PK_LOOP_ask_type

PK_ERROR_code_t       PK_LOOP_ask_type
(
--- received arguments ---
PK_LOOP_t             loop,     --- the loop

--- returned arguments ---
PK_LOOP_type_t *const loop_type --- the type of the loop
)


This function classifies the given loop.


The 'loop_type' is one of:

    PK_LOOP_type_vertex_c       loop is just a vertex without any edges
    PK_LOOP_type_wire_c         loop has no interior, e.g. from a wire body
    PK_LOOP_type_outer_c        simple peripheral loop
    PK_LOOP_type_inner_c        loop is a simple hole
    PK_LOOP_type_winding_c      winding loop on a periodic surface
                                e.g. a circle on a cylinder or doughnut
    PK_LOOP_type_inner_sing_c   loop is a hole around the surface singularity
                                e.g. chopping the top off a cone
    PK_LOOP_type_likely_outer_c an apparently peripheral loop on a doubly
                                closed surface
    PK_LOOP_type_likely_inner_c an apparent hole in a doubly closed surface
    PK_LOOP_type_unclear_c      a loop dividing a periodic degenerate surface
                                in two (contains just one pole), or in a face
                                with facet geometry
    PK_LOOP_type_error_c        invalid loop or algorithm failure

The range of types returned depends on the behaviour of the underlying
surface of the face to which the loop belongs.

PK_LOOP_type_vertex_c and PK_LOOP_type_wire_c are generic return
values that cover simple degenerate special cases; on solid and sheet
bodies these loop types correspond to edges and vertices scribed
inside a face.

If the surface is a mesh, PK_LOOP_type_unclear_c is returned.

For planes and other non-periodic surfaces, all 2D loops are either
PK_LOOP_type_outer_c or PK_LOOP_type_inner_c. Each face has one outer
peripheral loop (PK_LOOP_type_outer_c) and any number of holes
(PK_LOOP_type_inner_c).

For cylindrical topology, i.e. surfaces that are periodic in one
parameter only, closed loops that wind once around the periodic
parameter space are of type PK_LOOP_type_winding_c. To define the
periphery of a face in a body that is periodic in one parameter, you
need either:

 * A matched pair of PK_LOOP_type_winding_c loops
 * A PK_LOOP_type_outer_c loop

A surface that is periodic in one parameter may contain any number of
PK_LOOP_type_inner_c.

Surfaces such as cones, that are periodic in one parameter and
degenerate at one end, can be treated as if they were flattened into a
plane (with the degeneracy in the centre of the plane). Thus, a
cone contains just one PK_LOOP_type_outer_c. Conversely, such a
surface can also be treated as a tapered cylinder if it has a winding
loop that completely surrounds the degeneracy; such a loop is of type
PK_LOOP_type_inner_sing_c.

Doubly-periodic surfaces, such as doughnut torii, can have closed loops
that wind one or more times in either parameter; these are of type
PK_LOOP_type_winding_c. Any other loops form either the periphery of
open regions, or holes in the whole closed surface.

 * Those that could contain a winding loop are PK_LOOP_type_likely_inner_c
 * Those that do not wrap around completely in parameter space are
   PK_LOOP_type_likely_outer_c.

Closed loops in spherical topology (closed in one parameter and
degenerate in the other) split the whole surface in two. There are
three types of closed loop on a spherical surface:

 * A PK_LOOP_type_likely_outer_c loop contains neither pole
 * A PK_LOOP_type_likely_inner_c loop surrounds both poles.
 * A PK_LOOP_type_unclear_c loop is used for closed loops which surround
   only one pole, dividing the surface in two.

These types of loop apply to spherical topology in which the
degeneracy is either smooth or sharp in 3D.

Note that the doubly periodic topologies can support arbitrarily
complex loops that are not intuitively holes or peripheries; but
PK_LOOP_type_likely_outer_c and PK_LOOP_type_likely_inner_c are
meaningful for simple faces.