===== Primitive: Metaballs object [Nishimura et al. 1985] =====

**Definition**: <m>Sum{}{}{f(r)}-T</m>,\\
<m>f(r)=delim{lbrace}{matrix{3}{1}{{b(1-3*r^2/d^2), 0<r<=d/3} {1.5*b(1-r/d)^2, d/3<r<=d} {0, r>d}}}{ }</m>
\\
r - distance to the given point

**Call**: hfMetaball(x,x0,y0,z0,b,d,T);

**Parameters**:

x - point coordinates

x0,y0,z0 - arrays of mataball centers

b - array of b coefficients

d - array of d radii of influence

T - threshold value

**Test file**: [[http://hyperfun.org/metaball.hf|metaball.hf]]