1 Modeling Data {#occt_user_guides__modeling_data}
2 ========================
6 @section occt_modat_0 Introduction
8 Modeling Data supplies data structures to represent 2D and 3D geometric models.
10 This manual explains how to use Modeling Data. For advanced information on modeling data, see our <a href="http://www.opencascade.com/content/tutorial-learning">E-learning & Training</a> offerings.
12 @section occt_modat_1 Geometry Utilities
14 Geometry Utilities provide the following services:
15 * Creation of shapes by interpolation and approximation
16 * Direct construction of shapes
17 * Conversion of curves and surfaces to BSpline curves and surfaces
18 * Computation of the coordinates of points on 2D and 3D curves
19 * Calculation of extrema between shapes.
21 @subsection occt_modat_1_1 Interpolations and Approximations
23 In modeling, it is often required to approximate or interpolate points into curves and surfaces. In interpolation, the process is complete when the curve or surface passes through all the points; in approximation, when it is as close to these points as possible.
25 Approximation of Curves and Surfaces groups together a variety of functions used in 2D and 3D geometry for:
26 * the interpolation of a set of 2D points using a 2D BSpline or Bezier curve;
27 * the approximation of a set of 2D points using a 2D BSpline or Bezier curve;
28 * the interpolation of a set of 3D points using a 3D BSpline or Bezier curve, or a BSpline surface;
29 * the approximation of a set of 3D points using a 3D BSpline or Bezier curve, or a BSpline surface.
31 You can program approximations in two ways:
33 * Using high-level functions, designed to provide a simple method for obtaining approximations with minimal programming,
34 * Using low-level functions, designed for users requiring more control over the approximations.
36 @subsubsection occt_modat_1_1_1 Analysis of a set of points
38 The class *PEquation* from *GProp* package allows analyzing a collection or cloud of points and verifying if they are coincident, collinear or coplanar within a given precision. If they are, the algorithm computes the mean point, the mean line or the mean plane of the points. If they are not, the algorithm computes the minimal box, which includes all the points.
40 @subsubsection occt_modat_1_1_2 Basic Interpolation and Approximation
42 Packages *Geom2dAPI* and *GeomAPI* provide simple methods for approximation and interpolation with minimal programming
46 The class *Interpolate* from *Geom2dAPI* package allows building a constrained 2D BSpline curve, defined by a table of points through which the curve passes. If required, the parameter values and vectors of the tangents can be given for each point in the table.
50 The class *Interpolate* from *GeomAPI* package allows building a constrained 3D BSpline curve, defined by a table of points through which the curve passes. If required, the parameter values and vectors of the tangents can be given for each point in the table.
52 @image html /user_guides/modeling_data/images/modeling_data_image003.png "Approximation of a BSpline from scattered points"
53 @image latex /user_guides/modeling_data/images/modeling_data_image003.png "Approximation of a BSpline from scattered points"
55 This class may be instantiated as follows:
57 GeomAPI_Interpolate Interp(Points);
60 From this object, the BSpline curve may be requested as follows:
62 Handle(Geom_BSplineCurve) C = Interp.Curve();
67 The class *PointsToBSpline* from *Geom2dAPI* package allows building a 2DBSpline curve, which approximates a set of points. You have to define the lowest and highest degree of the curve, its continuity and a tolerance value for it.The tolerance value is used to check that points are not too close to each other, or tangential vectors not too small. The resulting BSpline curve will beC2 or second degree continuous, except where a tangency constraint is defined on a point through which the curve passes. In this case, it will be only C1continuous.
71 The class *PointsToBSpline* from GeomAPI package allows building a 3D BSplinecurve, which approximates a set of points. It is necessary to define the lowest and highest degree of the curve, its continuity and tolerance. The tolerance value is used to check that points are not too close to each other,or that tangential vectors are not too small.
73 The resulting BSpline curve will be C2 or second degree continuous, except where a tangency constraint is defined on a point, through which the curve passes. In this case, it will be only C1 continuous. This class is instantiated as follows:
76 GeomAPI_PointsToBSpline
77 Approx(Points,DegMin,DegMax,Continuity, Tol);
80 From this object, the BSpline curve may be requested as follows:
83 Handle(Geom_BSplineCurve) K = Approx.Curve();
86 #### Surface Approximation
88 The class **PointsToBSplineSurface** from GeomAPI package allows building a BSpline surface, which approximates or interpolates a set of points.
90 @subsubsection occt_modat_1_1_3 Advanced Approximation
92 Packages *AppDef* and *AppParCurves* provide low-level functions, allowing more control over the approximations.
94 The low-level functions provide a second API with functions to:
95 * Define compulsory tangents for an approximation. These tangents have origins and extremities.
96 * Approximate a set of curves in parallel to respect identical parameterization.
97 * Smooth approximations. This is to produce a faired curve.
99 You can also find functions to compute:
100 * The minimal box which includes a set of points
101 * The mean plane, line or point of a set of coplanar, collinear or coincident points.
103 #### Approximation by multiple point constraints
105 *AppDef* package provides low-level tools to allow parallel approximation of groups of points into Bezier or B-Spline curves using multiple point constraints.
107 The following low level services are provided:
109 * Definition of an array of point constraints:
111 The class *MultiLine* allows defining a given number of multi-point constraints in order to build the multi-line, multiple lines passing through ordered multiple point constraints.
113 @image html /user_guides/modeling_data/images/modeling_data_image004.png "Definition of a MultiLine using Multiple Point Constraints"
114 @image latex /user_guides/modeling_data/images/modeling_data_image004.png "Definition of a MultiLine using Multiple Point Constraints"
117 * *Pi*, *Qi*, *Ri* ... *Si* can be 2D or 3D points.
118 * Defined as a group: *Pn*, *Qn*, *Rn,* ... *Sn* form a MultipointConstraint. They possess the same passage, tangency and curvature constraints.
119 * *P1*, *P2*, ... *Pn*, or the *Q*, *R*, ... or *S* series represent the lines to be approximated.
121 * Definition of a set of point constraints:
123 The class *MultiPointConstraint* allows defining a multiple point constraint and computing the approximation of sets of points to several curves.
125 * Computation of an approximation of a Bezier curve from a set of points:
127 The class *Compute* allows making an approximation of a set of points to a Bezier curve
129 * Computation of an approximation of a BSpline curve from a set of points:
131 The class *BSplineCompute* allows making an approximation of a set of points to a BSpline curve.
133 * Definition of Variational Criteria:
135 The class *TheVariational* allows fairing the approximation curve to a given number of points using a least squares method in conjunction with a variational criterion, usually the weights at each constraint point.
137 #### Approximation by parametric or geometric constraints
140 *AppParCurves* package provides low-level tools to allow parallel approximation of groups of points into Bezier or B-Spline curve with parametric or geometric constraints, such as a requirement for the curve to pass through given points, or to have a given tangency or curvature at a particular point.
142 The algorithms used include:
143 - the least squares method
144 - a search for the best approximation within a given tolerance value.
146 The following low-level services are provided:
148 * Association of an index to an object:
150 The class *ConstraintCouple* allows you associating an index to an object to compute faired curves using *AppDef_TheVariational*.
152 * Definition of a set of approximations of Bezier curves:
154 The class *MultiCurve* allows defining the approximation of a multi-line made up of multiple Bezier curves.
156 * Definition of a set of approximations of BSpline curves:
158 The class *MultiBSpCurve* allows defining the approximation of a multi-line made up of multiple BSpline curves.
160 * Definition of points making up a set of point constraints
162 The class *MultiPoint* allows defining groups of 2D or 3D points making up a multi-line.
164 #### Example: How to approximate a curve with respect to tangency
166 To approximate a curve with respect to tangency, follow these steps:
168 1. Create an object of type <i> AppDef_MultiPointConstraints</i> from the set of points to approximate and use the method <i> SetTang </i>to set the tangency vectors.
169 2. Create an object of type <i> AppDef_MultiLine </i>from the <i> AppDef_MultiPointConstraint</i>.
170 3. Use <i> AppDef_BSplineCompute</i>, which instantiates <i>Approx_BSplineComputeLine</i> to perform the approximation.
172 @subsection occt_modat_1_2 Direct Construction
174 Direct Construction methods from *gce*, *GC* and *GCE2d* packages provide simplified algorithms to build elementary geometric entities such as lines, circles and curves. They complement the reference definitions provided by the *gp*, *Geom* and *Geom2d* packages.
176 The algorithms implemented by <i> gce</i>, <i> GCE2d</i> and <i> GC</i> packages are simple: there is no creation of objects defined by advanced positional constraints (for more information on this subject, see *Geom2dGcc* and *GccAna*, which describe geometry by constraints).
