Hi Davide,
the modified file works, thank you. In the german handbook there is a section on the gpc plugin, explaining the matrix object structure. Also, the functions of the plugin (all starting with gpc_) are listed and shortly explained in the function reference part (best access via index).
Perhaps the essential part is the matrix format.
mat(r,range(el(p.,1),el(p.,r)),range(range(el(x,1,1),el(x,1,el(p,1))),range(el(x,r,1),el(x,r,el(p,r)))),p,range(el(f.,1),el(f.,r)),range(range(el(y,1,1),el(y,1,el(p,1))),range(el(y,r,1),el(y,r,el(p,r)))),2,3)
r: number of boundaries
p: number vertices
p.i is the number of vertices in the i-th boundary
f.i is the hole-flag. 1=boundary defines a hole, 0 otherwise
x[i;j is the vertex co-ordinate of the j-th vertex in the i-th boundary.
The matrix has 1 + r + sum(p[i) columns.
some day I shall add some examples to the handbook, e.g. for area and centroid calculation.
Martin