case FALLOFF_SIGMOND:
                default:
                        //linear distance falloff
                        alpha = ( outer_radius - sqrt(mag_squared) ) / double_feather;
                        // inverse exponential of the linear falloff (asymptotes at 0 and 1)
                        // \frac{1.0}{ 1 + e^{- \( a*10-5 \)}}
                        alpha = 1.0 / (1 + exp(-(alpha*10-5)) );
                        break;
                }