double Ft(double ***memo,
              double xx,
              double yy,
              double fajlagos_t,
              double pixel_meret,
              int  xy,
              bool futhat)

    {
    //X irányú erő.
    if (xy==0 && futhat)
        {
        double dxk1,dxk2;

        double x_x2=xx/pixel_meret;
        double y_y2=yy/pixel_meret;

        dxk1=xx-floor(xx);
        dxk2=floor(xx+1.0)-xx;

        return (dxk1*memo[(int)x_x2][(int)y_y2][2] + dxk2*memo[(int)x_x2+1][(int)y_y2][2])*fajlagos_t;
        };

    //Y irányú erő.
    if (xy==1 && futhat)
        {
        double dyk1,dyk2;

        double xx2=xx/pixel_meret;
        double yy2=yy/pixel_meret;

        dyk1=yy-floor(yy);
        dyk2=floor(yy+1.0)-yy;

        return (dyk1*memo[(int)xx2][(int)yy2][3]+dyk2*memo[(int)xx2][(int)yy2+1][3])*fajlagos_t;
        };
    if (!futhat)
        return 0.0;
    }