178 For example, to construct a circle from a point and a radius using the *gp* package, it is necessary to construct axis *Ax2d* before creating the circle. If *gce* package is used, and *Ox* is taken for the axis, it is possible to create a circle directly from a point and a radius.
180 Another example is the class <i>gce_MakeCirc</i> providing a framework for defining eight problems encountered in the geometric construction of circles and implementing the eight related construction algorithms.
182 The object created (or implemented) is an algorithm which can be consulted to find out, in particular:
184 * its result, which is a <i>gp_Circ</i>, and
185 * its status. Here, the status indicates whether or not the construction was successful.
187 If it was unsuccessful, the status gives the reason for the failure.
190 gp_Pnt P1 (0.,0.,0.);
191 gp_Pnt P2 (0.,10.,0.);
192 gp_Pnt P3 (10.,0.,0.);
193 gce_MakeCirc MC (P1,P2,P3);
195 const gp_Circ& C = MC.Value();
199 In addition, <i> gce</i>, <i> GCE2d</i> and <i> GC</i> each have a <i>Root</i> class. This class is the root of all classes in the package, which return a status. The returned status (successful
200 construction or construction error) is described by the enumeration <i>gce_ErrorType</i>.
202 Note, that classes, which construct geometric transformations do not return a status, and therefore do not inherit from *Root*.
205 @subsubsection occt_modat_1_2_1 Simple geometric entities
207 The following algorithms used to build entities from *gp* package are provided by *gce* package.
208 - 2D line parallel to another at a distance,
209 - 2D line parallel to another passing through a point,
210 - 2D circle passing through two points,
211 - 2D circle parallel to another at a distance,
212 - 2D circle parallel to another passing through a point,
213 - 2D circle passing through three points,
214 - 2D circle from a center and a radius,
215 - 2D hyperbola from five points,
216 - 2D hyperbola from a center and two apexes,
217 - 2D ellipse from five points,
218 - 2D ellipse from a center and two apexes,
219 - 2D parabola from three points,
220 - 2D parabola from a center and an apex,
221 - line parallel to another passing through a point,
222 - line passing through two points,
223 - circle coaxial to another passing through a point,
224 - circle coaxial to another at a given distance,
225 - circle passing through three points,
226 - circle with its center, radius, and normal to the plane,
227 - circle with its axis (center + normal),
228 - hyperbola with its center and two apexes,
229 - ellipse with its center and two apexes,
230 - plane passing through three points,
231 - plane from its normal,
232 - plane parallel to another plane at a given distance,
233 - plane parallel to another passing through a point,
234 - plane from an array of points,
235 - cylinder from a given axis and a given radius,
236 - cylinder from a circular base,
237 - cylinder from three points,
238 - cylinder parallel to another cylinder at a given distance,
239 - cylinder parallel to another cylinder passing through a point,
240 - cone from four points,
241 - cone from a given axis and two passing points,
242 - cone from two points (an axis) and two radii,
243 - cone parallel to another at a given distance,
244 - cone parallel to another passing through a point,
245 - all transformations (rotations, translations, mirrors,scaling transformations, etc.).
247 Each class from *gp* package, such as *Circ, Circ2d, Mirror, Mirror2d*, etc., has the corresponding *MakeCirc, MakeCirc2d, MakeMirror, MakeMirror2d*, etc. class from *gce* package.
249 It is possible to create a point using a *gce* package class, then question it to recover the corresponding *gp* object.
251 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
252 gp_Pnt2d Point1,Point2;
254 //Initialization of Point1 and Point2
255 gce_MakeLin2d L = gce_MakeLin2d(Point1,Point2);
256 if (L.Status() == gce_Done() ){
257 gp_Lin2d l = L.Value();
259 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
261 This is useful if you are uncertain as to whether the arguments can create the *gp* object without raising an exception. In the case above, if *Point1* and *Point2* are closer than the tolerance value required by *MakeLin2d*, the function *Status* will return the enumeration *gce_ConfusedPoint*. This tells you why the *gp* object cannot be created. If you know that the points *Point1* and *Point2* are separated by the value exceeding the tolerance value, then you may create the *gp* object directly, as follows:
263 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
264 gp_Lin2d l = gce_MakeLin2d(Point1,Point2);
265 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
267 @subsubsection occt_modat_1_2_2 Geometric entities manipulated by handle
269 *GC* and *GCE2d* packages provides an implementation of algorithms used to build entities from *Geom* and *Geom2D* packages.
270 They implement the same algorithms as the *gce* package, and also contain algorithms for trimmed surfaces and curves.
271 The following algorithms are available:
272 - arc of a circle trimmed by two points,
273 - arc of a circle trimmed by two parameters,
274 - arc of a circle trimmed by one point and one parameter,
275 - arc of an ellipse from an ellipse trimmed by two points,
276 - arc of an ellipse from an ellipse trimmed by two parameters,
277 - arc of an ellipse from an ellipse trimmed by one point and one parameter,
278 - arc of a parabola from a parabola trimmed by two points,
279 - arc of a parabola from a parabola trimmed by two parameters,
280 - arc of a parabola from a parabola trimmed by one point and one parameter,
281 - arc of a hyperbola from a hyperbola trimmed by two points,
282 - arc of a hyperbola from a hyperbola trimmed by two parameters,
283 - arc of a hyperbola from a hyperbola trimmed by one point and one parameter,
284 - segment of a line from two points,
285 - segment of a line from two parameters,
286 - segment of a line from one point and one parameter,
287 - trimmed cylinder from a circular base and a height,
288 - trimmed cylinder from three points,
289 - trimmed cylinder from an axis, a radius, and a height,
290 - trimmed cone from four points,
291 - trimmed cone from two points (an axis) and a radius,
292 - trimmed cone from two coaxial circles.
294 Each class from *GCE2d* package, such as *Circle, Ellipse, Mirror*, etc., has the corresponding *MakeCircle, MakeEllipse, MakeMirror*, etc. class from *Geom2d* package.
295 Besides, the class *MakeArcOfCircle* returns an object of type *TrimmedCurve* from *Geom2d*.
297 Each class from *GC* package, such as *Circle, Ellipse, Mirror*, etc., has the corresponding *MakeCircle, MakeEllipse, MakeMirror*, etc. class from *Geom* package.
298 The following classes return objects of type *TrimmedCurve* from *Geom*:
301 - *MakeArcOfHyperbola*
302 - *MakeArcOfParabola*
305 @subsection occt_modat_1_3 Conversion to and from BSplines
307 The Conversion to and from BSplines component has two distinct purposes:
308 * Firstly, it provides a homogeneous formulation which can be used to describe any curve or surface.
309 This is useful for writing algorithms for a single data structure model.
310 The BSpline formulation can be used to represent most basic geometric objects provided
311 by the components which describe geometric data structures ("Fundamental Geometry Types", "2D Geometry Types" and "3D Geometry Types" components).
312 * Secondly, it can be used to divide a BSpline curve or surface into a series of curves or surfaces,
313 thereby providing a higher degree of continuity. This is useful for writing algorithms
314 which require a specific degree of continuity in the objects to which they are applied.
315 Discontinuities are situated on the boundaries of objects only.
317 The "Conversion to and from BSplines" component is composed of three packages.
319 The <i> Convert </i> package provides algorithms to convert the following into a BSpline curve or surface:
321 * a bounded curve based on an elementary 2D curve (line, circle or conic) from the <i> gp </i> package,
322 * a bounded surface based on an elementary surface (cylinder, cone, sphere or torus) from the <i> gp</i> package,
323 * a series of adjacent 2D or 3D Bezier curves defined by their poles.
325 These algorithms compute the data needed to define the resulting BSpline curve or surface.
326 This elementary data (degrees, periodic characteristics, poles and weights, knots and multiplicities)
327 may then be used directly in an algorithm, or can be used to construct the curve or the surface
328 by calling the appropriate constructor provided by the classes <i>Geom2d_BSplineCurve, Geom_BSplineCurve </i> or <i>Geom_BSplineSurface</i>.
330 The <i>Geom2dConvert</i> package provides the following:
332 * a global function which is used to construct a BSpline curve from a bounded curve based on a 2D curve from the Geom2d package,
333 * a splitting algorithm which computes the points at which a 2D BSpline curve should be cut in order to obtain arcs with the same degree of continuity,
334 * global functions used to construct the BSpline curves created by this splitting algorithm, or by other types of segmentation of the BSpline curve,
335 * an algorithm which converts a 2D BSpline curve into a series of adjacent Bezier curves.
