struct PK_blend_rib_control_s
    {
    PK_blend_output_rib_t output_rib;       --- whether and how ribs are made
                                            --- (PK_blend_output_rib_no_c)
    PK_blend_group_rib_t  group_rib;        --- how to group ribs
                                            --- (PK_blend_group_rib_no_c)
    int                   max_n_ribs;       --- limit ribs made per group (0)
    int                   n_interval_parms; --- number of parameters (0)
    double               *interval_parms;   --- parameters dividing the spine
                                            --- into intervals (NULL)
    };
typedef struct PK_blend_rib_control_s PK_blend_rib_control_t;




This structure controls how blend ribs are output.


                    Blend Rib Controls.

 output_rib       Control how ribs are produced.
                    This token controls under what circumstances face-face
                    blending will produce ribs.  The default is to not
                    output ribs.

 group_rib        Control how to group the ribs produced.
                    Grouping controls how the limit on the number of ribs
                    is applied.  This token defines whether the ribs are
                    grouped by blend face, or by the intervals along the
                    parameter curve defined by interval_parms,
                    or not at all (default).

 max_n_ribs       Limit on number of ribs produced per group.
                    The number of ribs produced in each group will be no
                    greater than this number. The default value is zero,
                    for which no ribs will be produced.

 n_interval_parms Number of interval_parms supplied.
                    Default is zero.

 interval_parms   Interval parameters.
                    These are used to divide the parameter curve into
                    intervals for grouping ribs.  They should have values
                    in the parameter range of the curve.

If output_rib is not set to PK_blend_output_rib_no_c then a curve must be
supplied in the parameter field of the shape structure.

If group_rib is set to PK_blend_group_by_parms and n_interval_parms is
zero, or either zero or one for closed blends, then the effect of max_n_ribs
will be identical to having no grouping.  If group_rib is set to another
value the interval_parms will have no effect.

max_n_ribs is an upper bound on the number of ribs produced per group, and
not necessarily the number that will be returned.

For further details on the use of these options please refer to the Face-Face
Blending Chapter of the Parasolid Functional Description.