111 KiB
Modeling Algorithms
@tableofcontents
@section occt_modalg_1 Introduction
@subsection occt_modalg_1_1 The Modeling Algorithms Module
This manual explains how to use the Modeling Algorithms. It provides basic documentation on modeling algorithms. For advanced information on Modeling Algorithms, see our offerings on our web site at www.opencascade.org/support/training/
The Modeling Algorithms module brings together a wide range of topological algorithms used in modeling. Along with these tools, you will find the geometric algorithms, which they call.
The algorithms available are divided into:
- Geometric tools
- Topological tools
- The Topology API
@subsection occt_modalg_1_2 The Topology API
The Topology API of Open CASCADE Technology (OCCT) includes the following six packages:
- BRepAlgoAPI
- BRepBuilderAPI
- BRepFilletAPI
- BRepFeat
- BRepOffsetAPI
- BRepPrimAPI
The classes in these six packages provide the user with a simple and powerful interface.
- A simple interface: a function call works ideally,
- A powerful interface: including error handling and access to extra information provided by the algorithms.
As an example, the class BRepBuilderAPI_MakeEdge can be used to create a linear edge from two points.
gp_Pnt P1(10,0,0), P2(20,0,0);
TopoDS_Edge E = BRepBuilderAPI_MakeEdge(P1,P2);
This is the simplest way to create edge E from two points P1, P2, but the developer can test for errors when he is not as confident of the data as in the previous example.
#include <gp_Pnt.hxx>
#include <TopoDS_Edge.hxx>
#include <BRepBuilderAPI_MakeEdge.hxx>
void EdgeTest()
{
gp_Pnt P1;
gp_Pnt P2;
BRepBuilderAPI_MakeEdge ME(P1,P2);
if (!ME.IsDone())
{
// doing ME.Edge() or E = ME here
// would raise StdFail_NotDone
Standard_DomainError::Raise
(“ProcessPoints::Failed to createan edge”);
}
TopoDS_Edge E = ME;
}
In this example an intermediary object ME has been introduced. This can be tested for the completion of the function before accessing the result. More information on error handling in the topology programming interface can be found in the next section.
BRepBuilderAPI_MakeEdge provides valuable information. For example, when creating an edge from two points, two vertices have to be created from the points. Sometimes you may be interested in getting these vertices quickly without exploring the new edge. Such information can be provided when using a class. The following example shows a function creating an edge and two vertices from two points.
void MakeEdgeAndVertices(const gp_Pnt& P1,
const gp_Pnt& P2,
TopoDS_Edge& E,
TopoDS_Vertex& V1,
TopoDS_Vertex& V2)
{
BRepBuilderAPI_MakeEdge ME(P1,P2);
if (!ME.IsDone()) {
Standard_DomainError::Raise
(“MakeEdgeAndVerices::Failed to create an edge”);
}
E = ME;
V1 = ME.Vextex1();
V2 = ME.Vertex2();
The BRepBuilderAPI_MakeEdge class provides the two methods Vertex1 and Vertex2, which return the two vertices used to create the edge.
How can BRepBuilderAPI_MakeEdge be both a function and a class? It can do this because it uses the casting capabilities of C++. The BRepBuilderAPI_MakeEdge class has a method called Edge; in the previous example the line E = ME could have been written.
E = ME.Edge();
This instruction tells the C++ compiler that there is an implicit casting of a BRepBuilderAPI_MakeEdge into a TopoDS_Edge using the Edge method. It means this method is automatically called when a BRepBuilderAPI_MakeEdge is found where a TopoDS_Edge is required.
This feature allows you to provide classes, which have the simplicity of function calls when required and the power of classes when advanced processing is necessary. All the benefits of this approach are explained when describing the topology programming interface classes.
@subsubsection occt_modalg_1_2_1 Error Handling in the Topology API
A method can report an error in the two following situations:
- The data or arguments of the method are incorrect, i.e. they do not respect the restrictions specified by the methods in its specifications. Typical example: creating a linear edge from two identical points is likely to lead to a zero divide when computing the direction of the line.
- Something unexpected happened. This situation covers every error not included in the first category. Including: interruption, programming errors in the method or in another method called by the first method, bad specifications of the arguments (i.e. a set of arguments that was not expected to fail).
The second situation is supposed to become increasingly exceptional as a system is debugged and it is handled by the exception mechanism. Using exceptions avoids handling error statuses in the call to a method: a very cumbersome style of programming.
In the first situation, an exception is also supposed to be raised because the calling method should have verified the arguments and if it did not do so, there is a bug. For example if before calling MakeEdge you are not sure that the two points are non-identical, this situation must be tested.
Making those validity checks on the arguments can be tedious to program and frustrating as you have probably correctly surmised that the method will perform the test twice. It does not trust you. As the test involves a great deal of computation, performing it twice is also time-consuming.
Consequently, you might be tempted to adopt the highly inadvisable style of programming illustrated in the following example:
#include <Standard_ErrorHandler.hxx>
try {
TopoDS_Edge E = BRepBuilderAPI_MakeEdge(P1,P2);
// go on with the edge
}
catch {
// process the error.
}
To help the user, the Topology API classes only raise the exception StdFail_NotDone. Any other exception means that something happened which was unforeseen in the design of this API.
The NotDone exception is only raised when the user tries to access the result of the computation and the original data is corrupted. At the construction of the class instance, if the algorithm cannot be completed, the internal flag NotDone is set. This flag can be tested and in some situations a more complete description of the error can be queried. If the user ignores the NotDone status and tries to access the result, an exception is raised.
BRepBuilderAPI_MakeEdge ME(P1,P2);
if (!ME.IsDone()) {
// doing ME.Edge() or E = ME here
// would raise StdFail_NotDone
Standard_DomainError::Raise
(“ProcessPoints::Failed to create an edge”);
}
TopoDS_Edge E = ME;
@section occt_modalg_2 Geometric Tools
@subsection occt_modalg_2_1 Overview
Open CASCADE Technology geometric tools include:
- Computation of intersections
- Interpolation laws
- Computation of curves and surfaces from constraints
- Computation of lines and circles from constraints
- Projections
@subsection occt_modalg_2_2 Intersections
The Geom2dAPI_InterCurveCurve class allows the evaluation of the intersection points (gp_Pnt2d) between two geometric curves (Geom2d_Curve) and the evaluation of the points of self-intersection of a curve.
@image html /user_guides/modeling_algos/images/modeling_algos_image003.png "Intersection and self-intersection of curves" @image latex /user_guides/modeling_algos/images/modeling_algos_image003.png "Intersection and self-intersection of curves"
In both cases, the algorithm requires a value for the tolerance (Standard_Real) for the confusion between two points. The default tolerance value used in all constructors is 1.0e-6.
@image html /user_guides/modeling_algos/images/modeling_algos_image004.png "Intersection and tangent intersection" @image latex /user_guides/modeling_algos/images/modeling_algos_image004.png "Intersection and tangent intersection"
The algorithm returns a point in the case of an intersection and a segment in the case of tangent intersection.
@subsubsection occt_modalg_2_2_1 Geom2dAPI_InterCurveCurve
This class may be instantiated either for intersection of curves C1 and C2.
Geom2dAPI_InterCurveCurve Intersector(C1,C2,tolerance);
or for self-intersection of curve C3.
Geom2dAPI_InterCurveCurve Intersector(C3,tolerance);
Standard_Integer N = Intersector.NbPoints();
Calls the number of intersection points
To select the desired intersection point, pass an integer index value in argument.
gp_Pnt2d P = Intersector.Point(Index);
To call the number of intersection segments, use
Standard_Integer M = Intersector.NbSegments();
To select the desired intersection segment pass integer index values in argument.
Handle(Geom2d_Curve) Seg1, Seg2;
Intersector.Segment(Index,Seg1,Seg2);
// if intersection of 2 curves
Intersector.Segment(Index,Seg1);
// if self-intersection of a curve
If you need access to a wider range of functionalities the following method will return the algorithmic object for the calculation of intersections:
Geom2dInt_GInter& TheIntersector = Intersector.Intersector();
@subsubsection occt_modalg_2_2_2 Intersection of Curves and Surfaces The GeomAPI_IntCS class is used to compute the intersection points between a curve and a surface.
This class is instantiated as follows:
GeomAPI_IntCS Intersector(C, S);
To call the number of intersection points, use:
Standard_Integer nb = Intersector.NbPoints();
gp_Pnt& P = Intersector.Point(Index);
Where Index is an integer between 1 and nb, calls the intersection points.
@subsubsection occt_modalg_2_2_3 Intersection of two Surfaces The GeomAPI_IntSS class is used to compute the intersection of two surfaces from Geom_Surface with respect to a given tolerance.
This class is instantiated as follows:
GeomAPI_IntSS Intersector(S1, S2, Tolerance);
Once the GeomAPI_IntSS object has been created, it can be interpreted.
Standard_Integer nb = Intersector. NbLines();
Calls the number of intersection curves.
Handle(Geom_Curve) C = Intersector.Line(Index)
Where Index is an integer between 1 and nb, calls the intersection curves.
@subsection occt_modalg_2_3 Interpolations Interpolation provides functionalities for interpolating BSpline curves, whether in 2D, using Geom2dAPI_Interpolate, or 3D using GeomAPI_Interpolate.
@subsubsection occt_modalg_2_3_1 Geom2dAPI_Interpolate This class is used to interpolate a BSplineCurve passing through an array of points. If tangency is not requested at the point of interpolation, continuity will be C2. If tangency is requested at the point, continuity will be C1. If Periodicity is requested, the curve will be closed and the junction will be the first point given. The curve will then have a continuity of C1 only. This class may be instantiated as follows:
Geom2dAPI_Interpolate
(const Handle_TColgp_HArray1OfPnt2d& Points,
const Standard_Boolean PeriodicFlag,
const Standard_Real Tolerance);
Geom2dAPI_Interpolate Interp(Points, Standard_False,
Precision::Confusion());
It is possible to call the BSpline curve from the object defined above it.
Handle(Geom2d_BSplineCurve) C = Interp.Curve();
Note that the Handle(Geom2d_BSplineCurve) operator has been redefined by the method Curve(). Consequently, it is unnecessary to pass via the construction of an intermediate object of the Geom2dAPI_Interpolate type and the following syntax is correct.
Handle(Geom2d_BSplineCurve) C =
Geom2dAPI_Interpolate(Points,
Standard_False,
Precision::Confusion());
@subsubsection occt_modalg_2_3_2 GeomAPI_Interpolate
This class may be instantiated as follows:
GeomAPI_Interpolate
(const Handle_TColgp_HArray1OfPnt& Points,
const Standard_Boolean PeriodicFlag,
const Standard_Real Tolerance);
GeomAPI_Interpolate Interp(Points, Standard_False,
Precision::Confusion());
It is possible to call the BSpline curve from the object defined above it.
Handle(Geom_BSplineCurve) C = Interp.Curve();
Note that the Handle(Geom_BSplineCurve) operator has been redefined by the method Curve(). Thus, it is unnecessary to pass via the construction of an intermediate object of the GeomAPI_Interpolate type and the following syntax is correct.
Handle(Geom_BSplineCurve) C =
GeomAPI_Interpolate(Points,
Standard_False,
1.0e-7);
Boundary conditions may be imposed with the method Load.
GeomAPI_Interpolate AnInterpolator
(Points, Standard_False, 1.0e-5);
AnInterpolator.Load (StartingTangent, EndingTangent);
@subsection occt_modalg_2_4 Lines and Circles from Constraints
There are two packages to create lines and circles from constraints: Geom2dGcc and GccAna. Geom2dGcc deals with reference-handled geometric objects from the Geom2d package, while GccAna deals with value-handled geometric objects from the gp package.
The Geom2dGcc package solves geometric constructions of lines and circles expressed by constraints such as tangency or parallelism, that is, a constraint expressed in geometric terms. As a simple example the following figure shows a line which is constrained to pass through a point and be tangent to a circle.
@image html /user_guides/modeling_algos/images/modeling_algos_image005.png "A constrained line" @image latex /user_guides/modeling_algos/images/modeling_algos_image005.png "A constrained line"
The Geom2dGcc package focuses on algorithms; it is useful for finding results, but it does not offer any management or modification functions, which could be applied to the constraints or their arguments. This package is designed to offer optimum performance, both in rapidity and precision. Trivial cases (for example, a circle centered on one point and passing through another) are not treated.
The Geom2dGcc package deals only with 2d objects from the Geom2d package. These objects are the points, lines and circles available.
All other lines such as Bezier curves and conic sections except for circles are considered general curves and must be differentiable twice.
The GccAna package deals with points, lines, and circles from the gp package. Apart from constructors for lines and circles, it also allows the creation of conics from the bisection of other geometric objects.
@subsection occt_modalg_2_5 Provided algorithms
The following analytic algorithms using value-handled entities for creation of 2D lines or circles with geometric constraints are available:
- circle tangent to three elements (lines, circles, curves, points),
- circle tangent to two elements and having a radius,
- circle tangent to two elements and centered on a third element,
- circle tangent to two elements and centered on a point,
- circle tangent to one element and centered on a second,
- bisector of two points,
- bisector of two lines,
- bisector of two circles,
- bisector of a line and a point,
- bisector of a circle and a point,
- bisector of a line and a circle,
- line tangent to two elements (points, circles, curves),
- line tangent to one element and parallel to a line,
- line tangent to one element and perpendicular to a line,
- line tangent to one element and forming angle with a line.
@subsection occt_modalg_2_6 Types of algorithms There are three categories of available algorithms, which complement each other:
- analytic,
- geometric,
- iterative.
An analytic algorithm will solve a system of equations, whereas a geometric algorithm works with notions of parallelism, tangency, intersection and so on.