337 The <i> GeomConvert</i> package also provides the following:
339 * a global function used to construct a BSpline curve from a bounded curve based on a curve from the Geom package,
340 * a splitting algorithm, which computes the points at which a BSpline curve should be cut in order to obtain arcs with the same degree of continuity,
341 * global functions to construct BSpline curves created by this splitting algorithm, or by other types of BSpline curve segmentation,
342 * an algorithm, which converts a BSpline curve into a series of adjacent Bezier curves,
343 * a global function to construct a BSpline surface from a bounded surface based on a surface from the Geom package,
344 * a splitting algorithm, which determines the curves along which a BSpline surface should be cut in order to obtain patches with the same degree of continuity,
345 * global functions to construct BSpline surfaces created by this splitting algorithm, or by other types of BSpline surface segmentation,
346 * an algorithm, which converts a BSpline surface into a series of adjacent Bezier surfaces,
347 * an algorithm, which converts a grid of adjacent Bezier surfaces into a BSpline surface.
349 @subsection occt_modat_1_4 Points on Curves
351 The Points on Curves component comprises high level functions providing an API for complex algorithms that compute points on a 2D or 3D curve.
353 The following characteristic points exist on parameterized curves in 3d space:
354 - points equally spaced on a curve,
355 - points distributed along a curve with equal chords,
356 - a point at a given distance from another point on a curve.
358 *GCPnts* package provides algorithms to calculate such points:
359 - *AbscissaPoint* calculates a point on a curve at a given distance from another point on the curve.
360 - *UniformAbscissa* calculates a set of points at a given abscissa on a curve.
361 - *UniformDeflection* calculates a set of points at maximum constant deflection between the curve and the polygon that results from the computed points.
363 ### Example: Visualizing a curve.
365 Let us take an adapted curve **C**, i.e. an object which is an interface between the services provided by either a 2D curve from the package Geom2d (in case of an Adaptor_Curve2d curve) or a 3D curve from the package Geom (in case of an Adaptor_Curve curve), and the services required on the curve by the computation algorithm. The adapted curve is created in the following way:
368 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
369 Handle(Geom2d_Curve) mycurve = ... ;
370 Geom2dAdaptor_Curve C (mycurve) ;
371 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
374 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
375 Handle(Geom_Curve) mycurve = ... ;
376 GeomAdaptor_Curve C (mycurve) ;
377 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
379 The algorithm is then constructed with this object:
381 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
382 GCPnts_UniformDeflection myAlgo () ;
383 Standard_Real Deflection = ... ;
384 myAlgo.Initialize ( C , Deflection ) ;
385 if ( myAlgo.IsDone() )
387 Standard_Integer nbr = myAlgo.NbPoints() ;
388 Standard_Real param ;
389 for ( Standard_Integer i = 1 ; i <= nbr ; i++ )
391 param = myAlgo.Parameter (i) ;
395 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
398 @subsection occt_modat_1_5 Extrema
400 The classes to calculate the minimum distance between points, curves, and surfaces in 2d and 3d are provided by *GeomAPI* and *Geom2dAPI* packages.
402 These packages calculate the extrema of distance between:
404 - point and a surface,
406 - a curve and a surface,
409 ### Extrema between Point and Curve / Surface
411 The *GeomAPI_ProjectPointOnCurve* class allows calculation of all extrema between a point and a curve. Extrema are the lengths of the segments orthogonal to the curve.
412 The *GeomAPI_ProjectPointOnSurface* class allows calculation of all extrema between a point and a surface. Extrema are the lengths of the segments orthogonal to the surface.
413 These classes use the "Projection" criteria for optimization.
415 ### Extrema between Curves
417 The *Geom2dAPI_ExtremaCurveCurve* class allows calculation of all minimal distances between two 2D geometric curves.
418 The *GeomAPI_ExtremaCurveCurve* class allows calculation of all minimal distances between two 3D geometric curves.
419 These classes use Euclidean distance as the criteria for optimization.
421 ### Extrema between Curve and Surface
423 The *GeomAPI_ExtremaCurveSurface* class allows calculation of one extrema between a 3D curve and a surface. Extrema are the lengths of the segments orthogonal to the curve and the surface.
424 This class uses the "Projection" criteria for optimization.
426 ### Extrema between Surfaces
428 The *GeomAPI_ExtremaSurfaceSurface* class allows calculation of one minimal and one maximal distance between two surfaces.
429 This class uses Euclidean distance to compute the minimum, and "Projection" criteria to compute the maximum.
431 @section occt_modat_2 2D Geometry
433 *Geom2d* package defines geometric objects in 2dspace. All geometric entities are STEP processed. The objects are handled by reference.
435 In particular, <i>Geom2d</i> package provides classes for:
436 * description of points, vectors and curves,
437 * their positioning in the plane using coordinate systems,
438 * their geometric transformation, by applying translations, rotations, symmetries, scaling transformations and combinations thereof.
440 The following objects are available:
445 - vector with magnitude,
449 - conic: circle, ellipse, hyperbola, parabola,
450 - rounded curve: trimmed curve, NURBS curve, Bezier curve,
453 Before creating a geometric object, it is necessary to decide how the object is handled.
454 The objects provided by *Geom2d* package are handled by reference rather than by value. Copying an instance copies the handle, not the object, so that a change to one instance is reflected in each occurrence of it.
455 If a set of object instances is needed rather than a single object instance, *TColGeom2d* package can be used. This package provides standard and frequently used instantiations of one-dimensional arrays and sequences for curves from *Geom2d* package. All objects are available in two versions:
456 - handled by reference and
459 The key characteristic of <i> Geom2d </i> curves is that they are parameterized.
460 Each class provides functions to work with the parametric equation of the curve,
461 and, in particular, to compute the point of parameter u on a curve and the derivative vectors of order 1, 2.., N at this point.
463 As a consequence of the parameterization, a <i> Geom2d </i> curve is naturally oriented.
465 Parameterization and orientation differentiate elementary <i>Geom2d</i>curves from their
466 equivalent as provided by <i> gp</i> package. <i>Geom2d</i> package provides conversion
467 functions to transform a <i> Geom2d</i> object into a <i> gp</i> object, and vice-versa, when this is possible.
469 Moreover, <i> Geom2d</i> package provides more complex curves, including Bezier curves,
470 BSpline curves, trimmed curves and offset curves.
472 <i> Geom2d </i> objects are organized according to an inheritance structure over several levels.
474 Thus, an ellipse (specific class <i> Geom2d_Ellipse</i>) is also a conical curve and inherits from the abstract class <i> Geom2d_Conic</i>, while a Bezier curve (concrete class <i> Geom2d_BezierCurve</i>) is also a bounded curve and inherits from the abstract class <i> Geom2d_BoundedCurve</i>; both these examples are also curves (abstract class <i>Geom2d_Curve</i>). Curves, points and vectors inherit from the abstract class <i> Geom2d_Geometry,</i> which describes the properties common to any geometric object from the <i>Geom2d</i> package.
476 This inheritance structure is open and it is possible to describe new objects, which inherit from those provided in the <i>Geom2d</i> package, provided that they respect the behavior of the classes from which they are to inherit.
478 Finally, <i> Geom2d</i> objects can be shared within more complex data structures. This is why they are used within topological data structures, for example.
480 <i>Geom2d</i>package uses the services of the <i> gp</i> package to:
481 * implement elementary algebraic calculus and basic analytic geometry,
482 * describe geometric transformations which can be applied to <i> Geom2d</i> objects,
483 * describe the elementary data structures of <i>Geom2d</i> objects.
485 However, the <i> Geom2d</i> package essentially provides data structures and not algorithms.
486 You can refer to the <i> GCE2d </i> package to find more evolved construction algorithms for <i> Geom2d </i> objects.
488 @section occt_modat_3 3D Geometry
490 The *Geom* package defines geometric objects in 3d space and contains all basic geometric transformations, such as identity, rotation, translation, mirroring, scale transformations, combinations of transformations, etc. as well as special functions depending on the reference definition of the geometric object (e.g. addition of a control point on a B-Spline curve,modification of a curve, etc.). All geometrical entities are STEP processed.
492 In particular, it provides classes for:
493 * description of points, vectors, curves and surfaces,
494 * their positioning in 3D space using axis or coordinate systems, and
495 * their geometric transformation, by applying translations, rotations, symmetries, scaling transformations and combinations thereof.
497 The following objects are available:
502 - Vector with magnitude
506 - Conic: circle, ellipse, hyperbola, parabola
508 - Elementary surface: plane, cylinder, cone, sphere, torus
509 - Bounded curve: trimmed curve, NURBS curve, Bezier curve
510 - Bounded surface: rectangular trimmed surface, NURBS surface,Bezier surface
511 - Swept surface: surface of linear extrusion, surface of revolution
514 The key characteristic of *Geom* curves and surfaces is that they are parameterized.
515 Each class provides functions to work with the parametric equation of the curve or
516 surface, and, in particular, to compute:
517 * the point of parameter u on a curve, or
518 * the point of parameters (u, v) on a surface.
