Once you get a handle to a metaball using metastart and metanext, you can query attributes of the metaball with metaimport.

There are three “special” attributes you can query:

float meta:density : The density of the current metaball

float meta:prim : The primitive number of the current metaball

matrix meta:transform : The transform associated with the current metaball. Applying the inverse of this transform will transform a point into the “space” of the metaball.

For example, the metaweight function can be expressed in the following way:

float
metaweight(string file; vector P)
{
int        handle;
float    density, tmp;

density = 0;
handle = metastart(file, P);
while (metanext(handle))
{
if (metaimport(handle, "meta:density", P, tmp))
density += tmp;
}
return density;
}

The attributes evaluated are un-premultiplied by the weight of the metaball at the position and must be multiplied for blending. For example, to evaluate a vector attribute (say color) on metaballs, the following function could be used:

vector
meta_attribute(string file, attrib_name; vector P)
{
int        handle;
vector    result, tmp;
float    density;

handle = metastart(file, P);
result = 0;
while (metanext(handle))
{
if (metaimport(handle, "meta:density", P, density))
{
if (metaimport(handle, attrib_name, P, tmp))
result += density * tmp;
}
return result;
}

In the i3d context, there is a default metaball geometry (specified by the -g option on the command line to the i3dgen program). If the filename is an empty string, the default geometry will be used.