Both methods can provide solutions. An iterative algorithm, however, seeks to refine an approximate solution.
@subsection occt_modalg_2_7 Performance factors
The appropriate algorithm is the one, which reaches a solution of the required accuracy in the least time. Only the solutions actually requested by the user should be calculated. A simple means to reduce the number of solutions is the notion of a "qualifier". There are four qualifiers, which are:
- Unqualified: the position of the solution is undefined with respect to this argument.
- Enclosing: the solution encompasses this argument.
- Enclosed: the solution is encompassed by this argument.
- Outside: the solution and argument are outside each other.
@subsection occt_modalg_2_8 Conventions
@subsubsection occt_modalg_2_8_1 Exterior/Interior It is not hard to define the interior and exterior of a circle. As is shown in the following diagram, the exterior is indicated by the sense of the binormal, that is to say the right side according to the sense of traversing the circle. The left side is therefore the interior (or "material").
@image html /user_guides/modeling_algos/images/modeling_algos_image006.png "Exterior/Interior of a Circle" @image latex /user_guides/modeling_algos/images/modeling_algos_image006.png "Exterior/Interior of a Circle"
By extension, the interior of a line or any open curve is defined as the left side according to the passing direction, as shown in the following diagram:
@image html /user_guides/modeling_algos/images/modeling_algos_image007.png "Exterior/Interior of a Line and a Curve" @image latex /user_guides/modeling_algos/images/modeling_algos_image007.png "Exterior/Interior of a Line and a Curve"
@subsubsection occt_modalg_2_8_2 Orientation of a Line It is sometimes necessary to define in advance the sense of travel along a line to be created. This sense will be from first to second argument.
The following figure shows a line, which is first tangent to circle C1 which is interior to the line, and then passes through point P1.
@image html /user_guides/modeling_algos/images/modeling_algos_image008.png "An Oriented Line" @image latex /user_guides/modeling_algos/images/modeling_algos_image008.png "An Oriented Line"
@subsection occt_modalg_2_9 Examples
@subsubsection occt_modalg_2_9_1 Line tangent to two circles The following four diagrams illustrate four cases of using qualifiers in the creation of a line. The fifth shows the solution if no qualifiers are given.
Note that the qualifier "Outside" is used to mean "Mutually exterior".
Example 1 Case 1
@image html /user_guides/modeling_algos/images/modeling_algos_image009.png "Both circles outside" @image latex /user_guides/modeling_algos/images/modeling_algos_image009.png "Both circles outside"
Constraints: Tangent and Exterior to C1. Tangent and Exterior to C2.
Syntax:
GccAna_Lin2d2Tan
Solver(GccEnt::Outside(C1),
GccEnt::Outside(C2),
Tolerance);
Example 1 Case 2
@image html /user_guides/modeling_algos/images/modeling_algos_image010.png "Both circles enclosed" @image latex /user_guides/modeling_algos/images/modeling_algos_image010.png "Both circles enclosed"
Constraints: Tangent and Including C1. Tangent and Including C2.
Syntax:
GccAna_Lin2d2Tan
Solver(GccEnt::Enclosing(C1),
GccEnt::Enclosing(C2),
Tolerance);
Example 1 Case 3
@image html /user_guides/modeling_algos/images/modeling_algos_image011.png "C1 enclosed, C2 outside" @image latex /user_guides/modeling_algos/images/modeling_algos_image011.png "C1 enclosed, C2 outside"
Constraints: Tangent and Including C1. Tangent and Exterior to C2.
Syntax:
GccAna_Lin2d2Tan
Solver(GccEnt::Enclosing(C1),
GccEnt::Outside(C2),
Tolerance);
Example 1 Case 4
@image html /user_guides/modeling_algos/images/modeling_algos_image012.png "C1 outside, C2 enclosed" @image latex /user_guides/modeling_algos/images/modeling_algos_image012.png "C1 outside, C2 enclosed" Constraints: Tangent and Exterior to C1. Tangent and Including C2.
Syntax:
GccAna_Lin2d2Tan
Solver(GccEnt::Outside(C1),
GccEnt::Enclosing(C2),
Tolerance);
Example 1 Case 5
@image html /user_guides/modeling_algos/images/modeling_algos_image013.png "With no qualifiers specified" @image latex /user_guides/modeling_algos/images/modeling_algos_image013.png "With no qualifiers specified"
Constraints: Tangent and Undefined with respect to C1. Tangent and Undefined with respect to C2.
Syntax:
GccAna_Lin2d2Tan
Solver(GccEnt::Unqualified(C1),
GccEnt::Unqualified(C2),
Tolerance);
@subsubsection occt_modalg_2_9_2 Circle of given radius tangent to two circles The following four diagrams show the four cases in using qualifiers in the creation of a circle.
Example 2 Case 1 @image html /user_guides/modeling_algos/images/modeling_algos_image014.png "Both solutions outside" @image latex /user_guides/modeling_algos/images/modeling_algos_image014.png "Both solutions outside"
Constraints: Tangent and Exterior to C1. Tangent and Exterior to C2.
Syntax:
GccAna_Circ2d2TanRad
Solver(GccEnt::Outside(C1),
GccEnt::Outside(C2), Rad, Tolerance);
Example 2 Case 2
@image html /user_guides/modeling_algos/images/modeling_algos_image015.png "C2 encompasses C1" @image latex /user_guides/modeling_algos/images/modeling_algos_image015.png "C2 encompasses C1"
Constraints: Tangent and Exterior to C1. Tangent and Included by C2.
Syntax:
GccAna_Circ2d2TanRad
Solver(GccEnt::Outside(C1),
GccEnt::Enclosed(C2), Rad, Tolerance);
Example 2 Case 3 @image html /user_guides/modeling_algos/images/modeling_algos_image016.png "Solutions enclose C2" @image latex /user_guides/modeling_algos/images/modeling_algos_image016.png "Solutions enclose C2"
Constraints: Tangent and Exterior to C1. Tangent and Including C2.
Syntax:
GccAna_Circ2d2TanRad
Solver(GccEnt::Outside(C1),
GccEnt::Enclosing(C2), Rad, Tolerance);
Example 2 Case 4 @image html /user_guides/modeling_algos/images/modeling_algos_image017.png "Solutions enclose C1" @image latex /user_guides/modeling_algos/images/modeling_algos_image017.png "Solutions enclose C1"
Constraints: Tangent and Enclosing C1. Tangent and Enclosing C2.
Syntax:
GccAna_Circ2d2TanRad
Solver(GccEnt::Enclosing(C1),
GccEnt::Enclosing(C2), Rad, Tolerance);
Example 2 Case 5
The following syntax will give all the circles of radius Rad, which are tangent to C1 and C2 without discrimination of relative position:
GccAna_Circ2d2TanRad Solver(GccEnt::Unqualified(C1),
GccEnt::Unqualified(C2),
Rad,Tolerance);
@subsection occt_modalg_2_10 Algorithms
The objects created by this toolkit are non-persistent.
@subsubsection occt_modalg_2_10_1 Qualifiers The GccEnt package contains the following package methods: * Unqualified, * Enclosing, * Enclosed, * Outside.
This enables creation of expressions, for example:
GccAna_Circ2d2TanRad
Solver(GccEnt::Outside(C1),
GccEnt::Enclosing(C2), Rad, Tolerance);
The objective in this case is to find all circles of radius Rad, which are tangent to both circle C1 and C2, C1 being outside and C2 being inside.
@subsubsection occt_modalg_2_10_2 General Remarks about Algorithms
We consider the following to be the case:
- If a circle passes through a point then the circle is tangential to it.
- A distinction is made between the trivial case of the center at a point and the complex case of the center on a line.
@subsubsection occt_modalg_2_10_3 Analytic Algorithms GccAna package implements analytic algorithms. It deals only with points, lines, and circles from gp package. Here is a list of the services offered:
Creation of a Line
Tangent ( point | circle ) & Parallel ( line )
Tangent ( point | circle ) & Perpendicular ( line | circle )
Tangent ( point | circle ) & Oblique ( line )
Tangent ( 2 { point | circle } )
Bisector( line | line )
Creation of Conics
Bisector ( point | point )
Bisector ( line | point )
Bisector ( circle | point )
Bisector ( line | line )
Bisector ( circle | line )
Bisector ( circle | circle )
Creation of a Circle
Tangent ( point | line | circle ) & Center ( point )
Tangent ( 3 { point | line | circle } )
Tangent ( 2 { point | line | circle } ) & Radius ( real )
Tangent ( 2 { point | line | circle } ) & Center ( line | circle )
Tangent ( point | line | circle ) & Center ( line | circle ) & Radius ( real )
For each algorithm, the tolerance (and angular tolerance if appropriate) is given as an argument. Calculation is done with the highest precision available from the hardware.
@subsubsection occt_modalg_2_10_4 Geometric Algorithms
Geom2dGcc package offers algorithms, which produce 2d lines or circles with geometric constraints. For arguments, it takes curves for which an approximate solution is not requested. A tolerance value on the result is given as a starting parameter. The following services are provided:
Creation of a Circle
Tangent ( curve ) & Center ( point )
Tangent ( curve , point | line | circle | curve ) & Radius ( real )
Tangent ( 2 {point | line | circle} ) & Center ( curve )
Tangent ( curve ) & Center ( line | circle | curve ) & Radius ( real )
Tangent ( point | line | circle ) & Center ( curve ) & Radius ( real )
All calculations will be done to the highest precision available from the hardware.
@subsubsection occt_modalg_2_10_5 Iterative Algorithms Geom2dGcc package offers iterative algorithms find a solution by refining an approximate solution. It produces 2d lines or circles with geometric constraints. For all geometric arguments except points, an approximate solution may be given as a starting parameter. The tolerance or angular tolerance value is given as an argument. The following services are provided:
Creation of a Line
Tangent ( curve ) & Oblique ( line )
Tangent ( curve , { point | circle | curve } )
Creation of a Circle
Tangent ( curve , 2 { point | circle | curve } )
Tangent ( curve , { point | circle | curve } )
& Center ( line | circle | curve )
@subsection occt_modalg_2_1 Curves and Surfaces from Constraints
@subsubsection occt_modalg_2_1_1 Fair Curve
FairCurve package provides a set of classes to create faired 2D curves or 2D curves with minimal variation in curvature.
Creation of Batten Curves
The class Batten allows producing faired curves defined on the basis of one or more constraints on each of the two reference points. These include point, angle of tangency and curvature settings. The following constraint orders are available:
- 0 the curve must pass through a point
- 1 the curve must pass through a point and have a given tangent
- 2 the curve must pass through a point, have a given tangent and a given curvature.
Only 0 and 1 constraint orders are used. The function Curve returns the result as a 2D BSpline curve.
Creation of Minimal Variation Curves
The class MinimalVariation allows producing curves with minimal variation in curvature at each reference point. The following constraint orders are available:
- 0 the curve must pass through a point
- 1 the curve must pass through a point and have a given tangent
- 2 the curve must pass through a point, have a given tangent and a given curvature.
Constraint orders of 0, 1 and 2 can be used. The algorithm minimizes tension, sagging and jerk energy.
The function Curve returns the result as a 2D BSpline curve.
Specifying the length of the curve
If you want to give a specific length to a batten curve, use:
b.SetSlidingFactor(L / b.SlidingOfReference())
where b is the name of the batten curve object
Limitations
Free sliding is generally more aesthetically pleasing than constrained sliding. However, the computation can fail with values such as angles greater than p/2, because in this case, the length is theoretically infinite.
In other cases, when sliding is imposed and the sliding factor is too large, the batten can collapse.
Computation Time
The constructor parameters, Tolerance and NbIterations, control how precise the computation is, and how long it will take.
@subsubsection occt_modalg_2_11_2 Surfaces from Boundary Curves
The GeomFill package provides the following services for creating surfaces from boundary curves:
Creation of Bezier surfaces
The class BezierCurves allows producing a Bezier surface from contiguous Bezier curves. Note that problems may occur with rational Bezier Curves.
Creation of BSpline surfaces
The class BSplineCurves allows producing a BSpline surface from contiguous BSpline curves. Note that problems may occur with rational BSplines.
Creation of a Pipe
The class Pipe allows producing a pipe by sweeping a curve (the section) along another curve (the path). The result is a BSpline surface.
Filling a contour
The class GeomFill_ConstrainedFilling allows filling a contour defined by two, three or four curves as well as by tangency constraints. The resulting surface is a BSpline.
Creation of a Boundary
The class GeomFill_SimpleBound allows you defining a boundary for the surface to be constructed.
Creation of a Boundary with an adjoining surface
The class GeomFill_BoundWithSurf allows defining a boundary for the surface to be constructed. This boundary will already be joined to another surface.
Filling styles
The enumerations FillingStyle specify the styles used to build the surface. These include:
- Stretch - the style with the flattest patches
- Coons - a rounded style with less depth than Curved
- Curved - the style with the most rounded patches.
@image html /user_guides/modeling_algos/images/modeling_algos_image018.png "Intersecting filleted edges with different radii leave a gap, is filled by a surface" @image latex /user_guides/modeling_algos/images/modeling_algos_image018.png "Intersecting filleted edges with different radii leave a gap, is filled by a surface"
@subsubsection occt_modalg_2_11_3 Surfaces from curve and point constraints The GeomPlate package provides the following services for creating surfaces respecting curve and point constraints:
Definition of a Framework
The class BuildPlateSurface allows creating a framework to build surfaces according to curve and point constraints as well as tolerance settings. The result is returned with the function Surface.
Note that you do not have to specify an initial surface at the time of construction. It can be added later or, if none is loaded, a surface will be computed automatically.