519 together with the derivative vectors of order 1, 2, ... N at this point.
521 As a consequence of this parameterization, a Geom curve or surface is naturally oriented.
523 Parameterization and orientation differentiate elementary Geom curves and surfaces from the classes of the same (or similar) names found in <i> gp</i> package.
524 <i>Geom</i> package also provides conversion functions to transform a Geom object into a <i> gp</i> object, and vice-versa, when such transformation is possible.
526 Moreover, <i> Geom </i>package provides more complex curves and surfaces, including:
527 * Bezier and BSpline curves and surfaces,
528 * swept surfaces, for example surfaces of revolution and surfaces of linear extrusion,
529 * trimmed curves and surfaces, and
530 * offset curves and surfaces.
532 Geom objects are organized according to an inheritance structure over several levels.
533 Thus, a sphere (concrete class <i> Geom_SphericalSurface</i>) is also an elementary surface and inherits from the abstract class <i> Geom_ElementarySurface</i>, while a Bezier surface (concrete class <i> Geom_BezierSurface</i>) is also a bounded surface and inherits from the abstract class <i> Geom_BoundedSurface</i>; both these examples are also surfaces (abstract class <i> Geom_Surface</i>). Curves, points and vectors inherit from the abstract class <i> Geom_Geometry,</i> which describes the properties common to any geometric object from the <i>Geom</i> package.
535 This inheritance structure is open and it is possible to describe new objects, which inherit from those provided in the Geom package, on the condition that they respect the behavior of the classes from which they are to inherit.
537 Finally, Geom objects can be shared within more complex data structures. This is why they are used within topological data structures, for example.
539 If a set of object instances is needed rather than a single object instance, *TColGeom* package can be used. This package provides instantiations of one- and two-dimensional arrays and sequences for curves from *Geom* package. All objects are available in two versions:
540 - handled by reference and
543 The <i> Geom</i> package uses the services of the <i> gp</i> package to:
544 * implement elementary algebraic calculus and basic analytic geometry,
545 * describe geometric transformations which can be applied to Geom objects,
546 * describe the elementary data structures of Geom objects.
548 However, the Geom package essentially provides data structures, not algorithms.
550 You can refer to the <i> GC</i> package to find more evolved construction algorithms for
553 @section occt_modat_4 Properties of Shapes
555 @subsection occt_modat_4_1 Local Properties of Shapes
557 <i>BRepLProp</i> package provides the Local Properties of Shapes component,
558 which contains algorithms computing various local properties on edges and faces in a BRep model.
560 The local properties which may be queried are:
562 * for a point of parameter u on a curve which supports an edge :
564 * the derivative vectors, up to the third degree,
565 * the tangent vector,
567 * the curvature, and the center of curvature;
568 * for a point of parameter (u, v) on a surface which supports a face :
570 * the derivative vectors, up to the second degree,
571 * the tangent vectors to the u and v isoparametric curves,
573 * the minimum or maximum curvature, and the corresponding directions of curvature;
574 * the degree of continuity of a curve which supports an edge, built by the concatenation of two other edges, at their junction point.
576 Analyzed edges and faces are described as <i> BRepAdaptor</i> curves and surfaces,
577 which provide shapes with an interface for the description of their geometric support.
578 The base point for local properties is defined by its u parameter value on a curve, or its (u, v) parameter values on a surface.
580 @subsection occt_modat_4_2 Local Properties of Curves and Surfaces
582 The "Local Properties of Curves and Surfaces" component provides algorithms for computing various local properties on a Geom curve (in 2D or 3D space) or a surface. It is composed of:
584 * <i> Geom2dLProp</i> package, which allows computing Derivative and Tangent vectors (normal and curvature) of a parametric point on a 2D curve;
585 * <i> GeomLProp </i> package, which provides local properties on 3D curves and surfaces
586 * <i> LProp </i> package, which provides an enumeration used to characterize a particular point on a 2D curve.
588 Curves are either <i> Geom_Curve </i> curves (in 3D space) or <i> Geom2d_Curve </i> curves (in the plane).
589 Surfaces are <i> Geom_Surface </i> surfaces. The point on which local properties are calculated
590 is defined by its u parameter value on a curve, and its (u,v) parameter values on a surface.
592 It is possible to query the same local properties for points as mentioned above, and additionally for 2D curves:
594 * the points corresponding to a minimum or a maximum of curvature;
595 * the inflection points.
598 #### Example: How to check the surface concavity
600 To check the concavity of a surface, proceed as follows:
602 1. Sample the surface and compute at each point the Gaussian curvature.
603 2. If the value of the curvature changes of sign, the surface is concave or convex depending on the point of view.
604 3. To compute a Gaussian curvature, use the class <i> SLprops</i> from <i> GeomLProp</i>, which instantiates the generic class <i> SLProps </i>from <i> LProp</i> and use the method <i> GaussianCurvature</i>.
606 @subsection occt_modat_4_2a Continuity of Curves and Surfaces
608 Types of supported continuities for curves and surfaces are described in *GeomAbs_Shape* enumeration.
610 In respect of curves, the following types of continuity are supported (see the figure below):
611 * C0 (*GeomAbs_C0*) - parametric continuity. It is the same as G0 (geometric continuity), so the last one is not represented by separate variable.
612 * G1 (*GeomAbs_G1*) - tangent vectors on left and on right are parallel.
613 * C1 (*GeomAbs_C1*) - indicates the continuity of the first derivative.
614 * G2 (*GeomAbs_G2*) - in addition to G1 continuity, the centers of curvature on left and on right are the same.
615 * C2 (*GeomAbs_C2*) - continuity of all derivatives till the second order.
616 * C3 (*GeomAbs_C3*) - continuity of all derivatives till the third order.
617 * CN (*GeomAbs_CN*) - continuity of all derivatives till the N-th order (infinite order of continuity).
619 *Note:* Geometric continuity (G1, G2) means that the curve can be reparametrized to have parametric (C1, C2) continuity.
621 @image html /user_guides/modeling_data/images/modeling_data_continuity_curves.svg "Continuity of Curves"
622 @image latex /user_guides/modeling_data/images/modeling_data_continuity_curves.svg "Continuity of Curves" width=\\textwidth
624 The following types of surface continuity are supported:
625 * C0 (*GeomAbs_C0*) - parametric continuity (the surface has no points or curves of discontinuity).
626 * G1 (*GeomAbs_G1*) - surface has single tangent plane in each point.
627 * C1 (*GeomAbs_C1*) - indicates the continuity of the first derivatives.
628 * G2 (*GeomAbs_G2*) - in addition to G1 continuity, principal curvatures and directions are continuous.
629 * C2 (*GeomAbs_C2*) - continuity of all derivatives till the second order.
630 * C3 (*GeomAbs_C3*) - continuity of all derivatives till the third order.
631 * CN (*GeomAbs_CN*) - continuity of all derivatives till the N-th order (infinite order of continuity).
633 @image html /user_guides/modeling_data/images/modeling_data_continuity_surfaces.svg "Continuity of Surfaces"
634 @image latex /user_guides/modeling_data/images/modeling_data_continuity_surfaces.svg "Continuity of Surfaces" width=\\textwidth
636 Against single surface, the connection of two surfaces (see the figure above) defines its continuity in each intersection point only. Smoothness of connection is a minimal value of continuities on the intersection curve.
639 @subsection occt_modat_4_2b Regularity of Shared Edges
641 Regularity of an edge is a smoothness of connection of two faces sharing this edge. In other words, regularity is a minimal continuity between connected faces in each point on edge.
643 Edge's regularity can be set by *BRep_Builder::Continuity* method. To get the regularity use *BRep_Tool::Continuity* method.
645 Some algorithms like @ref occt_modalg_6 "Fillet" set regularity of produced edges by their own algorithms. On the other hand, some other algorithms (like @ref occt_user_guides__boolean_operations "Boolean Operations", @ref occt_user_guides__shape_healing "Shape Healing", etc.) do not set regularity. If the regularity is needed to be set correctly on a shape, the method *BRepLib::EncodeRegularity* can be used. It calculates and sets correct values for all edges of the shape.
647 The regularity flag is extensively used by the following high level algorithms: @ref occt_modalg_6_1_2 "Chamfer", @ref occt_modalg_7_3 "Draft Angle", @ref occt_modalg_10 "Hidden Line Removal", @ref occt_modalg_9_2_3 "Gluer".
