceGrayImage.Size);
        vely = new Image<Gray, float>(faceNextGrayImage.Size);

        OpticalFlow.HS(faceGrayImage, faceNextGrayImage, true, velx, vely, 0.1d, new MCvTermCriteria(100));            

        #region Dense Optical Flow Drawing
        Size winSize = new Size(10, 10);
        vectorFieldX = (int)Math.Round((double)faceGrayImage.Width / winSize.Width);
        vectorFieldY = (int)Math.Round((double)faceGrayImage.Height / winSize.Height);
        sumVectorFieldX = 0f;
        sumVectorFieldY = 0f;
        vectorField = new PointF[vectorFieldX][];
        for (int i = 0; i < vectorFieldX; i++)
        {
            vectorField[i] = new PointF[vectorFieldY];
            for (int j = 0; j < vectorFieldY; j++)
            {
                Gray velx_gray = velx[j * winSize.Width, i * winSize.Width];
                float velx_float = (float)velx_gray.Intensity;
                Gray vely_gray = vely[j * winSize.Height, i * winSize.Height];
                float vely_float = (float)vely_gray.Intensity;
                sumVectorFieldX += velx_float;
                sumVectorFieldY += vely_float;
                vectorField[i][j] = new PointF(velx_float, vely_float);

                Cross2DF cr = new Cross2DF(
                    new PointF((i*winSize.Width) +trackingArea.X,
                               (j*winSize.Height)+trackingArea.Y),
                               1, 1);
                opticalFlowFrame.Draw(cr, new Bgr(Color.Red), 1);

                LineSegment2D ci = new LineSegment2D(
                    new Point((i*winSize.Width)+trackingArea.X,
                              (j * winSize.Height)+trackingArea.Y), 
                    new Point((int)((i * winSize.Width)  + trackingArea.X + velx_float),
                              (int)((j * winSize.Height) + trackingArea.Y + vely_float)));
                opticalFlowFrame.Draw(ci, new Bgr(Color.Yellow), 1);

            }
        }
        #endregion
    }