Definition of a Curve Constraint
The class CurveConstraint allows defining curves as constraints to the surface, which you want to build.
Definition of a Point Constraint
The class PointConstraint allows defining points as constraints to the surface, which you want to build.
Applying Geom_Surface to Plate Surfaces
The class Surface allows describing the characteristics of plate surface objects returned by BuildPlateSurface::Surface using the methods of Geom_Surface
Approximating a Plate surface to a BSpline
The class MakeApprox allows converting a GeomPlate surface into a Geom_BSplineSurface.
@image html /user_guides/modeling_algos/images/modeling_algos_image019.png "Surface generated from four curves and a point" @image latex /user_guides/modeling_algos/images/modeling_algos_image019.png "Surface generated from four curves and a point"
Let us create a Plate surface and approximate it from a polyline as a curve constraint and a point constraint
Standard_Integer NbCurFront=4,
NbPointConstraint=1;
gp_Pnt P1(0.,0.,0.);
gp_Pnt P2(0.,10.,0.);
gp_Pnt P3(0.,10.,10.);
gp_Pnt P4(0.,0.,10.);
gp_Pnt P5(5.,5.,5.);
BRepBuilderAPI_MakePolygon W;
W.Add(P1);
W.Add(P2);
W.Add(P3);
W.Add(P4);
W.Add(P1);
// Initialize a BuildPlateSurface
GeomPlate_BuildPlateSurface BPSurf(3,15,2);
// Create the curve constraints
BRepTools_WireExplorer anExp;
for(anExp.Init(W); anExp.More(); anExp.Next())
{
TopoDS_Edge E = anExp.Current();
Handle(BRepAdaptor_HCurve) C = new
BRepAdaptor_HCurve();
C-ChangeCurve().Initialize(E);
Handle(BRepFill_CurveConstraint) Cont= new
BRepFill_CurveConstraint(C,0);
BPSurf.Add(Cont);
}
// Point constraint
Handle(GeomPlate_PointConstraint) PCont= new
GeomPlate_PointConstraint(P5,0);
BPSurf.Add(PCont);
// Compute the Plate surface
BPSurf.Perform();
// Approximation of the Plate surface
Standard_Integer MaxSeg=9;
Standard_Integer MaxDegree=8;
Standard_Integer CritOrder=0;
Standard_Real dmax,Tol;
Handle(GeomPlate_Surface) PSurf = BPSurf.Surface();
dmax = Max(0.0001,10*BPSurf.G0Error());
Tol=0.0001;
GeomPlate_MakeApprox
Mapp(PSurf,Tol,MaxSeg,MaxDegree,dmax,CritOrder);
Handle (Geom_Surface) Surf (Mapp.Surface());
// create a face corresponding to the approximated Plate
Surface
Standard_Real Umin, Umax, Vmin, Vmax;
PSurf-Bounds( Umin, Umax, Vmin, Vmax);
BRepBuilderAPI_MakeFace MF(Surf,Umin, Umax, Vmin, Vmax);
@subsection occt_modalg_2_12 Projections This package provides functionality for projecting points onto 2D and 3D curves and surfaces.
@subsubsection occt_modalg_2_12_1 Projection of a Point onto a Curve Geom2dAPI_ProjectPointOnCurve allows calculation of all the normals projected from a point (gp_Pnt2d) onto a geometric curve (Geom2d_Curve). The calculation may be restricted to a given domain.
@image html /user_guides/modeling_algos/images/modeling_algos_image020.png "Normals from a point to a curve" @image latex /user_guides/modeling_algos/images/modeling_algos_image020.png "Normals from a point to a curve"
The curve does not have to be a Geom2d_TrimmedCurve. The algorithm will function with any class inheriting Geom2d_Curve.
@subsubsection occt_modalg_2_12_2 Geom2dAPI_ProjectPointOnCurve This class may be instantiated as in the following example:
gp_Pnt2d P;
Handle(Geom2d_BezierCurve) C =
new Geom2d_BezierCurve(args);
Geom2dAPI_ProjectPointOnCurve Projector (P, C);
To restrict the search for normals to a given domain [U1,U2], use the following constructor:
Geom2dAPI_ProjectPointOnCurve Projector (P, C, U1, U2);
Having thus created the Geom2dAPI_ProjectPointOnCurve object, we can now interrogate it.
Calling the number of solution points
Standard_Integer NumSolutions = Projector.NbPoints();
Calling the location of a solution point
The solutions are indexed in a range from 1 to Projector.NbPoints(). The point, which corresponds to a given Index may be found:
gp_Pnt2d Pn = Projector.Point(Index);
Calling the parameter of a solution point
For a given point corresponding to a given Index:
Standard_Real U = Projector.Parameter(Index);
This can also be programmed as:
Standard_Real U;
Projector.Parameter(Index,U);
Calling the distance between the start and end points
We can find the distance between the initial point and a point, which corresponds to the given Index:
Standard_Real D = Projector.Distance(Index);
Calling the nearest solution point
This class offers a method to return the closest solution point to the starting point. This solution is accessed as follows:
gp_Pnt2d P1 = Projector.NearestPoint();
Calling the parameter of the nearest solution point
Standard_Real U = Projector.LowerDistanceParameter();
Calling the minimum distance from the point to the curve
Standard_Real D = Projector.LowerDistance();
@subsubsection occt_modalg_2_12_3 Redefined operators
Some operators have been redefined to find the closest solution.
Standard_Real() returns the minimum distance from the point to the curve.
Standard_Real D = Geom2dAPI_ProjectPointOnCurve (P,C);
Standard_Integer() returns the number of solutions.
Standard_Integer N =
Geom2dAPI_ProjectPointOnCurve (P,C);
gp_Pnt2d() returns the nearest solution point.
gp_Pnt2d P1 = Geom2dAPI_ProjectPointOnCurve (P,C);
Using these operators makes coding easier when you only need the nearest point. Thus:
Geom2dAPI_ProjectPointOnCurve Projector (P, C);
gp_Pnt2d P1 = Projector.NearestPoint();
can be written more concisely as:
gp_Pnt2d P1 = Geom2dAPI_ProjectPointOnCurve (P,C);
However, note that in this second case no intermediate Geom2dAPI_ProjectPointOnCurve object is created, and thus it is impossible to have access to other solution points.
@subsubsection occt_modalg_2_12_4 Access to lower-level functionalities
If you want to use the wider range of functionalities available from the Extrema package, a call to the Extrema() method will return the algorithmic object for calculating extrema. For example:
Extrema_ExtPC2d& TheExtrema = Projector.Extrema();
@subsubsection occt_modalg_2_12_5 GeomAPI_ProjectPointOnCurve
This class is instantiated as in the following example:
gp_Pnt P;
Handle(Geom_BezierCurve) C =
new Geom_BezierCurve(args);
GeomAPI_ProjectPointOnCurve Projector (P, C);
If you wish to restrict the search for normals to the given domain [U1,U2], use the following constructor:
GeomAPI_ProjectPointOnCurve Projector (P, C, U1, U2);
Having thus created the GeomAPI_ProjectPointOnCurve object, you can now interrogate it.
Calling the number of solution points
Standard_Integer NumSolutions = Projector.NbPoints();
Calling the location of a solution point
The solutions are indexed in a range from 1 to Projector.NbPoints(). The point, which corresponds to a given index, may be found:
gp_Pnt Pn = Projector.Point(Index);
Calling the parameter of a solution point
For a given point corresponding to a given index:
Standard_Real U = Projector.Parameter(Index);
This can also be programmed as:
Standard_Real U;
Projector.Parameter(Index,U);
Calling the distance between the start and end point
The distance between the initial point and a point, which corresponds to a given index, may be found:
Standard_Real D = Projector.Distance(Index);
Calling the nearest solution point
This class offers a method to return the closest solution point to the starting point. This solution is accessed as follows:
gp_Pnt P1 = Projector.NearestPoint();
Calling the parameter of the nearest solution point
Standard_Real U = Projector.LowerDistanceParameter();
Calling the minimum distance from the point to the curve
Standard_Real D = Projector.LowerDistance();
Redefined operators
Some operators have been redefined to find the nearest solution.
Standard_Real() returns the minimum distance from the point to the curve.
Standard_Real D = GeomAPI_ProjectPointOnCurve (P,C);
Standard_Integer() returns the number of solutions.
Standard_Integer N = GeomAPI_ProjectPointOnCurve (P,C);
gp_Pnt2d() returns the nearest solution point.
gp_Pnt P1 = GeomAPI_ProjectPointOnCurve (P,C);
Using these operators makes coding easier when you only need the nearest point. In this way,
GeomAPI_ProjectPointOnCurve Projector (P, C);
gp_Pnt P1 = Projector.NearestPoint();
can be written more concisely as:
gp_Pnt P1 = GeomAPI_ProjectPointOnCurve (P,C);
In the second case, however, no intermediate GeomAPI_ProjectPointOnCurve object is created, and it is impossible to access other solutions points.
Access to lower-level functionalities
If you want to use the wider range of functionalities available from the Extrema package, a call to the Extrema() method will return the algorithmic object for calculating the extrema. For example:
Extrema_ExtPC& TheExtrema = Projector.Extrema();
@subsubsection occt_modalg_2_12_6 Projection of a Point on a Surface
GeomAPI_ProjectPointOnSurf class allows calculation of all normals projected from a point from gp_Pnt onto a geometric surface from Geom_Surface.
@image html /user_guides/modeling_algos/images/modeling_algos_image021.png "Projection of normals from a point to a surface" @image latex /user_guides/modeling_algos/images/modeling_algos_image021.png "Projection of normals from a point to a surface"
Note that the surface does not have to be of Geom_RectangularTrimmedSurface type.
The algorithm will function with any class inheriting Geom_Surface.
GeomAPI_ProjectPointOnSurf is instantiated as in the following example:
gp_Pnt P;
Handle (Geom_Surface) S = new Geom_BezierSurface(args);
GeomAPI_ProjectPointOnSurf Proj (P, S);
To restrict the search for normals within the given rectangular domain [U1, U2, V1, V2], use the following constructor:
GeomAPI_ProjectPointOnSurf Proj (P, S, U1, U2, V1, V2);
The values of U1, U2, V1 and V2 lie at or within their maximum and minimum limits, i.e.:
Umin <= U1 < U2 <= Umax
Vmin <= V1 < V2 <= Vmax
Having thus created the GeomAPI_ProjectPointOnSurf object, you can interrogate it.
Calling the number of solution points
Standard_Integer NumSolutions = Proj.NbPoints();
Calling the location of a solution point
The solutions are indexed in a range from 1 to Proj.NbPoints(). The point corresponding to the given index may be found:
gp_Pnt Pn = Proj.Point(Index);
Calling the parameters of a solution point
For a given point corresponding to the given index:
Standard_Real U,V;
Proj.Parameters(Index, U, V);
Calling the distance between the start and end point
The distance between the initial point and a point corresponding to the given index may be found:
Standard_Real D = Projector.Distance(Index);
Calling the nearest solution point
This class offers a method, which returns the closest solution point to the starting point. This solution is accessed as follows:
gp_Pnt P1 = Proj.NearestPoint();
Calling the parameters of the nearest solution point
Standard_Real U,V;
Proj.LowerDistanceParameters (U, V);
Calling the minimum distance from a point to the surface
Standard_Real D = Proj.LowerDistance();
Redefined operators
Some operators have been redefined to help you find the nearest solution.
Standard_Real() returns the minimum distance from the point to the surface.
Standard_Real D = GeomAPI_ProjectPointOnSurf (P,S);
Standard_Integer() returns the number of solutions.
Standard_Integer N = GeomAPI_ProjectPointOnSurf (P,S);
gp_Pnt2d() returns the nearest solution point.
gp_Pnt P1 = GeomAPI_ProjectPointOnSurf (P,S);
Using these operators makes coding easier when you only need the nearest point. In this way,
GeomAPI_ProjectPointOnSurface Proj (P, S);
gp_Pnt P1 = Proj.NearestPoint();
can be written more concisely as:
gp_Pnt P1 = GeomAPI_ProjectPointOnSurface (P,S);
In the second case, however, no intermediate GeomAPI_ProjectPointOnSurf object is created, and it is impossible to access other solution points.
@subsubsection occt_modalg_2_12_7 Access to lower-level functionalities
If you want to use the wider range of functionalities available from the Extrema package, a call to the Extrema() method will return the algorithmic object for calculating the extrema as follows:
Extrema_ExtPS& TheExtrema = Proj.Extrema();
@subsubsection occt_modalg_2_12_8 Switching from 2d and 3d Curves The To2d and To3d methods are used to;
- build a 2d curve from a 3d Geom_Curve lying on a gp_Pln plane
- build a 3d curve from a Geom2d_Curve and a gp_Pln plane.
These methods are called as follows:
Handle(Geom2d_Curve) C2d = GeomAPI::To2d(C3d, Pln);
Handle(Geom_Curve) C3d = GeomAPI::To3d(C2d, Pln);
@section occt_modalg_3 Topological Tools
Open CASCADE Technology topological tools include:
- Standard topological objects combining topological data structure and boundary representation
- Geometric Transformations
- Conversion to NURBS geometry
- Finding Planes
- Duplicating Shapes
- Checking Validity
@subsection occt_modalg_3_1 Creation of Standard Topological Objects
The standard topological objects include
- Vertices
- Edges
- Wires
- Faces
- Shells
- Solids.
There are two root classes for their construction and modification:
- The deferred class BRepBuilderAPI_MakeShape is the root of all BRepBuilderAPI classes, which build shapes. It inherits from the class BRepBuilderAPI_Command and provides a field to store the constructed shape.