650 @subsection occt_modat_4_3 Global Properties of Shapes
652 The Global Properties of Shapes component provides algorithms for computing the global
653 properties of a composite geometric system in 3D space, and frameworks to query the computed results.
655 The global properties computed for a system are :
659 * moment about an axis,
660 * radius of gyration about an axis,
661 * principal properties of inertia such as principal axis, principal moments, and principal radius of gyration.
663 Geometric systems are generally defined as shapes. Depending on the way they are analyzed, these shapes will give properties of:
665 * lines induced from the edges of the shape,
666 * surfaces induced from the faces of the shape, or
667 * volumes induced from the solid bounded by the shape.
669 The global properties of several systems may be brought together to give the global properties of the system composed of the sum of all individual systems.
671 The Global Properties of Shapes component is composed of:
672 * seven functions for computing global properties of a shape: one function for lines, two functions for surfaces and four functions for volumes. The choice of functions depends on input parameters and algorithms used for computation (<i>BRepGProp</i> global functions),
673 * a framework for computing global properties for a set of points (<i>GProp_PGProps</i>),
674 * a general framework to bring together the global properties retained by several more elementary frameworks, and provide a general programming interface to consult computed global properties.
676 Packages *GeomLProp* and *Geom2dLProp* provide algorithms calculating the local properties of curves and surfaces
678 A curve (for one parameter) has the following local properties:
684 - Center of curvature.
686 A surface (for two parameters U and V) has the following local properties:
688 - derivative for U and V)
689 - tangent line (for U and V)
693 - main directions of curvature
697 The following methods are available:
698 * *CLProps* -- calculates the local properties of a curve (tangency, curvature,normal);
699 * *CurAndInf2d* -- calculates the maximum and minimum curvatures and the inflection points of 2d curves;
700 * *SLProps* -- calculates the local properties of a surface (tangency, the normal and curvature).
701 * *Continuity* -- calculates regularity at the junction of two curves.
703 Note that the B-spline curve and surface are accepted but they are not cut into pieces of the desired continuity. It is the global continuity, which is seen.
705 @subsection occt_modat_4_4 Adaptors for Curves and Surfaces
707 Some Open CASCADE Technology general algorithms may work theoretically on numerous types of curves or surfaces.
709 To do this, they simply get the services required of the analyzed curve or surface through an interface so as to a single API, whatever the type of curve or surface. These interfaces are called adaptors.
711 For example, <i> Adaptor3d_Curve </i> is the abstract class which provides the required services by an algorithm which uses any 3d curve.
713 <i> GeomAdaptor </i> package provides interfaces:
715 * On a curve lying on a Geom surface;
718 <i> Geom2dAdaptor</i> package provides interfaces :
719 * On a <i>Geom2d</i> curve.
721 <i> BRepAdaptor </i> package provides interfaces:
725 When you write an algorithm which operates on geometric objects, use <i> Adaptor3d</i> (or <i> Adaptor2d</i>) objects.
727 As a result, you can use the algorithm with any kind of object, if you provide for this object an interface derived from *Adaptor3d* or *Adaptor2d*.
728 These interfaces are easy to use: simply create an adapted curve or surface from a *Geom2d* curve, and then use this adapted curve as an argument for the algorithm? which requires it.
731 @section occt_modat_5 Topology
733 OCCT Topology allows accessing and manipulating data of objects without dealing with their 2D or 3D representations. Whereas OCCT Geometry provides a description of objects in terms of coordinates or parametric values, Topology describes data structures of objects in parametric space. These descriptions use location in and restriction of parts of this space.
735 Topological library allows you to build pure topological data structures. Topology defines relationships between simple geometric entities. In this way, you can model complex shapes as assemblies of simpler entities. Due to a built-in non-manifold (or mixed-dimensional) feature, you can build models mixing:
736 * 0D entities such as points;
737 * 1D entities such as curves;
738 * 2D entities such as surfaces;
739 * 3D entities such as volumes.
741 You can, for example, represent a single object made of several distinct bodies containing embedded curves and surfaces connected or non-connected to an outer boundary.
743 Abstract topological data structure describes a basic entity -- a shape, which can be divided into the following component topologies:
744 * Vertex -- a zero-dimensional shape corresponding to a point in geometry;
745 * Edge -- a shape corresponding to a curve, and bound by a vertex at each extremity;
746 * Wire -- a sequence of edges connected by their vertices;
747 * Face -- part of a plane (in 2D geometry) or a surface (in 3D geometry) bounded by a closed wire;
748 * Shell -- a collection of faces connected by some edges of their wire boundaries;
749 * Solid -- a part of 3D space bound by a shell;
750 * Compound solid -- a collection of solids.
752 The wire and the solid can be either infinite or closed.
754 A face with 3D underlying geometry may also refer to a collection of connected triangles that approximate the underlying surface. The surfaces can be undefined leaving the faces represented by triangles only. If so, the model is purely polyhedral.
756 Topology defines the relationship between simple geometric entities, which can thus be linked together to represent complex shapes.
758 Abstract Topology is provided by six packages.
759 The first three packages describe the topological data structure used in Open CASCADE Technology:
761 * <i> TopAbs</i> package provides general resources for topology-driven applications. It contains enumerations that are used to describe basic topological notions: topological shape, orientation and state. It also provides methods to manage these enumerations.
762 * <i> TopLoc </i>package provides resources to handle 3D local coordinate systems: <i> Datum3D</i>and <i> Location</i>. <i> Datum3D</i> describes an elementary coordinate system, while <i> Location</i> comprises a series of elementary coordinate systems.
763 * <i> TopoDS</i> package describes classes to model and build data structures that are purely topological.
765 Three additional packages provide tools to access and manipulate this abstract topology:
767 * <i> TopTools</i> package provides basic tools to use on topological data structures.
768 * <i> TopExp</i> package provides classes to explore and manipulate the topological data structures described in the TopoDS package.
769 * <i> BRepTools </i> package provides classes to explore, manipulate, read and write BRep data structures. These more complex data structures combine topological descriptions with additional geometric information, and include rules for evaluating equivalence of different possible representations of the same object, for example, a point.
771 @subsection occt_modat_5_1 Shape Location
773 A local coordinate system can be viewed as either of the following:
774 - A right-handed trihedron with an origin and three orthonormal vectors. The *gp_Ax2* package corresponds to this definition.
775 - A transformation of a +1 determinant, allowing the transformation of coordinates between local and global references frames. This corresponds to the *gp_Trsf*.
777 *TopLoc* package distinguishes two notions:
778 - *TopLoc_Datum3D* class provides the elementary reference coordinate, represented by a right-handed orthonormal system of axes or by a right-handed unitary transformation.
779 - *TopLoc_Location* class provides the composite reference coordinate made from elementary ones. It is a marker composed of a chain of references to elementary markers. The resulting cumulative transformation is stored in order to avoid recalculating the sum of the transformations for the whole list.
781 @image html /user_guides/modeling_data/images/modeling_data_image005.png "Structure of TopLoc_Location"
782 @image latex /user_guides/modeling_data/images/modeling_data_image005.png "Structure of TopLoc_Location"
784 Two reference coordinates are equal if they are made up of the same elementary coordinates in the same order. There is no numerical comparison. Two coordinates can thus correspond to the same transformation without being equal if they were not built from the same elementary coordinates.
786 For example, consider three elementary coordinates:
788 The composite coordinates are:
794 **NOTE** C3 and C4 are equal because they are both R1 * R2 * R3.
796 The *TopLoc* package is chiefly targeted at the topological data structure, but it can be used for other purposes.
798 Change of coordinates
799 ---------------------
801 *TopLoc_Datum3D* class represents a change of elementary coordinates. Such changes must be shared so this class inherits from *MMgt_TShared*. The coordinate is represented by a transformation *gp_Trsfpackage*. This transformation has no scaling factor.
803 @subsection occt_modat_5_2 Naming shapes, sub-shapes, their orientation and state
805 The **TopAbs** package provides general enumerations describing the basic concepts of topology and methods to handle these enumerations. It contains no classes. This package has been separated from the rest of the topology because the notions it contains are sufficiently general to be used by all topological tools. This avoids redefinition of enumerations by remaining independent of modeling resources. The TopAbs package defines three notions:
806 - **Type** *TopAbs_ShapeEnum*;
807 - **Orientation** *TopAbs_Orientation* ;
808 - **State** *StateTopAbs_State*
810 @subsubsection occt_modat_5_2_1 Topological types
812 TopAbs contains the *TopAbs_ShapeEnum* enumeration,which lists the different topological types:
813 - COMPOUND -- a group of any type of topological objects.
