How to draw Optical flow images from ocl::PyrLKOpticalFlow::dense()

Question

How to draw Optical flow images from ocl::PyrLKOpticalFlow::dense() Which actually calculates both horizontal and vertical component of the Optical flow? So I don't know how to draw them. I'm new to opencv . Can anyone help me?

Syntax :

ocl::PyrLKOpticalFlow::dense(oclMat &prevImg, oclMat& nextImg, oclMat& u, oclMat &v,oclMat &err)

Answer 1

A well establische method used in the optical flow community is to display a motion vector field as a color coded image as you can see at one of the various data sets. E.g MPI dataset or the Middlebury dataset.

Therefor you estimate the length and the angle of your motion vector. And use a HSV to RGB colorspace transformation (see OpenCV cvtColor function) to create your color coded image. Use the angle of your motion vector as H (Hue) - channel and the normalized length as the S (Saturation) - channel and set V (Value) to 1. The the color of your image will show you the direction of your motion and the saturation the length ( speed ).

The code will should like this ( Note if use_value == true, the Saturation will be set to 1 and the Value channel is related to the motion vector length):

    void FlowToRGB(const cv::Mat & inpFlow,
                cv::Mat & rgbFlow,
                const float & max_size ,
                bool use_value)
{
    if(inpFlow.empty()) return;
    if( inpFlow.depth() != CV_32F)
        throw(std::exception("FlowToRGB: error inpFlow wrong data type ( has be CV_32FC2"));
    const float grad2deg = (float)(90/3.141);
    cv::Mat pol(inpFlow.size(), CV_32FC2);

    float mean_val = 0, min_val = 1000, max_val = 0;
    float _dx, _dy;

    for(int r = 0; r < inpFlow.rows; r++)
    {
        for(int c = 0; c < inpFlow.cols; c++)
        {
            cv::Point2f polar = cvmath::toPolar(inpFlow.at<cv::Point2f>(r,c));
            polar.y *= grad2deg;
            mean_val +=polar.x;
            max_val = MAX(max_val, polar.x);
            min_val = MIN(min_val, polar.x);
            pol.at<cv::Point2f>(r,c) = cv::Point2f(polar.y,polar.x);
        }
    }
    mean_val /= inpFlow.size().area();
    float scale = max_val - min_val;
    float shift = -min_val;//-mean_val + scale;
    scale = 255.f/scale;
    if( max_size > 0)
    {
        scale = 255.f/max_size;
        shift = 0;
    }

    //calculate the angle, motion value 
    cv::Mat hsv(inpFlow.size(), CV_8UC3);
    uchar * ptrHSV = hsv.ptr<uchar>();
    int idx_val  = (use_value) ? 2:1;
    int idx_sat  = (use_value) ? 1:2;


    for(int r = 0; r < inpFlow.rows; r++, ptrHSV += hsv.step1())
    {
        uchar * _ptrHSV = ptrHSV;
        for(int c = 0; c < inpFlow.cols; c++, _ptrHSV+=3)
        {
            cv::Point2f vpol = pol.at<cv::Point2f>(r,c);

            _ptrHSV[0] = cv::saturate_cast<uchar>(vpol.x);
            _ptrHSV[idx_val] = cv::saturate_cast<uchar>( (vpol.y + shift) * scale);  
            _ptrHSV[idx_sat] = 255;
        }
    }   
    cv::Mat rgbFlow32F;
    cv::cvtColor(hsv, rgbFlow32F, CV_HSV2BGR);
    rgbFlow32F.convertTo(rgbFlow, CV_8UC3);}
 }