- The deferred BRepBuilderAPI_ModifyShape is used as a root for the shape modifications. It inherits BRepBuilderAPI_MakeShape and implements the methods used to trace the history of all sub-shapes.
@subsubsection occt_modalg_3_1_1 Vertex
BRepBuilderAPI_MakeVertex creates a new vertex from a 3D point from gp.
gp_Pnt P(0,0,10);
TopoDS_Vertex V = BRepBuilderAPI_MakeVertex(P);
This class always creates a new vertex and has no other methods.
@subsubsection occt_modalg_3_1_2 Edge
Use BRepBuilderAPI_MakeEdge to create from a curve and vertices. The basic method is to construct an edge from a curve, two vertices, and two parameters. All other constructions are derived from this one. The basic method and its arguments are described first, followed by the other methods. The BRepBuilderAPI_MakeEdge class can provide extra information and return an error status.
Basic Edge construction
Handle(Geom_Curve) C = ...; // a curve
TopoDS_Vertex V1 = ...,V2 = ...;// two Vertices
Standard_Real p1 = ..., p2 = ..;// two parameters
TopoDS_Edge E = BRepBuilderAPI_MakeEdge(C,V1,V2,p1,p2);
where C is the domain of the edge; V1 is the first vertex oriented FORWARD; V2 is the second vertex oriented REVERSED; p1 and p2 are the parameters for the vertices V1 and V2 on the curve. The default tolerance is associated with this edge.
@image html /user_guides/modeling_algos/images/modeling_algos_image022.png "Basic Edge Construction" @image latex /user_guides/modeling_algos/images/modeling_algos_image022.png "Basic Edge Construction"
The following rules apply to the arguments:
The curve
- Must not be a Null Handle.
- If the curve is a trimmed curve, the basis curve is used.
The vertices
- Can be null shapes. When V1 or V2 is Null the edge is open in the corresponding direction and the corresponding parameter p1 or p2 must be infinite (i.e p1 is RealFirst(), p2 is RealLast()).
- Must be different vertices if they have different 3d locations and identical vertices if they have the same 3d location (identical vertices are used when the curve is closed).
The parameters
- Must be increasing and in the range of the curve, i.e.:
C->FirstParameter() <= p1 < p2 <= C->LastParameter()
- If the parameters are decreasing, the Vertices are switched, i.e. V2 becomes V1 and V1 becomes V2.
- On a periodic curve the parameters p1 and p2 are adjusted by adding or subtracting the period to obtain p1 in the range of the curve and p2 in the range p1 < p2 <= p1+ Period. So on a parametric curve p2 can be greater than the second parameter, see the figure below.
- Can be infinite but the corresponding vertex must be Null (see above).
- The distance between the Vertex 3d location and the point evaluated on the curve with the parameter must be lower than the default precision.
The figure below illustrates two special cases, a semi-infinite edge and an edge on a periodic curve.
@image html /user_guides/modeling_algos/images/modeling_algos_image023.png "Infinite and Periodic Edges" @image latex /user_guides/modeling_algos/images/modeling_algos_image023.png "Infinite and Periodic Edges"
Other Edge constructions
BRepBuilderAPI_MakeEdge class provides methods, which are all simplified calls of the previous one:
- The parameters can be omitted. They are computed by projecting the vertices on the curve.
- 3d points (Pnt from gp) can be given in place of vertices. Vertices are created from the points. Giving vertices is useful when creating connected vertices.
- The vertices or points can be omitted if the parameters are given. The points are computed by evaluating the parameters on the curve.
- The vertices or points and the parameters can be omitted. The first and the last parameters of the curve are used.
The five following methods are thus derived from the basic construction:
Handle(Geom_Curve) C = ...; // a curve
TopoDS_Vertex V1 = ...,V2 = ...;// two Vertices
Standard_Real p1 = ..., p2 = ..;// two parameters
gp_Pnt P1 = ..., P2 = ...;// two points
TopoDS_Edge E;
// project the vertices on the curve
E = BRepBuilderAPI_MakeEdge(C,V1,V2);
// Make vertices from points
E = BRepBuilderAPI_MakeEdge(C,P1,P2,p1,p2);
// Make vertices from points and project them
E = BRepBuilderAPI_MakeEdge(C,P1,P2);
// Computes the points from the parameters
E = BRepBuilderAPI_MakeEdge(C,p1,p2);
// Make an edge from the whole curve
E = BRepBuilderAPI_MakeEdge(C);
Six methods (the five above and the basic method) are also provided for curves from the gp package in place of Curve from Geom. The methods create the corresponding Curve from Geom and are implemented for the following classes:
gp_Lin creates a Geom_Line gp_Circ creates a Geom_Circle gp_Elips creates a Geom_Ellipse gp_Hypr creates a Geom_Hyperbola gp_Parab creates a Geom_Parabola
There are also two methods to construct edges from two vertices or two points. These methods assume that the curve is a line; the vertices or points must have different locations.
TopoDS_Vertex V1 = ...,V2 = ...;// two Vertices
gp_Pnt P1 = ..., P2 = ...;// two points
TopoDS_Edge E;
// linear edge from two vertices
E = BRepBuilderAPI_MakeEdge(V1,V2);
// linear edge from two points
E = BRepBuilderAPI_MakeEdge(P1,P2);
Other information and error status
If BRepBuilderAPI_MakeEdge is used as a class, it can provide two vertices. This is useful when the vertices were not provided as arguments, for example when the edge was constructed from a curve and parameters. The two methods Vertex1 and Vertex2 return the vertices. Note that the returned vertices can be null if the edge is open in the corresponding direction.
The Error method returns a term of the BRepBuilderAPI_EdgeError enumeration. It can be used to analyze the error when IsDone method returns False. The terms are:
- EdgeDone - No error occurred, IsDone returns True.
- PointProjectionFailed - No parameters were given, but the projection of the 3D points on the curve failed. This happens if the point distance to the curve is greater than the precision.
- ParameterOutOfRange - The given parameters are not in the range C->FirstParameter(), C->LastParameter()
- DifferentPointsOnClosedCurve - The two vertices or points have different locations but they are the extremities of a closed curve.
- PointWithInfiniteParameter - A finite coordinate point was associated with an infinite parameter (see the Precision package for a definition of infinite values).
- DifferentsPointAndParameter - The distance of the 3D point and the point evaluated on the curve with the parameter is greater than the precision.
- LineThroughIdenticPoints - Two identical points were given to define a line (construction of an edge without curve), gp::Resolution is used to test confusion .
The following example creates a rectangle centered on the origin of dimensions H, L with fillets of radius R. The edges and the vertices are stored in the arrays theEdges and theVertices. We use class Array1OfShape (i.e. not arrays of edges or vertices). See the image below.
@image html /user_guides/modeling_algos/images/modeling_algos_image024.png "Creating a Wire" @image latex /user_guides/modeling_algos/images/modeling_algos_image024.png "Creating a Wire"
#include <BRepBuilderAPI_MakeEdge.hxx>
#include <TopoDS_Shape.hxx>
#include <gp_Circ.hxx>
#include <gp.hxx>
#include <TopoDS_Wire.hxx>
#include <TopTools_Array1OfShape.hxx>
#include <BRepBuilderAPI_MakeWire.hxx>
// Use MakeArc method to make an edge and two vertices
void MakeArc(Standard_Real x,Standard_Real y,
Standard_Real R,
Standard_Real ang,
TopoDS_Shape& E,
TopoDS_Shape& V1,
TopoDS_Shape& V2)
{
gp_Ax2 Origin = gp::XOY();
gp_Vec Offset(x, y, 0.);
Origin.Translate(Offset);
BRepBuilderAPI_MakeEdge
ME(gp_Circ(Origin,R), ang, ang+PI/2);
E = ME;
V1 = ME.Vertex1();
V2 = ME.Vertex2();
}
TopoDS_Wire MakeFilletedRectangle(const Standard_Real H,
const Standard_Real L,
const Standard_Real R)
{
TopTools_Array1OfShape theEdges(1,8);
TopTools_Array1OfShape theVertices(1,8);
// First create the circular edges and the vertices
// using the MakeArc function described above.
void MakeArc(Standard_Real, Standard_Real,
Standard_Real, Standard_Real,
TopoDS_Shape&, TopoDS_Shape&, TopoDS_Shape&);
Standard_Real x = L/2 - R, y = H/2 - R;
MakeArc(x,-y,R,3.*PI/2.,theEdges(2),theVertices(2),
theVertices(3));
MakeArc(x,y,R,0.,theEdges(4),theVertices(4),
theVertices(5));
MakeArc(-x,y,R,PI/2.,theEdges(6),theVertices(6),
theVertices(7));
MakeArc(-x,-y,R,PI,theEdges(8),theVertices(8),
theVertices(1));
// Create the linear edges
for (Standard_Integer i = 1; i <= 7; i += 2)
{
theEdges(i) = BRepBuilderAPI_MakeEdge
(TopoDS::Vertex(theVertices(i)),TopoDS::Vertex
(theVertices(i+1)));
}
// Create the wire using the BRepBuilderAPI_MakeWire
BRepBuilderAPI_MakeWire MW;
for (i = 1; i <= 8; i++)
{
MW.Add(TopoDS::Edge(theEdges(i)));
}
return MW.Wire();
}
@subsubsection occt_modalg_3_1_3 Edge 2D
Use BRepBuilderAPI_MakeEdge2d class to make edges on a working plane from 2d curves. The working plane is a default value of the BRepBuilderAPI package (see the Plane methods).
BRepBuilderAPI_MakeEdge2d class is strictly similar to BRepBuilderAPI_MakeEdge, but it uses 2D geometry from gp and Geom2d instead of 3D geometry.
@subsubsection occt_modalg_3_1_4 Polygon
BRepBuilderAPI_MakePolygon class is used to build polygonal wires from vertices or points. Points are automatically changed to vertices as in BRepBuilderAPI_MakeEdge.
The basic usage of BRepBuilderAPI_MakePolygon is to create a wire by adding vertices or points using the Add method. At any moment, the current wire can be extracted. The close method can be used to close the current wire. In the following example, a closed wire is created from an array of points.
#include <TopoDS_Wire.hxx>
#include <BRepBuilderAPI_MakePolygon.hxx>
#include <TColgp_Array1OfPnt.hxx>
TopoDS_Wire ClosedPolygon(const TColgp_Array1OfPnt& Points)
{
BRepBuilderAPI_MakePolygon MP;
for(Standard_Integer i=Points.Lower();i=Points.Upper();i++)
{
MP.Add(Points(i));
}
MP.Close();
return MP;
}
Short-cuts are provided for 2, 3, or 4 points or vertices. Those methods have a Boolean last argument to tell if the polygon is closed. The default value is False.
Two examples:
Example of a closed triangle from three vertices:
TopoDS_Wire W = BRepBuilderAPI_MakePolygon(V1,V2,V3,Standard_True);
Example of an open polygon from four points:
TopoDS_Wire W = BRepBuilderAPI_MakePolygon(P1,P2,P3,P4);
BRepBuilderAPI_MakePolygon class maintains a current wire. The current wire can be extracted at any moment and the construction can proceed to a longer wire. After each point insertion, the class maintains the last created edge and vertex, which are returned by the methods Edge, FirstVertex and LastVertex.
When the added point or vertex has the same location as the previous one it is not added to the current wire but the most recently created edge becomes Null. The Added method can be used to test this condition. The MakePolygon class never raises an error. If no vertex has been added, the Wire is Null. If two vertices are at the same location, no edge is created.
@subsubsection occt_modalg_3_1_5 Face
Use BRepBuilderAPI_MakeFace class to create a face from a surface and wires. An underlying surface is constructed from a surface and optional parametric values. Wires can be added to the surface. A planar surface can be constructed from a wire. An error status can be returned after face construction.
Basic face construction
A face can be constructed from a surface and four parameters to determine a limitation of the UV space. The parameters are optional, if they are omitted the natural bounds of the surface are used. Up to four edges and vertices are created with a wire. No edge is created when the parameter is infinite.
Handle(Geom_Surface) S = ...; // a surface
Standard_Real umin,umax,vmin,vmax; // parameters
TopoDS_Face F = BRepBuilderAPI_MakeFace(S,umin,umax,vmin,vmax);
@image html /user_guides/modeling_algos/images/modeling_algos_image025.png "Basic Face Construction" @image latex /user_guides/modeling_algos/images/modeling_algos_image025.png "Basic Face Construction"
To make a face from the natural boundary of a surface, the parameters are not required:
Handle(Geom_Surface) S = ...; // a surface
TopoDS_Face F = BRepBuilderAPI_MakeFace(S);
Constraints on the parameters are similar to the constraints in BRepBuilderAPI_MakeEdge.
- umin,umax (vmin,vmax) must be in the range of the surface and must be increasing.
- On a U (V) periodic surface umin and umax (vmin,vmax) are adjusted.
- umin, umax, vmin, vmax can be infinite. There will be no edge in the corresponding direction.
Other face constructions
The two basic constructions (from a surface and from a surface and parameters) are implemented for all the gp package surfaces, which are transformed in the corresponding Surface from Geom.
| gp package surface | Geom package surface | |
|---|---|---|
| gp_Pln | Geom_Plane | |
| gp_Cylinder | Geom_CylindricalSurface | |
| gp_Cone | creates a | Geom_ConicalSurface |
| gp_Sphere | Geom_SphericalSurface | |
| gp_Torus | Geom_ToroidalSurface |
Once a face has been created, a wire can be added using the Add method. For example, the following code creates a cylindrical surface and adds a wire.
gp_Cylinder C = ..; // a cylinder
TopoDS_Wire W = ...;// a wire
BRepBuilderAPI_MakeFace MF(C);
MF.Add(W);
TopoDS_Face F = MF;
More than one wire can be added to a face, provided that they do not cross each other and they define only one area on the surface. (Note that this is not checked). The edges on a Face must have a parametric curve description.
