Function sdf_smooth_union 

pub fn sdf_smooth_union(d1: f64, d2: f64, k: f64) -> f64

Polynomial smooth-minimum of d1 and d2 with blend radius k.

k == 0 degenerates to min(d1, d2). Larger values produce a softer blend between the two surfaces.

Reference: Inigo Quilez — https://iquilezles.org/articles/smin/