814 - COMPSOLID -- a composite solid is a set of solids connected by their faces. It expands the notions of WIRE and SHELL to solids.
815 - SOLID -- a part of space limited by shells. It is three dimensional.
816 - SHELL -- a set of faces connected by their edges. A shell can be open or closed.
817 - FACE -- in 2D it is a part of a plane; in 3D it is a part of a surface. Its geometry is constrained (trimmed) by contours. It is two dimensional.
818 - WIRE -- a set of edges connected by their vertices. It can be an open or closed contour depending on whether the edges are linked or not.
819 - EDGE -- a topological element corresponding to a restrained curve. An edge is generally limited by vertices. It has one dimension.
820 - VERTEX -- a topological element corresponding to a point. It has zero dimension.
821 - SHAPE -- a generic term covering all of the above.
823 A topological model can be considered as a graph of objects with adjacency relationships. When modeling a part in 2D or 3D space it must belong to one of the categories listed in the ShapeEnum enumeration. The TopAbspackage lists all the objects, which can be found in any model. It cannot be extended but a subset can be used. For example, the notion of solid is useless in 2D.
825 The terms of the enumeration appear in order from the most complex to the most simple, because objects can contain simpler objects in their description. For example, a face references its wires, edges, and vertices.
826 @image html /user_guides/modeling_data/images/modeling_data_image006.png "ShapeEnum"
827 @image latex /user_guides/modeling_data/images/modeling_data_image006.png "ShapeEnum"
829 @subsubsection occt_modat_5_2_2 Orientation
831 The notion of orientation is represented by the **TopAbs_Orientation** enumeration. Orientation is a generalized notion of the sense of direction found in various modelers. This is used when a shape limits a geometric domain; and is closely linked to the notion of boundary. The three cases are the following:
832 - Curve limited by a vertex.
833 - Surface limited by an edge.
834 - Space limited by a face.
836 In each case the topological form used as the boundary of a geometric domain of a higher dimension defines two local regions of which one is arbitrarily considered as the **default region**.
838 For a curve limited by a vertex the default region is the set of points with parameters greater than the vertex. That is to say it is the part of the curve after the vertex following the natural direction along the curve.
840 For a surface limited by an edge the default region is on the left of the edge following its natural direction. More precisely it is the region pointed to by the vector product of the normal vector to the surface and the vector tangent to the curve.
842 For a space limited by a face the default region is found on the negative side of the normal to the surface.
844 Based on this default region the orientation allows definition of the region to be kept, which is called the *interior* or *material*. There are four orientations defining the interior.
846 | Orientation | Description |
847 | :--------- | :--------------------------------- |
848 | FORWARD | The interior is the default region. |
849 | REVERSED | The interior is the region complementary to the default. |
850 | INTERNAL | The interior includes both regions. The boundary lies inside the material. For example a surface inside a solid. |
851 | EXTERNAL | The interior includes neither region. The boundary lies outside the material. For example an edge in a wire-frame model. |
853 @image html /user_guides/modeling_data/images/modeling_data_image007.png "Four Orientations"
854 @image latex /user_guides/modeling_data/images/modeling_data_image007.png "Four Orientations"
856 The notion of orientation is a very general one, and it can be used in any context where regions or boundaries appear. Thus, for example, when describing the intersection of an edge and a contour it is possible to describe not only the vertex of intersection but also how the edge crosses the contour considering it as a boundary. The edge would therefore be divided into two regions: exterior and interior and the intersection vertex would be the boundary. Thus an orientation can be associated with an intersection vertex as in the following figure:
858 | Orientation | Association |
859 | :-------- | :-------- |
860 | FORWARD | Entering |
861 | REVERSED | Exiting |
862 | INTERNAL | Touching from inside |
863 | EXTERNAL | Touching from outside |
865 @image html /user_guides/modeling_data/images/modeling_data_image008.png "Four orientations of intersection vertices"
866 @image latex /user_guides/modeling_data/images/modeling_data_image008.png "Four orientations of intersection vertices"
869 Along with the Orientation enumeration the *TopAbs* package defines four methods:
871 @subsubsection occt_modat_5_2_3 State
873 The **TopAbs_State** enumeration described the position of a vertex or a set of vertices with respect to a region. There are four terms:
875 |Position | Description |
876 | :------ | :------- |
877 |IN | The point is interior. |
878 |OUT | The point is exterior. |
879 |ON | The point is on the boundary(within tolerance). |
880 |UNKNOWN | The state of the point is indeterminate. |
882 The UNKNOWN term has been introduced because this enumeration is often used to express the result of a calculation, which can fail. This term can be used when it is impossible to know if a point is inside or outside, which is the case with an open wire or face.
884 @image html /user_guides/modeling_data/images/modeling_data_image009.png "The four states"
885 @image latex /user_guides/modeling_data/images/modeling_data_image009.png "The four states"
887 The State enumeration can also be used to specify various parts of an object. The following figure shows the parts of an edge intersecting a face.
889 @image html /user_guides/modeling_data/images/modeling_data_image010.png "State specifies the parts of an edge intersecting a face"
890 @image latex /user_guides/modeling_data/images/modeling_data_image010.png "State specifies the parts of an edge intersecting a face"
892 @subsection occt_modat_5_3 Manipulating shapes and sub-shapes
894 The *TopoDS* package describes the topological data structure with the following characteristics:
895 - reference to an abstract shape with neither orientation nor location.
896 - Access to the data structure through the tool classes.
898 As stated above, OCCT Topology describes data structures of objects in parametric space. These descriptions use localization in and restriction of parts of this space. The types of shapes, which can be described in these terms, are the vertex, the face and the shape. The vertex is defined in terms of localization in parametric space, and the face and shape, in terms of restriction of this space.
900 OCCT topological descriptions also allow the simple shapes defined in these terms to be combined into sets. For example, a set of edges forms a wire; a set of faces forms a shell, and a set of solids forms a composite solid (CompSolid in Open CASCADE Technology). You can also combine shapes of either sort into compounds. Finally, you can give a shape an orientation and a location.
902 Listing shapes in order of complexity from vertex to composite solid leads us to the notion of the data structure as knowledge of how to break a shape down into a set of simpler shapes. This is in fact, the purpose of the *TopoDS* package.
904 The model of a shape is a shareable data structure because it can be used by other shapes. (An edge can be used by more than one face of a solid). A shareable data structure is handled by reference. When a simple reference is insufficient, two pieces of information are added: an orientation and a local coordinate reference.
905 - An orientation tells how the referenced shape is used in a boundary (*Orientation* from *TopAbs*).
906 - A local reference coordinate (*Location* from *TopLoc*) allows referencing a shape at a position different from that of its definition.
908 The **TopoDS_TShape** class is the root of all shape descriptions. It contains a list of shapes. Classes inheriting **TopoDS_TShape** can carry the description of a geometric domain if necessary (for example, a geometric point associated with a TVertex). A **TopoDS_TShape** is a description of a shape in its definition frame of reference. This class is manipulated by reference.
910 The **TopoDS_Shape** class describes a reference to a shape. It contains a reference to an underlying abstract shape, an orientation,and a local reference coordinate. This class is manipulated by value and thus cannot be shared.
912 The class representing the underlying abstract shape is never referenced directly. The *TopoDS_Shape* class is always used to refer to it.
914 The information specific to each shape (the geometric support) is always added by inheritance to classes deriving from **TopoDS_TShape**. The following figures show the example of a shell formed from two faces connected by an edge.
916 @image html /user_guides/modeling_data/images/modeling_data_image011.png "Structure of a shell formed from two faces"
917 @image latex /user_guides/modeling_data/images/modeling_data_image011.png "Structure of a shell formed from two faces"
919 @image html /user_guides/modeling_data/images/modeling_data_image012.png "Data structure of the above shell"
920 @image latex /user_guides/modeling_data/images/modeling_data_image012.png "Data structure of the above shell"
922 In the previous diagram, the shell is described by the underlying shape TS, and the faces by TF1 and TF2. There are seven edges from TE1 to TE7 and six vertices from TV1 to TV6.
924 The wire TW1 references the edges from TE1 to TE4; TW2 references from TE4 to TE7.
926 The vertices are referenced by the edges as follows:TE1(TV1,TV4), TE2(TV1,TV2), TE3(TV2,TV3), TE4(TV3,TV4), TE5(TV4,TV5), TE6(T5,TV6),TE7(TV3,TV6).
928 **Note** that this data structure does not contain any *back references*. All references go from more complex underlying shapes to less complex ones. The techniques used to access the information are described later. The data structure is as compact as possible. Sub-objects can be shared among different objects.