If there is no parametric curve for an edge of the wire on the Face it is computed by projection.
For one wire, a simple syntax is provided to construct the face from the surface and the wire. The above lines could be written:
TopoDS_Face F = BRepBuilderAPI_MakeFace(C,W);
A planar face can be created from only a wire, provided this wire defines a plane. For example, to create a planar face from a set of points you can use BRepBuilderAPI_MakePolygon and BRepBuilderAPI_MakeFace.
#include <TopoDS_Face.hxx>
#include <TColgp_Array1OfPnt.hxx>
#include <BRepBuilderAPI_MakePolygon.hxx>
#include <BRepBuilderAPI_MakeFace.hxx>
TopoDS_Face PolygonalFace(const TColgp_Array1OfPnt& thePnts)
{
BRepBuilderAPI_MakePolygon MP;
for(Standard_Integer i=thePnts.Lower();
i<=thePnts.Upper(); i++)
{
MP.Add(thePnts(i));
}
MP.Close();
TopoDS_Face F = BRepBuilderAPI_MakeFace(MP.Wire());
return F;
}
The last use of MakeFace is to copy an existing face to add new wires. For example the following code adds a new wire to a face:
TopoDS_Face F = ...; // a face
TopoDS_Wire W = ...; // a wire
F = BRepBuilderAPI_MakeFace(F,W);
To add more than one wire an instance of the BRepBuilderAPI_MakeFace class can be created with the face and the first wire and the new wires inserted with the Add method.
Error status
The Error method returns an error status, which is a term from the BRepBuilderAPI_FaceError enumeration.
- FaceDone - no error occurred.
- NoFace - no initialization of the algorithm; an empty constructor was used.
- NotPlanar - no surface was given and the wire was not planar.
- CurveProjectionFailed - no curve was found in the parametric space of the surface for an edge.
- ParametersOutOfRange - the parameters umin, umax, vmin, vmax are out of the surface.
@subsubsection occt_modalg_3_1_6 Wire The wire is a composite shape built not from a geometry, but by the assembly of edges. BRepBuilderAPI_MakeWire class can build a wire from one or more edges or connect new edges to an existing wire.
Up to four edges can be used directly, for example:
TopoDS_Wire W = BRepBuilderAPI_MakeWire(E1,E2,E3,E4);
For a higher or unknown number of edges the Add method must be used; for example, to build a wire from an array of shapes (to be edges).
TopTools_Array1OfShapes theEdges;
BRepBuilderAPI_MakeWire MW;
for (Standard_Integer i = theEdge.Lower();
i <= theEdges.Upper(); i++)
MW.Add(TopoDS::Edge(theEdges(i));
TopoDS_Wire W = MW;
The class can be constructed with a wire. A wire can also be added. In this case, all the edges of the wires are added. For example to merge two wires:
#include <TopoDS_Wire.hxx>
#include <BRepBuilderAPI_MakeWire.hxx>
TopoDS_Wire MergeWires (const TopoDS_Wire& W1,
const TopoDS_Wire& W2)
{
BRepBuilderAPI_MakeWire MW(W1);
MW.Add(W2);
return MW;
}
BRepBuilderAPI_MakeWire class connects the edges to the wire. When a new edge is added if one of its vertices is shared with the wire it is considered as connected to the wire. If there is no shared vertex, the algorithm searches for a vertex of the edge and a vertex of the wire, which are at the same location (the tolerances of the vertices are used to test if they have the same location). If such a pair of vertices is found, the edge is copied with the vertex of the wire in place of the original vertex. All the vertices of the edge can be exchanged for vertices from the wire. If no connection is found the wire is considered to be disconnected. This is an error.
BRepBuilderAPI_MakeWire class can return the last edge added to the wire (Edge method). This edge can be different from the original edge if it was copied.
The Error method returns a term of the BRepBuilderAPI_WireError enumeration: WireDone - no error occurred. EmptyWire - no initialization of the algorithm, an empty constructor was used. DisconnectedWire - the last added edge was not connected to the wire. NonManifoldWire - the wire with some singularity.
@subsubsection occt_modalg_3_1_7 Shell The shell is a composite shape built not from a geometry, but by the assembly of faces. Use BRepBuilderAPI_MakeShell class to build a Shell from a set of Faces. What may be important is that each face should have the required continuity. That is why an initial surface is broken up into faces.
@subsubsection occt_modalg_3_1_8 Solid The solid is a composite shape built not from a geometry, but by the assembly of shells. Use BRepBuilderAPI_MakeSolid class to build a Solid from a set of Shells. Its use is similar to the use of the MakeWire class: shells are added to the solid in the same way that edges are added to the wire in MakeWire.
@subsubsection occt_modalg_3_2 Modification Operators
@subsubsection occt_modalg_3_2_1 Transformation BRepBuilderAPI_Transform class can be used to apply a transformation to a shape (see class gp_Trsf). The methods have a boolean argument to copy or share the original shape, as long as the transformation allows (it is only possible for direct isometric transformations). By default, the original shape is shared.
The following example deals with the rotation of shapes.
TopoDS_Shape myShape1 = ...;
// The original shape 1
TopoDS_Shape myShape2 = ...;
// The original shape2
gp_Trsf T;
T.SetRotation(gp_Ax1(gp_Pnt(0.,0.,0.),gp_Vec(0.,0.,1.)),
2.*PI/5.);
BRepBuilderAPI_Transformation theTrsf(T);
theTrsf.Perform(myShape1);
TopoDS_Shape myNewShape1 = theTrsf.Shape()
theTrsf.Perform(myShape2,Standard_True);
// Here duplication is forced
TopoDS_Shape myNewShape2 = theTrsf.Shape()
@subsubsection occt_modalg_3_2_2 Duplication
Use the BRepBuilderAPI_Copy class to duplicate a shape. A new shape is thus created. In the following example, a solid is copied:
TopoDS Solid MySolid;
....// Creates a solid
TopoDS_Solid myCopy = BRepBuilderAPI_Copy(mySolid);
@section occt_modalg_4 Construction of Primitives @subsection occt_modalg_4_1 Making Primitives @subsubsection occt_modalg_4_1_1 Box
BRepPrimAPI_MakeBox class allows building a parallelepiped box. The result is either a Shell or a Solid. There are four ways to build a box:
- From three dimensions dx,dy,dz. The box is parallel to the axes and extends for [0,dx] [0,dy] [0,dz].
- From a point and three dimensions. The same as above but the point is the new origin.
- From two points, the box is parallel to the axes and extends on the intervals defined by the coordinates of the two points.
- From a system of axes (gp_Ax2) and three dimensions. Same as the first way but the box is parallel to the given system of axes.
An error is raised if the box is flat in any dimension using the default precision. The following code shows how to create a box:
TopoDS_Solid theBox = BRepPrimAPI_MakeBox(10.,20.,30.);
The four methods to build a box are shown in the figure:
@image html /user_guides/modeling_algos/images/modeling_algos_image026.png "Making Boxes" @image latex /user_guides/modeling_algos/images/modeling_algos_image026.png "Making Boxes"
@subsubsection occt_modalg_4_1_2 Wedge BRepPrimAPI_MakeWedge class allows building a wedge, which is a slanted box, i.e. a box with angles. The wedge is constructed in much the same way as a box i.e. from three dimensions dx,dy,dz plus arguments or from an axis system, three dimensions, and arguments.
The following figure shows two ways to build wedges. One is to add an ltx dimension, which is the length in x of the face at dy. The second is to add xmin, xmax, zmin, zmax to describe the face at dy.
The first method is a particular case of the second with xmin = 0, xmax = ltx, zmin = 0, zmax = dz. To make a centered pyramid you can use xmin = xmax = dx / 2, zmin = zmax = dz / 2.
@image html /user_guides/modeling_algos/images/modeling_algos_image027.png "Making Wedges" @image latex /user_guides/modeling_algos/images/modeling_algos_image027.png "Making Wedges"
@subsubsection occt_modalg_4_1_3 Rotation object BRepPrimAPI_MakeOneAxis is a deferred class used as a root class for all classes constructing rotational primitives. Rotational primitives are created by rotating a curve around an axis. They cover the cylinder, the cone, the sphere, the torus, and the revolution, which provides all other curves.
The particular constructions of these primitives are described, but they all have some common arguments, which are:
- A system of coordinates, where the Z axis is the rotation axis..
- An angle in the range [0,2*PI].
- A vmin, vmax parameter range on the curve.
The result of the OneAxis construction is a Solid, a Shell, or a Face. The face is the face covering the rotational surface. Remember that you will not use the OneAxis directly but one of the derived classes, which provide improved constructions. The following figure illustrates the OneAxis arguments.
@image html /user_guides/modeling_algos/images/modeling_algos_image028.png "MakeOneAxis arguments" @image latex /user_guides/modeling_algos/images/modeling_algos_image028.png "MakeOneAxis arguments"
@subsubsection occt_modalg_4_1_4 Cylinder BRepPrimAPI_MakeCylinder class allows creating cylindrical primitives. A cylinder is created either in the default coordinate system or in a given coordinate system (gp_Ax2). There are two constructions:
- Radius and height, to build a full cylinder.
- Radius, height and angle to build a portion of a cylinder.
The following code builds the cylindrical face of the figure, which is a quarter of cylinder along the Y axis with the origin at X,Y,Z, a length of DY, and a radius R.
Standard_Real X = 20, Y = 10, Z = 15, R = 10, DY = 30;
// Make the system of coordinates
gp_Ax2 axes = gp::ZOX();
axes.Translate(gp_Vec(X,Y,Z));
TopoDS_Face F =
BRepPrimAPI_MakeCylinder(axes,R,DY,PI/2.);
@image html /user_guides/modeling_algos/images/modeling_algos_image029.png "Cylinder" @image latex /user_guides/modeling_algos/images/modeling_algos_image029.png "Cylinder"
@subsubsection occt_modalg_4_1_5 Cone BRepPrimAPI_MakeCone class allows creating conical primitives. Like a cylinder, a cone is created either in the default coordinate system or in a given coordinate system (gp_Ax2). There are two constructions:
- Two radii and height, to build a full cone. One of the radii can be null to make a sharp cone.
- Radii, height and angle to build a truncated cone.
The following code builds the solid cone of the figure, which is located in the default system with radii R1 and R2 and height H.
Standard_Real R1 = 30, R2 = 10, H = 15;
TopoDS_Solid S = BRepPrimAPI_MakeCone(R1,R2,H);
@image html /user_guides/modeling_algos/images/modeling_algos_image030.png "Cone" @image latex /user_guides/modeling_algos/images/modeling_algos_image030.png "Cone"
@subsubsection occt_modalg_4_1_6 Sphere BRepPrimAPI_MakeSphere class allows creating spherical primitives. Like a cylinder, a sphere is created either in the default coordinate system or in a given coordinate system (gp_Ax2). There are four constructions:
- From a radius - builds a full sphere.
- From a radius and an angle - builds a lune (digon).
- From a radius and two angles - builds a wraparound spherical segment between two latitudes. The angles a1, a2 must follow the relation: PI/2 <= a1 < a2 <= PI/2.
- From a radius and three angles - a combination of two previous methods builds a portion of spherical segment.
The following code builds four spheres from a radius and three angles.
Standard_Real R = 30, ang =
PI/2, a1 = -PI/2.3, a2 = PI/4;
TopoDS_Solid S1 = BRepPrimAPI_MakeSphere(R);
TopoDS_Solid S2 = BRepPrimAPI_MakeSphere(R,ang);
TopoDS_Solid S3 = BRepPrimAPI_MakeSphere(R,a1,a2);
TopoDS_Solid S4 = BRepPrimAPI_MakeSphere(R,a1,a2,ang);
Note that we could equally well choose to create Shells instead of Solids.
@image html /user_guides/modeling_algos/images/modeling_algos_image031.png "Examples of Spheres" @image latex /user_guides/modeling_algos/images/modeling_algos_image031.png "Examples of Spheres"
@subsubsection occt_modalg_4_1_7 Torus BRepPrimAPI_MakeTorus class allows creating toroidal primitives. Like the other primitives, a torus is created either in the default coordinate system or in a given coordinate system (gp_Ax2). There are four constructions similar to the sphere constructions:
- Two radii - builds a full torus.
- Two radii and an angle - builds an angular torus segment.
- Two radii and two angles - builds a wraparound torus segment between two radial planes. The angles a1, a2 must follow the relation 0 < a2 - a1 < 2*PI.
- Two radii and three angles - a combination of two previous methods builds a portion of torus segment.
@image html /user_guides/modeling_algos/images/modeling_algos_image032.png "Examples of Tori" @image latex /user_guides/modeling_algos/images/modeling_algos_image032.png "Examples of Tori"
The following code builds four toroidal shells from two radii and three angles.
Standard_Real R1 = 30, R2 = 10, ang = PI, a1 = 0,
a2 = PI/2;
TopoDS_Shell S1 = BRepPrimAPI_MakeTorus(R1,R2);
TopoDS_Shell S2 = BRepPrimAPI_MakeTorus(R1,R2,ang);
TopoDS_Shell S3 = BRepPrimAPI_MakeTorus(R1,R2,a1,a2);
TopoDS_Shell S4 =
BRepPrimAPI_MakeTorus(R1,R2,a1,a2,ang);
Note that we could equally well choose to create Solids instead of Shells.
@subsubsection occt_modalg_4_1_8 Revolution BRepPrimAPI_MakeRevolution class allows building a uniaxial primitive from a curve. As other uniaxial primitives it can be created in the default coordinate system or in a given coordinate system.
