.X_T File Format – Geometry nodes

Common fields

Every curve node

Every surface node

The same seven fields, in the same order, with owner being a face, body, assembly or world.

Senses

The natural tangent of a curve is in the direction of increasing parameter, and the natural normal of a surface is the cross product of ∂P/∂u and ∂P/∂v. In the file, the edge and curve orientation is the curve’s sense field. The face and surface orientation is made from the face’s and the surface’s sense fields: the face normal is parallel to the surface’s natural normal if neither or both senses are positive.

GEOMETRIC_OWNER

Geometry that depends on other geometry points back to it through a doubly linked ring of GEOMETRIC_OWNER nodes. The dependants are: an intersection curve (two surfaces), an SP-curve (its surface), a trimmed curve (its basis curve), a blended edge (two supporting surfaces, two blend-boundary surfaces and a spine curve), an offset surface (its surface), a swept surface (its section curve) and a spun surface (its profile curve). The 2D B-curve of an SP-curve is not a dependant in this sense.

Curves

In each table, the fields after the common seven are shown.

B_CURVE data

NURBS_CURVE node:

The expanded knot vector, in which a knot of multiplicity m appears m times, has n_vertices + degree + 1 values. Knots must not decrease. Multiplicity may be as high as the degree in the interior and degree + 1 at the first and last knot. The curve is fully defined from knot index degree to the last knot index minus degree. A rational curve stores its weights with the vertices as an extra dimension: for a curve in 3D space vertex_dim is 4, the x, y and z values are multiplied by the vertex’s weight and the fourth value is the weight. A periodic curve must also be closed; a closed non-periodic curve needs only positional continuity and cannot be attached to topology.

CURVE_DATA node (the B_CURVE’s data field, auxiliary and optional): self_int : u, and analytic_form : p, which if not null points to a HELIX_CU_FORM node: axis_pt : v; axis_dir : v; point : v; hand : c (+ right-handed, - left-handed); turns : i (the extent relative to the point, e.g. [0 10]); pitch : f (distance along the axis in one turn); tol : f (accuracy of the fit).

Intersection curves

An intersection curve is stored as its two surfaces, an ordered chart of points along it and two limits. The tangent at a point is the cross product of the surface normals there, taking the surface senses into account. Points where that cross product vanishes, or where a surface is degenerate, are terminators; a curve has none in its interior.

CHART node: Base_parameter : f; Base_scale : f; Chart_count : d; Chordal_error : f (may be null); Angular_error : f (may be null); Parameter_error[2] : f (always null, null); Hvec[] : h (the points). Only the position of each hvec is written, though the structure also holds the surface parameters u[2] and v[2], the tangent and the curve parameter t.

LIMIT node: type : c; hvec[] : h.

The curve’s parameterisation is derived from the chart: with chart points P_i, tangents T_i, chord lengths C_i = |P_i+1 − P_i|, cos a_i = T_i·(P_i+1 − P_i) and cos b_i = T_i·(P_i − P_i−1) (with the chords normalised), take f₀ = Base_scale, f_i = (cos b_i / cos a_i) f_i−1, t₀ = Base_parameter and t_i = t_i−1 + C_i−1 f_i−1. The reference states the normalisation only implicitly. A point between chart points is parameterised by projecting onto the tangent line at the previous chart point, and the f_i make the parameterisation C1.

Surfaces

For a spun surface the reference writes the field comments “at low u” and “at low v” for start and end, then “high u” for end_param; treat start and end as the low and high u degeneracies.

Blended edge

R(u,v) = C(u) + r X(u) cos(v a(u)) + r Y(u) sin(v a(u)), where C is the spine curve, r the blend radius, X and Y unit vectors normal to the spine’s tangent and a the angle the boundary points subtend at the spine. u comes from the spine; v is 0 on the first surface’s blend boundary and 1 on the second’s (the other way round if the spine’s sense is negative).

• blend_type: R rolling ball, E cliff edge.
• range: offsets applied to the supporting surfaces. For a rolling ball blend the spine is the intersection of the two surfaces offset by their range entries, each along the natural normal times the range, times −1 if the surface sense is negative.
• thumb_weight is always (1, 1) and boundary always (0, 0) in the reference.
• For a cliff edge, one supporting surface is a blended edge of range (0, 0) whose spine is the cliff edge curve; its type is R.
• start and end are LIMIT nodes (type L) used only when the spine is periodic but the blended edge curve has terminators, to give the extent of the spine.

B_SURFACE data

NURBS_SURF node:

SURFACE_DATA node (data, auxiliary, optional and recommended to be set only by Parasolid): original_uint : i; original_vint : i; extended_uint : i; extended_vint : i; self_int : u; original_u_start, original_u_end, original_v_start, original_v_end, extended_u_start, extended_u_end, extended_v_start, extended_v_end : c; analytic_form_type, swept_form_type, spun_form_type, blend_form_type : c; analytic_form, swept_form, spun_form, blend_form : p. The original and extended intervals record a B-surface extended by Parasolid, explicitly by adding rows or columns of control points, or implicitly (the nurbs data unchanged, out-of-range evaluation allowed). The character fields describe each boundary: D degenerate, P periodic, B bounded, C closed but not periodic, and ? for unset, which is what the extended fields hold if the surface was not extended (with null extended intervals). The swept, spun and blend forms are for future use and must be ? and null. If analytic_form is not null it points to a HELIX_SU_FORM node and analytic_form_type is H: axis_pt : v; axis_dir : v; hand : c; turns : i; pitch : f; gap : f (always zero for now); tol : f. The v parameter increases along the axis.

Point and transform

Not confirmed on this page

• Node type numbers for CYLINDER onwards are those the reference tabulates, paired by position in its table; the table’s extracted columns are mixed, so CHART (40), LIMIT (41), BSPLINE_VERTICES (45), PLANE (50), CYLINDER (51), CONE (52), SPHERE (53), TORUS (54), BLENDED_EDGE (56), BLEND_BOUND (59), OFFSET_SURF (60), SWEPT_SURF (67) and SPUN_SURF (68) should be checked against a real file. The numbers of B_CURVE, B_SURFACE, SP_CURVE, TRIMMED_CURVE, PE_CURVE, PE_SURF, NURBS_CURVE, NURBS_SURF, CURVE_DATA, SURFACE_DATA, KNOT_MULT, KNOT_SET, GEOMETRIC_OWNER, EXT_PE_DATA, INT_PE_DATA, KEY and TRANSFORM are not given here.
• The shape of PE (foreign) data, KEY nodes and the EXT_PE_DATA contents beyond their field names.

Sources

• Parasolid XT Format Reference, October 2006 – UGS Corp.; the Geometry chapter