930 Two very similar objects, perhaps two versions of the same object, might share identical sub-objects. The usage of local coordinates in the data structure allows the description of a repetitive sub-structure to be shared.
932 The compact data structure avoids the loss of information associated with copy operations which are usually used in creating a new version of an object or when applying a coordinate change.
934 The following figure shows a data structure containing two versions of a solid. The second version presents a series of identical holes bored at different positions. The data structure is compact and yet keeps all information on the sub-elements.
936 The three references from *TSh2* to the underlying face *TFcyl* have associated local coordinate systems, which correspond to the successive positions of the hole.
937 @image html /user_guides/modeling_data/images/modeling_data_image013.png "Data structure containing two versions of a solid"
938 @image latex /user_guides/modeling_data/images/modeling_data_image013.png "Data structure containing two versions of a solid"
940 Classes inheriting TopoDS_Shape
941 ------------------------------
942 *TopoDS* is based on class *TopoDS_Shape* and the class defining its underlying shape. This has certain advantages, but the major drawback is that these classes are too general. Different shapes they could represent do not type them (Vertex, Edge, etc.) hence it is impossible to introduce checks to avoid incoherences such as inserting a face in an edge.
944 *TopoDS* package offers two sets of classes, one set inheriting the underlying shape with neither orientation nor location and the other inheriting *TopoDS_Shape*, which represent the standard topological shapes enumerated in *TopAbs* package.
946 The following classes inherit Shape : *TopoDS_Vertex, TopoDS_Edge, TopoDS_Wire, TopoDS_Face, TopoDS_Shell, TopoDS_Solid,TopoDS_CompSolid,* and *TopoDS_Compound*. In spite of the similarity of names with those inheriting from **TopoDS_TShape** there is a profound difference in the way they are used.
948 *TopoDS_Shape* class and the classes, which inherit from it, are the natural means to manipulate topological objects. *TopoDS_TShape* classes are hidden. *TopoDS_TShape* describes a class in its original local coordinate system without orientation. *TopoDS_Shape* is a reference to *TopoDS_TShape* with an orientation and a local reference.
950 *TopoDS_TShape* class is deferred; *TopoDS_Shape* class is not. Using *TopoDS_Shape* class allows manipulation of topological objects without knowing their type. It is a generic form. Purely topological algorithms often use the *TopoDS_Shape* class.
952 *TopoDS_TShape* class is manipulated by reference; TopoDS_Shape class by value. A TopoDS_Shape is nothing more than a reference enhanced with an orientation and a local coordinate. The sharing of *TopoDS_Shapes* is meaningless. What is important is the sharing of the underlying *TopoDS_TShapes*. Assignment or passage in argument does not copy the data structure: this only creates new *TopoDS_Shapes* which refer to the same *TopoDS_TShape*.
954 Although classes inheriting *TopoDS_TShape* are used for adding extra information, extra fields should not be added in a class inheriting from TopoDS_Shape. Classes inheriting from TopoDS_Shape serve only to specialize a reference in order to benefit from static type control (carried out by the compiler). For example, a routine that receives a *TopoDS_Face* in argument is more precise for the compiler than the one, which receives a *TopoDS_Shape*. It is pointless to derive other classes than those found inTopoDS. All references to a topological data structure are made with the Shape class and its inheritors defined in *TopoDS*.
956 There are no constructors for the classes inheriting from the *TopoDS_Shape* class, otherwise the type control would disappear through **implicit casting** (a characteristic of C++). The TopoDS package provides package methods for **casting** an object of the TopoDS_Shape class in one of these sub-classes, with type verification.
958 The following example shows a routine receiving an argument of the *TopoDS_Shape* type, then putting it into a variable V if it is a vertex or calling the method ProcessEdge if it is an edge.
961 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
962 #include <TopoDS_Vertex.hxx>
963 #include <TopoDS_Edge.hxx>
964 #include <TopoDS_Shape.hxx>
967 void ProcessEdge(const TopoDS_Edge&);
969 void Process(const TopoDS_Shape& aShape) {
970 if (aShape.Shapetype() == TopAbs_VERTEX) {
972 V = TopoDS::Vertex(aShape); // Also correct
973 TopoDS_Vertex V2 = aShape; // Rejected by the compiler
974 TopoDS_Vertex V3 = TopoDS::Vertex(aShape); // Correct
976 else if (aShape.ShapeType() == TopAbs_EDGE){
977 ProcessEdge(aShape) ; // This is rejected
978 ProcessEdge(TopoDS::Edge(aShape)) ; // Correct
981 cout <<"Neither a vertex nor an edge ?";
982 ProcessEdge(TopoDS::Edge(aShape)) ;
983 // OK for compiler but an exception will be raised at run-time
986 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
990 @subsection occt_modat_5_4 Exploration of Topological Data Structures
992 The *TopExp* package provides tools for exploring the data structure described with the *TopoDS* package. Exploring a topological structure means finding all sub-objects of a given type, for example, finding all the faces of a solid.
994 The TopExp package provides the class *TopExp_Explorer* to find all sub-objects of a given type. An explorer is built with:
995 - The shape to be explored.
996 - The type of shapes to be found e.g. VERTEX, EDGE with the exception of SHAPE, which is not allowed.
997 - The type of Shapes to avoid. e.g. SHELL, EDGE. By default, this type is SHAPE. This default value means that there is no restriction on the exploration.
1001 The Explorer visits the whole structure in order to find the shapes of the requested type not contained in the type to avoid. The example below shows how to find all faces in the shape *S*:
1004 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1008 for (Ex.Init(S,TopAbs_FACE); Ex.More(); Ex.Next()) {
1009 ProcessFace(Ex.Current());
1012 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1014 Find all the vertices which are not in an edge
1016 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1017 for (Ex.Init(S,TopAbs_VERTEX,TopAbs_EDGE); ...)
1018 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1021 Find all the faces in a SHELL, then all the faces not in a SHELL:
1024 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1026 TopExp_Explorer Ex1, Ex2;
1028 for (Ex1.Init(S,TopAbs_SHELL);Ex1.More(); Ex1.Next()){
1030 for (Ex2.Init(Ex1.Current(),TopAbs_FACE);Ex2.More();
1032 //visit all the faces of the current shell
1033 ProcessFaceinAshell(Ex2.Current());
1037 for(Ex1.Init(S,TopAbs_FACE,TopAbs_SHELL);Ex1.More(); Ex1.Next()){
1038 // visit all faces not ina shell.
1039 ProcessFace(Ex1.Current());
1042 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1044 The Explorer presumes that objects contain only objects of an equal or inferior type. For example, if searching for faces it does not look at wires, edges, or vertices to see if they contain faces.
1046 The *MapShapes* method from *TopExp* package allows filling a Map. An exploration using the Explorer class can visit an object more than once if it is referenced more than once. For example, an edge of a solid is generally referenced by two faces. To process objects only once, they have to be placed in a Map.
1049 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1050 void TopExp::MapShapes (const TopoDS_Shape& S,
1051 const TopAbs_ShapeEnum T,
1052 TopTools_IndexedMapOfShape& M)
1054 TopExp_Explorer Ex(S,T);
1056 M.Add(Ex.Current());
1060 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1062 In the following example all faces and all edges of an object are drawn in accordance with the following rules:
1063 - The faces are represented by a network of *NbIso* iso-parametric lines with *FaceIsoColor* color.
1064 - The edges are drawn in a color, which indicates the number of faces sharing the edge:
1065 - *FreeEdgeColor* for edges, which do not belong to a face (i.e. wireframe element).
1066 - *BorderEdgeColor* for an edge belonging to a single face.
1067 - *SharedEdgeColor* for an edge belonging to more than one face.
1068 - The methods *DrawEdge* and *DrawFaceIso* are also available to display individual edges and faces.
1070 The following steps are performed:
1071 1. Storing the edges in a map and create in parallel an array of integers to count the number of faces sharing the edge. This array is initialized to zero.
1072 2. Exploring the faces. Each face is drawn.
1073 3. Exploring the edges and for each of them increment the counter of faces in the array.
1074 4. From the Map of edges, drawing each edge with the color corresponding to the number of faces.
1076 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1077 void DrawShape ( const TopoDS_Shape& aShape,
1078 const Standard_Integer nbIsos,
1079 const Color FaceIsocolor,
1080 const Color FreeEdgeColor,
1081 const Color BorderEdgeColor,
1082 const Color SharedEdgeColor)
1084 // Store the edges in aMap.
1085 TopTools_IndexedMapOfShape edgemap;
1086 TopExp::MapShapes(aShape,TopAbs_EDGE,edgeMap);
1087 // Create an array set to zero.
1088 TColStd_Array1OfInteger faceCount(1,edgeMap.Extent());
1090 // Explore the faces.