The curve can be any Geom_Curve, provided it is planar and lies in the same plane as the Z-axis of local coordinate system. There are four modes of construction:
- From a curve, use the full curve and make a full rotation.
- From a curve and an angle of rotation.
- From a curve and two parameters to trim the curve. The two parameters must be growing and within the curve range.
- From a curve, two parameters, and an angle. The two parameters must be growing and within the curve range.
@subsection occt_modalg_4_2 Sweeping: Prism, Revolution and Pipe @subsubsection occt_modalg_4_2_1 Sweeping
Sweeps are the objects you obtain by sweeping a profile along a path. The profile can be of any topology. The path is usually a curve or a wire. The profile generates objects according to the following rules:
- Vertices generate Edges
- Edges generate Faces.
- Wires generate Shells.
- Faces generate Solids.
- Shells generate Composite Solids
It is forbidden to sweep Solids and Composite Solids. A Compound generates a Compound with the sweep of all its elements.
@image html /user_guides/modeling_algos/images/modeling_algos_image033.png "Generating a sweep" @image latex /user_guides/modeling_algos/images/modeling_algos_image033.png "Generating a sweep"
BRepPrimAPI_MakeSweep class is a deferred class used as a root of the the following sweep classes:
- BRepPrimAPI_MakePrism - produces a linear sweep
- BRepPrimAPI_MakeRevol - produces a rotational sweep
- BRepPrimAPI_MakePipe - produces a general sweep.
@subsubsection occt_modalg_4_2_2 Prism BRepPrimAPI_MakePrism class allows creating a linear prism from a shape and a vector or a direction.
- A vector allows creating a finite prism;
- A direction allows creating an infinite or semi-infinite prism. The semi-infinite or infinite prism is toggled by a Boolean argument. All constructors have a boolean argument to copy the original shape or share it (by default).
The following code creates a finite, an infinite and a semi-infinite solid using a face, a direction and a length.
TopoDS_Face F = ..; // The swept face
gp_Dir direc(0,0,1);
Standard_Real l = 10;
// create a vector from the direction and the length
gp_Vec v = direc;
v *= l;
TopoDS_Solid P1 = BRepPrimAPI_MakePrism(F,v);
// finite
TopoDS_Solid P2 = BRepPrimAPI_MakePrism(F,direc);
// infinite
TopoDS_Solid P3 = BRepPrimAPI_MakePrism(F,direc,Standard_False);
// semi-infinite
@image html /user_guides/modeling_algos/images/modeling_algos_image034.png "Finite, infinite, and semi-infinite prisms" @image latex /user_guides/modeling_algos/images/modeling_algos_image034.png "Finite, infinite, and semi-infinite prisms"
@subsubsection occt_modalg_4_2_3 Rotation BRepPrimAPI_MakeRevol class allows creating a rotational sweep from a shape, an axis (gp_Ax1), and an angle. The angle has a default value of 2*PI which means a closed revolution.
BRepPrimAPI_MakeRevol constructors have a last argument to copy or share the original shape. The following code creates a a full and a partial rotation using a face, an axis and an angle.
TopoDS_Face F = ...; // the profile
gp_Ax1 axis(gp_Pnt(0,0,0),gp_Dir(0,0,1));
Standard_Real ang = PI/3;
TopoDS_Solid R1 = BRepPrimAPI_MakeRevol(F,axis);
// Full revol
TopoDS_Solid R2 = BRepPrimAPI_MakeRevol(F,axis,ang);
@image html /user_guides/modeling_algos/images/modeling_algos_image035.png "Full and partial rotation" @image latex /user_guides/modeling_algos/images/modeling_algos_image035.png "Full and partial rotation"
@section occt_modalg_5 Boolean Operations
Boolean operations are used to create new shapes from the combinations of two shapes. See @ref occt_user_guides__boolean_operations "Boolean Operations" for detailed documentation.
| Operation | Result |
|---|---|
| Fuse | all points in S1 or S2 |
| Common | all points in S1 and S2 |
| Cut S1 by S2 | all points in S1 and not in S2 |
BRepAlgoAPI_BooleanOperation class is the deferred root class for Boolean operations.
@image html /user_guides/modeling_algos/images/modeling_algos_image036.png "Boolean Operations" @image latex /user_guides/modeling_algos/images/modeling_algos_image036.png "Boolean Operations"
@subsection occt_modalg_5_1 Fuse
BRepAlgoAPI_Fuse performs the Fuse operation.
TopoDS_Shape A = ..., B = ...;
TopoDS_Shape S = BRepAlgoAPI_Fuse(A,B);
@subsection occt_modalg_5_2 Common
BRepAlgoAPI_Common performs the Common operation.
TopoDS_Shape A = ..., B = ...;
TopoDS_Shape S = BRepAlgoAPI_Common(A,B);
@subsection occt_modalg_5_3 Cut BRepAlgoAPI_Cut performs the Cut operation.
TopoDS_Shape A = ..., B = ...;
TopoDS_Shape S = BRepAlgoAPI_Cut(A,B);
@subsection occt_modalg_5_4 Section
BRepAlgoAPI_Section performs the section, described as a TopoDS_Compound made of TopoDS_Edge.
@image html /user_guides/modeling_algos/images/modeling_algos_image037.png "Section operation" @image latex /user_guides/modeling_algos/images/modeling_algos_image037.png "Section operation"
TopoDS_Shape A = ..., TopoDS_ShapeB = ...;
TopoDS_Shape S = BRepAlgoAPI_Section(A,B);
@section occt_modalg_6 Fillets and Chamfers
@subsection occt_modalg_6_1 Fillets
@subsection occt_modalg_6_1_1 Fillet on shape
A fillet is a smooth face replacing a sharp edge.
BRepFilletAPI_MakeFillet class allows filleting a shape.
To produce a fillet, it is necessary to define the filleted shape at the construction of the class and add fillet descriptions using the Add method.
A fillet description contains an edge and a radius. The edge must be shared by two faces. The fillet is automatically extended to all edges in a smooth continuity with the original edge. It is not an error to add a fillet twice, the last description holds.
@image html /user_guides/modeling_algos/images/modeling_algos_image038.png "Filleting two edges using radii r1 and r2." @image latex /user_guides/modeling_algos/images/modeling_algos_image038.png "Filleting two edges using radii r1 and r2."
In the following example a filleted box with dimensions a,b,c and radius r is created.
Constant radius
#include <TopoDS_Shape.hxx>
#include <TopoDS.hxx>
#include <BRepPrimAPI_MakeBox.hxx>
#include <TopoDS_Solid.hxx>
#include <BRepFilletAPI_MakeFillet.hxx>
#include <TopExp_Explorer.hxx>
TopoDS_Shape FilletedBox(const Standard_Real a,
const Standard_Real b,
const Standard_Real c,
const Standard_Real r)
{
TopoDS_Solid Box = BRepPrimAPI_MakeBox(a,b,c);
BRepFilletAPI_MakeFillet MF(Box);
// add all the edges to fillet
TopExp_Explorer ex(Box,TopAbs_EDGE);
while (ex.More())
{
MF.Add(r,TopoDS::Edge(ex.Current()));
ex.Next();
}
return MF.Shape();
}
@image html /user_guides/modeling_algos/images/modeling_algos_image039.png "Fillet with constant radius" @image latex /user_guides/modeling_algos/images/modeling_algos_image039.png "Fillet with constant radius"
Changing radius
void CSampleTopologicalOperationsDoc::OnEvolvedblend1()
{
TopoDS_Shape theBox = BRepPrimAPI_MakeBox(200,200,200);
BRepFilletAPI_MakeFillet Rake(theBox);
ChFi3d_FilletShape FSh = ChFi3d_Rational;
Rake.SetFilletShape(FSh);
TColgp_Array1OfPnt2d ParAndRad(1, 6);
ParAndRad(1).SetCoord(0., 10.);
ParAndRad(1).SetCoord(50., 20.);
ParAndRad(1).SetCoord(70., 20.);
ParAndRad(1).SetCoord(130., 60.);
ParAndRad(1).SetCoord(160., 30.);
ParAndRad(1).SetCoord(200., 20.);
TopExp_Explorer ex(theBox,TopAbs_EDGE);
Rake.Add(ParAndRad, TopoDS::Edge(ex.Current()));
TopoDS_Shape evolvedBox = Rake.Shape();
}
@image html /user_guides/modeling_algos/images/modeling_algos_image040.png "Fillet with changing radius" @image latex /user_guides/modeling_algos/images/modeling_algos_image040.png "Fillet with changing radius"
@subsection occt_modalg_6_1_2 Chamfer
A chamfer is a rectilinear edge replacing a sharp vertex of the face.
The use of BRepFilletAPI_MakeChamfer class is similar to the use of BRepFilletAPI_MakeFillet, except for the following:
- The surfaces created are ruled and not smooth.
- The Add syntax for selecting edges requires one or two distances, one edge and one face (contiguous to the edge).
Add(dist, E, F)
Add(d1, d2, E, F) with d1 on the face F.
@image html /user_guides/modeling_algos/images/modeling_algos_image041.png "Chamfer" @image latex /user_guides/modeling_algos/images/modeling_algos_image041.png "Chamfer"
@subsection occt_modalg_6_1_3 Fillet on a planar face
BRepFilletAPI_MakeFillet2d class allows constructing fillets and chamfers on planar faces. To create a fillet on planar face: define it, indicate, which vertex is to be deleted, and give the fillet radius with AddFillet method.
A chamfer can be calculated with AddChamfer method. It can be described by
- two edges and two distances
- one edge, one vertex, one distance and one angle. Fillets and chamfers are calculated when addition is complete.
If face F2 is created by 2D fillet and chamfer builder from face F1, the builder can be rebuilt (the builder recovers the status it had before deletion). To do so, use the following syntax:
BRepFilletAPI_MakeFillet2d builder;
builder.Init(F1,F2);
Planar Fillet
#include “BRepPrimAPI_MakeBox.hxx”
#include “TopoDS_Shape.hxx”
#include “TopExp_Explorer.hxx”
#include “BRepFilletAPI_MakeFillet2d.hxx”
#include “TopoDS.hxx”
#include “TopoDS_Solid.hxx”
TopoDS_Shape FilletFace(const Standard_Real a,
const Standard_Real b,
const Standard_Real c,
const Standard_Real r)
{
TopoDS_Solid Box = BRepPrimAPI_MakeBox (a,b,c);
TopExp_Explorer ex1(Box,TopAbs_FACE);
const TopoDS_Face& F = TopoDS::Face(ex1.Current());
BRepFilletAPI_MakeFillet2d MF(F);
TopExp_Explorer ex2(F, TopAbs_VERTEX);
while (ex2.More())
{
MF.AddFillet(TopoDS::Vertex(ex2.Current()),r);
ex2.Next();
}
// while...
return MF.Shape();
}
@section occt_modalg_7 Offsets, Drafts, Pipes and Evolved shapes @subsection occt_modalg_7_1 Shelling
Shelling is used to offset given faces of a solid by a specific value. It rounds or intersects adjacent faces along its edges depending on the convexity of the edge.
The constructor BRepOffsetAPI_MakeThickSolid shelling operator takes the solid, the list of faces to remove and an offset value as input.
TopoDS_Solid SolidInitial = ...;
Standard_Real Of = ...;
TopTools_ListOfShape LCF;
TopoDS_Shape Result;
Standard_Real Tol = Precision::Confusion();
for (Standard_Integer i = 1 ;i <= n; i++) {
TopoDS_Face SF = ...; // a face from SolidInitial
LCF.Append(SF);
}
Result = BRepOffsetAPI_MakeThickSolid (SolidInitial,
LCF,
Of,
Tol);
@image html /user_guides/modeling_algos/images/modeling_algos_image042.png "Shelling" @image latex /user_guides/modeling_algos/images/modeling_algos_image042.png "Shelling"
@subsection occt_modalg_7_2 Draft Angle
BRepOffsetAPI_DraftAngle class allows modifying a shape by applying draft angles to its planar, cylindrical and conical faces.
The class is created or initialized from a shape, then faces to be modified are added; for each face, three arguments are used:
- Direction: the direction with which the draft angle is measured
- Angle: value of the angle
- Neutral plane: intersection between the face and the neutral plane is invariant.
The following code places a draft angle on several faces of a shape; the same direction, angle and neutral plane are used for each face:
TopoDS_Shape myShape = ...
// The original shape
TopTools_ListOfShape ListOfFace;
// Creation of the list of faces to be modified
...
gp_Dir Direc(0.,0.,1.);
// Z direction
Standard_Real Angle = 5.*PI/180.;
// 5 degree angle
gp_Pln Neutral(gp_Pnt(0.,0.,5.), Direc);
// Neutral plane Z=5
BRepOffsetAPI_DraftAngle theDraft(myShape);
TopTools_ListIteratorOfListOfShape itl;
for (itl.Initialize(ListOfFace); itl.More(); itl.Next()) {
theDraft.Add(TopoDS::Face(itl.Value()),Direc,Angle,Neutral);
if (!theDraft.AddDone()) {
// An error has occurred. The faulty face is given by // ProblematicShape
break;
}
}
if (!theDraft.AddDone()) {
// An error has occurred
TopoDS_Face guilty = theDraft.ProblematicShape();
...
}
theDraft.Build();
if (!theDraft.IsDone()) {
// Problem encountered during reconstruction
...
}
else {
TopoDS_Shape myResult = theDraft.Shape();
...
