/*!
    Inverse square falloff.

    \param lightDist The distance from the light source.
    \param lightRadius The light source radius.

    \return The falloff factor.
 */
float $(float lightDistance, float lightRadius)
{
    float t = lightDistance / lightRadius;
    float t2 = t * t;
    float t4 = t2 * t2;
    float r = saturate(1.0 - t4);
    return ::functions::math::sqr(r) / (::functions::math::sqr(lightDistance) + 1.0);
}