1091 TopExp_Explorer expFace(aShape,TopAbs_FACE);
1092 while (expFace.More()) {
1093 //Draw the current face.
1094 DrawFaceIsos(TopoDS::Face(expFace.Current()),nbIsos,FaceIsoColor);
1095 // Explore the edges ofthe face.
1096 TopExp_Explorer expEdge(expFace.Current(),TopAbs_EDGE);
1097 while (expEdge.More()) {
1098 //Increment the face count for this edge.
1099 faceCount(edgemap.FindIndex(expEdge.Current()))++;
1104 //Draw the edges of theMap
1106 for (i=1;i<=edgemap.Extent();i++) {
1107 switch (faceCount(i)) {
1109 DrawEdge(TopoDS::Edge(edgemap(i)),FreeEdgeColor);
1112 DrawEdge(TopoDS::Edge(edgemap(i)),BorderEdgeColor);
1115 DrawEdge(TopoDS::Edge(edgemap(i)),SharedEdgeColor);
1120 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1122 @subsection occt_modat_5_5 Lists and Maps of Shapes
1124 **TopTools** package contains tools for exploiting the *TopoDS* data structure. It is an instantiation of the tools from *TCollection* package with the Shape classes of *TopoDS*.
1127 * *TopTools_Array1OfShape, HArray1OfShape* -- instantiation of the *TCollection_Array1* and *TCollection_HArray1* with *TopoDS_Shape*.
1128 * *TopTools_SequenceOfShape* -- instantiation of the *TCollection_Sequence* with *TopoDS_Shape*.
1129 * *TopTools_MapOfShape* - instantiation of the *TCollection_Map*. Allows the construction of sets of shapes.
1130 * *TopTools_IndexedMapOfShape* - instantiation of the *TCollection_IndexedMap*. Allows the construction of tables of shapes and other data structures.
1132 With a *TopTools_Map*, a set of references to Shapes can be kept without duplication.
1133 The following example counts the size of a data structure as a number of *TShapes*.
1136 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1137 #include <TopoDS_Iterator.hxx>
1138 Standard_Integer Size(const TopoDS_Shape& aShape)
1140 // This is a recursive method.
1141 // The size of a shape is1 + the sizes of the subshapes.
1143 Standard_Integer size = 1;
1144 for (It.Initialize(aShape);It.More();It.Next()) {
1145 size += Size(It.Value());
1149 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1151 This program is incorrect if there is sharing in the data structure.
1153 Thus for a contour of four edges it should count 1 wire + 4 edges +4 vertices with the result 9, but as the vertices are each shared by two edges this program will return 13. One solution is to put all the Shapes in a Map so as to avoid counting them twice, as in the following example:
1156 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1157 #include <TopoDS_Iterator.hxx>
1158 #include <TopTools_MapOfShape.hxx>
1160 void MapShapes(const TopoDS_Shape& aShape,
1161 TopTools_MapOfShape& aMap)
1163 //This is a recursive auxiliary method. It stores all subShapes of aShape in a Map.
1164 if (aMap.Add(aShape)) {
1165 //Add returns True if aShape was not already in the Map.
1167 for (It.Initialize(aShape);It.More();It.Next()){
1168 MapShapes(It.Value(),aMap);
1173 Standard_Integer Size(const TopoDS_Shape& aShape)
1175 // Store Shapes in a Mapand return the size.
1176 TopTools_MapOfShape M;
1177 MapShapes(aShape,M);
1180 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1182 **Note** For more details about Maps please, refer to the TCollection documentation. (Foundation Classes Reference Manual)
1184 The following example is more ambitious and writes a program which copies a data structure using an *IndexedMap*. The copy is an identical structure but it shares nothing with the original. The principal algorithm is as follows:
1185 - All Shapes in the structure are put into an *IndexedMap*.
1186 - A table of Shapes is created in parallel with the map to receive the copies.
1187 - The structure is copied using the auxiliary recursive function,which copies from the map to the array.
1189 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1190 #include <TopoDS_Shape.hxx>
1191 #include <TopoDS_Iterator.hxx>
1192 #include <TopTools_IndexedMapOfShape.hxx>
1193 #include <TopTools_Array1OfShape.hxx>
1194 #include <TopoDS_Location.hxx>
1196 TopoDS_Shape Copy(const TopoDS_Shape& aShape,
1197 const TopoDS_Builder& aBuilder)
1199 // Copies the wholestructure of aShape using aBuilder.
1200 // Stores all thesub-Shapes in an IndexedMap.
1201 TopTools_IndexedMapOfShape theMap;
1205 TopLoc_Location Identity;
1207 S.Location(Identity);
1208 S.Orientation(TopAbs_FORWARD);
1210 for (i=1; i<= theMap.Extent(); i++) {
1211 for(It.Initialize(theMap(i)); It.More(); It.Next()) {
1213 S.Location(Identity);
1214 S.Orientation(TopAbs_FORWARD);
1219 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1222 In the above example, the index *i* is that of the first object not treated in the Map. When *i* reaches the same size as the Map this means that everything has been treated. The treatment consists in inserting in the Map all the sub-objects, if they are not yet in the Map, they are inserted with an index greater than *i*.
1224 **Note** that the objects are inserted with a local reference set to the identity and a FORWARD orientation. Only the underlying TShape is of great interest.
1227 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1228 //Create an array to store the copies.
1229 TopTools_Array1OfShapetheCopies(1,theMap.Extent());
1231 // Use a recursivefunction to copy the first element.
1232 void AuxiliaryCopy (Standard_Integer,
1233 const TopTools_IndexedMapOfShape &,
1234 TopTools_Array1OfShape &,
1235 const TopoDS_Builder&);
1237 AuxiliaryCopy(1,theMap,theCopies,aBuilder);
1239 // Get the result with thecorrect local reference and orientation.
1241 S.Location(aShape.Location());
1242 S.Orientation(aShape.Orientation());
1244 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1246 Below is the auxiliary function, which copies the element of rank *i* from the map to the table. This method checks if the object has been copied; if not copied, then an empty copy is performed into the table and the copies of all the sub-elements are inserted by finding their rank in the map.
1249 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1250 void AuxiliaryCopy(Standard_Integer index,
1251 const TopTools_IndexedMapOfShapes& sources,
1252 TopTools_Array1OfShape& copies,
1253 const TopoDS_Builder& aBuilder)
1255 //If the copy is a null Shape the copy is not done.
1256 if (copies(index).IsNull()) {
1257 copies(index) =sources(index).EmptyCopied();
1258 //Insert copies of the sub-shapes.
1261 TopLoc_Location Identity;
1262 for(It.Initialize(sources(index)),It.More(), It.Next ()) {
1264 S.Location(Identity);
1265 S.Orientation(TopAbs_FORWARD);
1266 AuxiliaryCopy(sources.FindIndex(S),sources,copies,aBuilder);
1267 S.Location(It.Value().Location());S.Orientation(It.Value().Orientation()); aBuilder.Add(copies(index),S);
1271 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1273 @subsubsection occt_modat_5_5_1 Wire Explorer
1275 *BRepTools_WireExplorer* class can access edges of a wire in their order of connection.
1277 For example, in the wire in the image we want to recuperate the edges in the order {e1, e2, e3,e4, e5} :
1279 @image html /user_guides/modeling_data/images/modeling_data_image014.png "A wire composed of 6 edges."
1280 @image latex /user_guides/modeling_data/images/modeling_data_image014.png "A wire composed of 6 edges."
1282 *TopExp_Explorer*, however, recuperates the lines in any order.
1284 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1285 TopoDS_Wire W = ...;
1286 BRepTools_WireExplorer Ex;
1287 for(Ex.Init(W); Ex.More(); Ex.Next()) {
1288 ProcessTheCurrentEdge(Ex.Current());
1289 ProcessTheVertexConnectingTheCurrentEdgeToThePrevious
1290 One(Ex.CurrentVertex());
1292 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1294 @subsection occt_modat_5_6 Storage of shapes
1296 *BRepTools* and *BinTools* packages contain methods *Read* and *Write* allowing to read and write a Shape to/from a stream or a file.
1297 The methods provided by *BRepTools* package use ASCII storage format; *BinTools* package uses binary format.
1298 Each of these methods has two arguments:
1299 - a *TopoDS_Shape* object to be read/written;
1300 - a stream object or a file name to read from/write to.
1302 The following sample code reads a shape from ASCII file and writes it to a binary one:
1304 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
1305 TopoDS_Shape aShape;
1306 if (BRepTools::Read (aShape, "source_file.txt")) {
1307 BinTools::Write (aShape, "result_file.bin");
1309 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~