}
@image html /user_guides/modeling_algos/images/modeling_algos_image043.png "DraftAngle" @image latex /user_guides/modeling_algos/images/modeling_algos_image043.png "DraftAngle"
@subsection occt_modalg_7_3 Pipe Constructor
BRepOffsetAPI_MakePipe class allows creating a pipe from a Spine, which is a Wire and a Profile which is a Shape. This implementation is limited to spines with smooth transitions, sharp transitions are precessed by BRepOffsetAPI_MakePipeShell. To be more precise the continuity must be G1, which means that the tangent must have the same direction, though not necessarily the same magnitude, at neighboring edges.
The angle between the spine and the profile is preserved throughout the pipe.
TopoDS_Wire Spine = ...;
TopoDS_Shape Profile = ...;
TopoDS_Shape Pipe = BRepOffsetAPI_MakePipe(Spine,Profile);
@image html /user_guides/modeling_algos/images/modeling_algos_image044.png "Example of a Pipe" @image latex /user_guides/modeling_algos/images/modeling_algos_image044.png "Example of a Pipe"
@subsection occt_modalg_7_4 Evolved Solid
BRepOffsetAPI_MakeEvolved class allows creating an evolved solid from a Spine (planar face or wire) and a profile (wire).
The evolved solid is an unlooped sweep generated by the spine and the profile.
The evolved solid is created by sweeping the profile’s reference axes on the spine. The origin of the axes moves to the spine, the X axis and the local tangent coincide and the Z axis is normal to the face.
The reference axes of the profile can be defined following two distinct modes:
- The reference axes of the profile are the origin axes.
- The references axes of the profile are calculated as follows:
- the origin is given by the point on the spine which is the closest to the profile
- the X axis is given by the tangent to the spine at the point defined above
- the Z axis is the normal to the plane which contains the spine.
TopoDS_Face Spine = ...;
TopoDS_Wire Profile = ...;
TopoDS_Shape Evol =
BRepOffsetAPI_MakeEvolved(Spine,Profile);
@section occt_modalg_8 Sewing operators @subsection occt_modalg_8_1 Sewing
BRepOffsetAPI_Sewing class allows sewing TopoDS Shapes together along their common edges. The edges can be partially shared as in the following example.
@image html /user_guides/modeling_algos/images/modeling_algos_image045.png "Shapes with partially shared edges" @image latex /user_guides/modeling_algos/images/modeling_algos_image045.png "Shapes with partially shared edges"
The constructor takes as arguments the tolerance (default value is 10-6) and a flag, which is used to mark the degenerate shapes.
The following methods are used in this class:
- Add adds shapes, as it is needed;
- Perform forces calculation of the sewed shape.
- SewedShape returns the result.
Additional methods can be used to give information on the number of free boundaries, multiple edges and degenerate shapes.
@subsection occt_modalg_8_2 Find Contiguous Edges BRepOffsetAPI_FindContiguousEdges class is used to find edges, which coincide among a set of shapes within the given tolerance; these edges can be analyzed for tangency, continuity (C1, G2, etc.)...
The constructor takes as arguments the tolerance defining the edge proximity (10-6 by default) and a flag used to mark degenerated shapes.
The following methods are used in this class:
- Add adds shapes, which are to be analyzed;
- NbEdges returns the total number of edges;
- NbContiguousEdges returns the number of contiguous edges within the given tolerance as defined above;
- ContiguousEdge takes an edge number as an argument and returns the TopoDS edge contiguous to another edge;
- ContiguousEdgeCouple gives all edges or sections, which are common to the edge with the number given above.
- SectionToBoundary finds the original edge on the original shape from the section.
@section occt_modalg_9 Features
@subsection occt_modalg_9_1 The BRepFeat Classes and their use BRepFeat package is used to manipulate extensions of the classical boundary representation of shapes closer to features. In that sense, BRepFeat is an extension of BRepBuilderAPI package.
@subsubsection occt_modalg_9_1_1 Form classes The Form from BRepFeat class is a deferred class used as a root for form features. It inherits MakeShape from BRepBuilderAPI and provides implementation of methods keep track of all sub-shapes.
MakePrism
MakePrism from BRepFeat class is used to build a prism interacting with a shape. It is created or initialized from
- a shape (the basic shape),
- the base of the prism,
- a face (the face of sketch on which the base has been defined and used to determine whether the base has been defined on the basic shape or not),
- a direction,
- a Boolean indicating the type of operation (fusion=protrusion or cut=depression) on the basic shape,
- another Boolean indicating if the self-intersections have to be found (not used in every case).
There are six Perform methods:
| Method | Description |
|---|---|
| Perform(Height) | The resulting prism is of the given length. |
| Perform(Until) | The prism is defined between the position of the base and the given face. |
| Perform(From, Until) | The prism is defined between the two faces From and Until. |
| PerformUntilEnd() | The prism is semi-infinite, limited by the actual position of the base. |
| PerformFromEnd(Until) | The prism is semi-infinite, limited by the face Until. |
| PerformThruAll() | The prism is infinite. In the case of a depression, the result is similar to a cut with an infinite prism. In the case of a protrusion, infinite parts are not kept in the result. |
Note that Add method can be used before Perform methods to indicate that a face generated by an edge slides onto a face of the base shape.
In the following sequence, a protrusion is performed, i.e. a face of the shape is changed into a prism.
TopoDS_Shape Sbase = ...; // an initial shape
TopoDS_Face Fbase = ....; // a base of prism
gp_Dir Extrusion (.,.,.);
// An empty face is given as the sketch face
BRepFeat_MakePrism thePrism(Sbase, Fbase, TopoDS_Face(), Extrusion, Standard_True, Standard_True);
thePrism, Perform(100.);
if (thePrism.IsDone()) {
TopoDS_Shape theResult = thePrism;
...
}
@image html /user_guides/modeling_algos/images/modeling_algos_image047.png "Fusion with MakePrism" @image latex /user_guides/modeling_algos/images/modeling_algos_image047.png "Fusion with MakePrism"
@image html /user_guides/modeling_algos/images/modeling_algos_image048.png "Creating a prism between two faces with Perform(From, Until)" @image latex /user_guides/modeling_algos/images/modeling_algos_image048.png "Creating a prism between two faces with Perform(From, Until)"
MakeDPrism
MakeDPrism from BRepFeat class is used to build draft prism topologies interacting with a basis shape . These can be depressions or protrusions. A class object is created or initialized from
- a shape (basic shape),
- the base of the prism,
- a face (face of sketch on which the base has been defined and used to determine whether the base has been defined on the basic shape or not),
- an angle,
- a Boolean indicating the type of operation (fusion=protrusion or cut=depression) on the basic shape,
- another Boolean indicating if self-intersections have to be found (not used in every case).
Evidently the input data for MakeDPrism are the same as for MakePrism except for a new parameter Angle and a missing parameter Direction: the direction of the prism generation is determined automatically as the normal to the base of the prism. The semantics of draft prism feature creation is based on the construction of shapes:
- along a length
- up to a limiting face
- from a limiting face to a height.
The shape defining construction of the draft prism feature can be either the supporting edge or the concerned area of a face.
In case of the supporting edge, this contour can be attached to a face of the basis shape by binding. When the contour is bound to this face, the information that the contour will slide on the face becomes available to the relevant class methods. In case of the concerned area of a face, it is possible to cut it out and move it to a different height, which will define the limiting face of a protrusion or depression direction .
The Perform methods are the same as for MakePrism.
TopoDS_Shape S = BRepPrimAPI_MakeBox(400.,250.,300.);
TopExp_Explorer Ex;
Ex.Init(S,TopAbs_FACE);
Ex.Next();
Ex.Next();
Ex.Next();
Ex.Next();
Ex.Next();
TopoDS_Face F = TopoDS::Face(Ex.Current());
Handle(Geom_Surface) surf = BRep_Tool::Surface(F);
gp_Circ2d
c(gp_Ax2d(gp_Pnt2d(200.,130.),gp_Dir2d(1.,0.)),50.);
BRepBuilderAPI_MakeWire MW;
Handle(Geom2d_Curve) aline = new Geom2d_Circle(c);
MW.Add(BRepBuilderAPI_MakeEdge(aline,surf,0.,PI));
MW.Add(BRepBuilderAPI_MakeEdge(aline,surf,PI,2.*PI));
BRepBuilderAPI_MakeFace MKF;
MKF.Init(surf,Standard_False);
MKF.Add(MW.Wire());
TopoDS_Face FP = MKF.Face();
BRepLib::BuildCurves3d(FP);
BRepFeat_MakeDPrism MKDP (S,FP,F,10*PI180,Standard_True,
Standard_True);
MKDP.Perform(200);
TopoDS_Shape res1 = MKDP.Shape();
@image html /user_guides/modeling_algos/images/modeling_algos_image049.png "A tapered prism" @image latex /user_guides/modeling_algos/images/modeling_algos_image049.png "A tapered prism"
MakeRevol
The MakeRevol from BRepFeat class is used to build a revolution interacting with a shape. It is created or initialized from
- a shape (the basic shape,)
- the base of the revolution,
- a face (the face of sketch on which the base has been defined and used to determine whether the base has been defined on the basic shape or not),
- an axis of revolution,
- a boolean indicating the type of operation (fusion=protrusion or cut=depression) on the basic shape,
- another boolean indicating whether the self-intersections have to be found (not used in every case).
There are four Perform methods:
| Method | Description |
|---|---|
| Perform(Angle) | The resulting revolution is of the given magnitude. |
| Perform(Until) | The revolution is defined between the actual position of the base and the given face. |
| Perform(From, Until) | The revolution is defined between the two faces, From and Until. |
| PerformThruAll() | The result is similar to Perform(2*PI). |
Note that Add method can be used before Perform methods to indicate that a face generated by an edge slides onto a face of the base shape.
In the following sequence, a face is revolved and the revolution is limited by a face of the base shape.
TopoDS_Shape Sbase = ...; // an initial shape
TopoDS_Face Frevol = ....; // a base of prism
TopoDS_Face FUntil = ....; // face limiting the revol
gp_Dir RevolDir (.,.,.);
gp_Ax1 RevolAx(gp_Pnt(.,.,.), RevolDir);
// An empty face is given as the sketch face
BRepFeat_MakeRevol theRevol(Sbase, Frevol, TopoDS_Face(), RevolAx, Standard_True, Standard_True);
theRevol.Perform(FUntil);
if (theRevol.IsDone()) {
TopoDS_Shape theResult = theRevol;
...
}
MakePipe
This method constructs compound shapes with pipe features: depressions or protrusions. A class object is created or initialized from
- a shape (basic shape),
- a base face (profile of the pipe)
- a face (face of sketch on which the base has been defined and used to determine whether the base has been defined on the basic shape or not),
- a spine wire
- a Boolean indicating the type of operation (fusion=protrusion or cut=depression) on the basic shape,
- another Boolean indicating if self-intersections have to be found (not used in every case).
There are three Perform methods:
| Method | Description |
|---|---|
| Perform() | The pipe is defined along the entire path (spine wire) |
| Perform(Until) | The pipe is defined along the path until a given face |
| Perform(From, Until) | The pipe is defined between the two faces From and Until |
TopoDS_Shape S = BRepPrimAPI_MakeBox(400.,250.,300.);
TopExp_Explorer Ex;
Ex.Init(S,TopAbs_FACE);
Ex.Next();
Ex.Next();
TopoDS_Face F1 = TopoDS::Face(Ex.Current());
Handle(Geom_Surface) surf = BRep_Tool::Surface(F1);
BRepBuilderAPI_MakeWire MW1;
gp_Pnt2d p1,p2;
p1 = gp_Pnt2d(100.,100.);
p2 = gp_Pnt2d(200.,100.);
Handle(Geom2d_Line) aline = GCE2d_MakeLine(p1,p2).Value();
MW1.Add(BRepBuilderAPI_MakeEdge(aline,surf,0.,p1.Distance(p2)));
p1 = p2;
p2 = gp_Pnt2d(150.,200.);
aline = GCE2d_MakeLine(p1,p2).Value();
MW1.Add(BRepBuilderAPI_MakeEdge(aline,surf,0.,p1.Distance(p2)));
p1 = p2;
p2 = gp_Pnt2d(100.,100.);
aline = GCE2d_MakeLine(p1,p2).Value();
MW1.Add(BRepBuilderAPI_MakeEdge(aline,surf,0.,p1.Distance(p2)));
BRepBuilderAPI_MakeFace MKF1;
MKF1.Init(surf,Standard_False);
MKF1.Add(MW1.Wire());
TopoDS_Face FP = MKF1.Face();
BRepLib::BuildCurves3d(FP);
TColgp_Array1OfPnt CurvePoles(1,3);
gp_Pnt pt = gp_Pnt(150.,0.,150.);
CurvePoles(1) = pt;
pt = gp_Pnt(200.,100.,150.);
CurvePoles(2) = pt;
pt = gp_Pnt(150.,200.,150.);
CurvePoles(3) = pt;
Handle(Geom_BezierCurve) curve = new Geom_BezierCurve
(CurvePoles);
TopoDS_Edge E = BRepBuilderAPI_MakeEdge(curve);
TopoDS_Wire W = BRepBuilderAPI_MakeWire(E);
BRepFeat_MakePipe MKPipe (S,FP,F1,W,Standard_False,
Standard_True);
MKPipe.Perform();
TopoDS_Shape res1 = MKPipe.Shape();
@image html /user_guides/modeling_algos/images/modeling_algos_image050.png "Pipe depression" @image latex /user_guides/modeling_algos/images/modeling_algos_image050.png "Pipe depression"
@subsubsection occt_modalg_9_1_2 Linear Form
MakeLinearForm class creates a rib or a groove along a planar surface.
The semantics of mechanical features is built around giving thickness to a contour. This thickness can either be symmetrical - on one side of the contour - or dissymmetrical - on both sides. As in the semantics of form features, the thickness is defined by construction of shapes in specific contexts.
