// from mathfuncs_core.cpp of opencv
void fastAtan2(const float *Y, const float *X, float *angle, int len, bool angleInDegrees )
{
    int i = 0;
    float scale = angleInDegrees ? 1 : (float)(CV_PI/180);

    for( ; i < len; i++ )
    {
        float x = X[i], y = Y[i];
        float ax = std::abs(x), ay = std::abs(y);
        float a, c, c2;
        if( ax >= ay )
        {
            c = ay/(ax + (float)DBL_EPSILON);
            c2 = c*c;
            a = (((atan2_p7*c2 + atan2_p5)*c2 + atan2_p3)*c2 + atan2_p1)*c;
        }
        else
        {
            c = ax/(ay + (float)DBL_EPSILON);
            c2 = c*c;
            a = 90.f - (((atan2_p7*c2 + atan2_p5)*c2 + atan2_p3)*c2 + atan2_p1)*c;
        }
        if( x < 0 )
            a = 180.f - a;
        if( y < 0 )
            a = 360.f - a;
        angle[i] = (float)(a*scale);
    }
}