The development contexts differ, however, in the case of mechanical features. Here they include extrusion:
- to a limiting face of the basis shape;
- to or from a limiting plane;
- to a height. A class object is created or initialized from
- a shape (basic shape);
- a wire (base of rib or groove);
- a plane (plane of the wire);
- direction1 (a vector along which thickness will be built up);
- direction2 (vector opposite to the previous one along which thickness will be built up, may be null);
- a Boolean indicating the type of operation (fusion=rib or cut=groove) on the basic shape;
- another Boolean indicating if self-intersections have to be found (not used in every case).
There is one Perform() method, which performs a prism from the wire along the direction1 and direction2 interacting with base shape Sbase. The height of the prism is Magnitude(Direction1)+Magnitude(direction2).
BRepBuilderAPI_MakeWire mkw;
gp_Pnt p1 = gp_Pnt(0.,0.,0.);
gp_Pnt p2 = gp_Pnt(200.,0.,0.);
mkw.Add(BRepBuilderAPI_MakeEdge(p1,p2));
p1 = p2;
p2 = gp_Pnt(200.,0.,50.);
mkw.Add(BRepBuilderAPI_MakeEdge(p1,p2));
p1 = p2;
p2 = gp_Pnt(50.,0.,50.);
mkw.Add(BRepBuilderAPI_MakeEdge(p1,p2));
p1 = p2;
p2 = gp_Pnt(50.,0.,200.);
mkw.Add(BRepBuilderAPI_MakeEdge(p1,p2));
p1 = p2;
p2 = gp_Pnt(0.,0.,200.);
mkw.Add(BRepBuilderAPI_MakeEdge(p1,p2));
p1 = p2;
mkw.Add(BRepBuilderAPI_MakeEdge(p2,gp_Pnt(0.,0.,0.)));
TopoDS_Shape S = BRepBuilderAPI_MakePrism(BRepBuilderAPI_MakeFace
(mkw.Wire()),gp_Vec(gp_Pnt(0.,0.,0.),gp_P
nt(0.,100.,0.)));
TopoDS_Wire W = BRepBuilderAPI_MakeWire(BRepBuilderAPI_MakeEdge(gp_Pnt
(50.,45.,100.),
gp_Pnt(100.,45.,50.)));
Handle(Geom_Plane) aplane =
new Geom_Plane(gp_Pnt(0.,45.,0.), gp_Vec(0.,1.,0.));
BRepFeat_MakeLinearForm aform(S, W, aplane, gp_Dir
(0.,5.,0.), gp_Dir(0.,-3.,0.), 1, Standard_True);
aform.Perform();
TopoDS_Shape res = aform.Shape();
@image html /user_guides/modeling_algos/images/modeling_algos_image051.png "Creating a rib" @image latex /user_guides/modeling_algos/images/modeling_algos_image051.png "Creating a rib"
@subsubsection occt_modalg_9_1_3 Gluer
The Gluer from BRepFeat class allows gluing two solids along faces. The contact faces of the glued shape must not have parts outside the contact faces of the basic shape. The class is created or initialized from two shapes: the “glued” shape and the basic shape (on which the other shape is glued). Two Bind methods are used to bind a face of the glued shape to a face of the basic shape and an edge of the glued shape to an edge of the basic shape.
Note that every face and edge has to be bounded, if two edges of two glued faces are coincident they must be explicitly bounded.
TopoDS_Shape Sbase = ...; // the basic shape
TopoDS_Shape Sglued = ...; // the glued shape
TopTools_ListOfShape Lfbase;
TopTools_ListOfShape Lfglued;
// Determination of the glued faces
...
BRepFeat_Gluer theGlue(Sglue, Sbase);
TopTools_ListIteratorOfListOfShape itlb(Lfbase);
TopTools_ListIteratorOfListOfShape itlg(Lfglued);
for (; itlb.More(); itlb.Next(), itlg(Next()) {
const TopoDS_Face& f1 = TopoDS::Face(itlg.Value());
const TopoDS_Face& f2 = TopoDS::Face(itlb.Value());
theGlue.Bind(f1,f2);
// for example, use the class FindEdges from LocOpe to
// determine coincident edges
LocOpe_FindEdge fined(f1,f2);
for (fined.InitIterator(); fined.More(); fined.Next()) {
theGlue.Bind(fined.EdgeFrom(),fined.EdgeTo());
}
}
theGlue.Build();
if (theGlue.IsDone() {
TopoDS_Shape theResult = theGlue;
...
}
@subsubsection occt_modalg_9_1_4 Split Shape
SplitShape from BRepFeat class is used to split faces of a shape with wires or edges. The shape containing the new entities is rebuilt, sharing the unmodified ones.
The class is created or initialized from a shape (the basic shape). Three Add methods are available:
- Add(Wire, Face) - adds a new wire on a face of the basic shape.
- Add(Edge, Face) - adds a new edge on a face of the basic shape.
- Add(EdgeNew, EdgeOld) - adds a new edge on an existing one (the old edge must contain the new edge).
Note The added wires and edges must define closed wires on faces or wires located between two existing edges. Existing edges must not be intersected.
TopoDS_Shape Sbase = ...; // basic shape
TopoDS_Face Fsplit = ...; // face of Sbase
TopoDS_Wire Wsplit = ...; // new wire contained in Fsplit
BRepFeat_SplitShape Spls(Sbase);
Spls.Add(Wsplit, Fsplit);
TopoDS_Shape theResult = Spls;
...
@section occt_modalg_10 Hidden Line Removal
To provide the precision required in industrial design, drawings need to offer the possibility of removing lines, which are hidden in a given projection.
For this the Hidden Line Removal component provides two algorithms: HLRBRep_Algo and HLRBRep_PolyAlgo.
These algorithms are based on the principle of comparing each edge of the shape to be visualized with each of its faces, and calculating the visible and the hidden parts of each edge. Note that these are not the algorithms used in generating shading, which calculate the visible and hidden parts of each face in a shape to be visualized by comparing each face in the shape with every other face in the same shape. These algorithms operate on a shape and remove or indicate edges hidden by faces. For a given projection, they calculate a set of lines characteristic of the object being represented. They are also used in conjunction with extraction utilities, which reconstruct a new, simplified shape from a selection of the results of the calculation. This new shape is made up of edges, which represent the shape visualized in the projection.
HLRBRep_Algo takes the shape itself into account whereas HLRBRep_PolyAlgo works with a polyhedral simplification of the shape. When you use HLRBRep_Algo, you obtain an exact result, whereas, when you use HLRBRep_PolyAlgo, you reduce the computation time.
No smoothing algorithm is provided. Consequently, a polyhedron will be treated as such and the algorithms will give the results in form of line segments conforming to the mathematical definition of the polyhedron. This is always the case with HLRBRep_PolyAlgo.
HLRBRep_Algo and HLRBRep_PolyAlgo can deal with any kind of object, for example, assemblies of volumes, surfaces, and lines, as long as there are no unfinished objects or points within it.
However, there some restrictions in HLR use:
- Points are not processed;
- Z-clipping planes are not used;
- Infinite faces or lines are not processed.
@image html /user_guides/modeling_algos/images/modeling_algos_image052.png "Sharp, smooth and sewn edges in a simple screw shape" @image latex /user_guides/modeling_algos/images/modeling_algos_image052.png "Sharp, smooth and sewn edges in a simple screw shape"
@image html /user_guides/modeling_algos/images/modeling_algos_image053.png "Outline edges and isoparameters in the same shape" @image latex /user_guides/modeling_algos/images/modeling_algos_image053.png "Outline edges and isoparameters in the same shape"
@image html /user_guides/modeling_algos/images/modeling_algos_image054.png "A simple screw shape seen with shading" @image latex /user_guides/modeling_algos/images/modeling_algos_image054.png "A simple screw shape seen with shading"
@image html /user_guides/modeling_algos/images/modeling_algos_image055.png "An extraction showing hidden sharp edges" @image latex /user_guides/modeling_algos/images/modeling_algos_image055.png "An extraction showing hidden sharp edges"
The following services are related to Hidden Lines Removal :
Loading Shapes
To pass a TopoDS_Shape to an HLRBRep_Algo object, use HLRBRep_Algo::Add. With an HLRBRep_PolyAlgo object, use HLRBRep_PolyAlgo::Load. If you wish to add several shapes, use Add or Load as often as necessary.
Setting view parameters
HLRBRep_Algo::Projector and HLRBRep_PolyAlgo::Projector set a projector object which defines the parameters of the view. This object is an HLRAlgo_Projector.
Computing the projections
HLRBRep_PolyAlgo::Update launches the calculation of outlines of the shape visualized by the HLRBRep_PolyAlgo framework.
In the case of HLRBRep_Algo, use HLRBRep_Algo::Update. With this algorithm, you must also call the method HLRBRep_Algo::Hide to calculate visible and hidden lines of the shape to be visualized. With an HLRBRep_PolyAlgo object, visible and hidden lines are computed by HLRBRep_PolyHLRToShape.
Extracting edges
The classes HLRBRep_HLRToShape and HLRBRep_PolyHLRToShape present a range of extraction filters for an HLRBRep_Algo object and an HLRBRep_PolyAlgo object, respectively. They highlight the type of edge from the results calculated by the algorithm on a shape. With both extraction classes, you can highlight the following types of output:
- visible/hidden sharp edges;
- visible/hidden smooth edges;
- visible/hidden sewn edges;
- visible/hidden outline edges.
To perform extraction on an HLRBRep_PolyHLRToShape object, use HLRBRep_PolyHLRToShape::Update function.
For an HLRBRep_HLRToShape object built from an HLRBRepAlgo object you can also highlight:
- visible isoparameters and
- hidden isoparameters.
@subsection occt_modalg_10_1 Examples
HLRBRep_Algo
// Build The algorithm object
myAlgo = new HLRBRep_Algo();
// Add Shapes into the algorithm
TopTools_ListIteratorOfListOfShape anIterator(myListOfShape);
for (;anIterator.More();anIterator.Next())
myAlgo-Add(anIterator.Value(),myNbIsos);
// Set The Projector (myProjector is a
HLRAlgo_Projector)
myAlgo-Projector(myProjector);
// Build HLR
myAlgo->Update();
// Set The Edge Status
myAlgo->Hide();
// Build the extraction object :
HLRBRep_HLRToShape aHLRToShape(myAlgo);
// extract the results :
TopoDS_Shape VCompound = aHLRToShape.VCompound();
TopoDS_Shape Rg1LineVCompound =
aHLRToShape.Rg1LineVCompound();
TopoDS_Shape RgNLineVCompound =
aHLRToShape.RgNLineVCompound();
TopoDS_Shape OutLineVCompound =
aHLRToShape.OutLineVCompound();
TopoDS_Shape IsoLineVCompound =
aHLRToShape.IsoLineVCompound();
TopoDS_Shape HCompound = aHLRToShape.HCompound();
TopoDS_Shape Rg1LineHCompound =
aHLRToShape.Rg1LineHCompound();
TopoDS_Shape RgNLineHCompound =
aHLRToShape.RgNLineHCompound();
TopoDS_Shape OutLineHCompound =
aHLRToShape.OutLineHCompound();
TopoDS_Shape IsoLineHCompound =
aHLRToShape.IsoLineHCompound();
HLRBRep_PolyAlgo
// Build The algorithm object
myPolyAlgo = new HLRBRep_PolyAlgo();
// Add Shapes into the algorithm
TopTools_ListIteratorOfListOfShape
anIterator(myListOfShape);
for (;anIterator.More();anIterator.Next())
myPolyAlgo-Load(anIterator.Value());
// Set The Projector (myProjector is a
HLRAlgo_Projector)
myPolyAlgo->Projector(myProjector);
// Build HLR
myPolyAlgo->Update();
// Build the extraction object :
HLRBRep_PolyHLRToShape aPolyHLRToShape;
aPolyHLRToShape.Update(myPolyAlgo);
// extract the results :
TopoDS_Shape VCompound =
aPolyHLRToShape.VCompound();
TopoDS_Shape Rg1LineVCompound =
aPolyHLRToShape.Rg1LineVCompound();
TopoDS_Shape RgNLineVCompound =
aPolyHLRToShape.RgNLineVCompound();
TopoDS_Shape OutLineVCompound =
aPolyHLRToShape.OutLineVCompound();
TopoDS_Shape HCompound =
aPolyHLRToShape.HCompound();
TopoDS_Shape Rg1LineHCompound =
aPolyHLRToShape.Rg1LineHCompound();
TopoDS_Shape RgNLineHCompound =
aPolyHLRToShape.RgNLineHCompound();
TopoDS_Shape OutLineHCompound =
aPolyHLRToShape.OutLineHCompound();
@section occt_modalg_10_2 Meshing of Shapes
The HLRBRep_PolyAlgo algorithm works with triangulation of shapes. This is provided by the function BRepMesh::Mesh, which adds a triangulation of the shape to its topological data structure. This triangulation is computed with a given deflection.
Standard_Real radius=10. ;
Standard_Real height=25. ;
BRepBuilderAPI_MakeCylinder myCyl (radius, height) ;
TopoDS_Shape myShape = myCyl.Shape() ;
Standard_Real Deflection = 0.01 ;
BRepMesh::Mesh (myShape, Deflection);
Meshing covers a shape with a triangular mesh. Other than hidden line removal, you can use meshing to transfer the shape to another tool: a manufacturing tool, a shading algorithm, a finite element algorithm, or a collision algorithm, for example.
You can obtain information on the shape by first exploring it. To then access triangulation of a face in the shape, use BRepTool::Triangulation. To access a polygon which is the approximation of an edge of the face, use BRepTool::PolygonOnTriangulation.