Package gp in the OpenCASCADE

gp是幾何處理程序包（Geometric Processor package），簡稱gp。包gp提供以下功能：

這些實體同時在二維和三維空間中定義，且包中的類都是非持續的（non-persistent），即這些類的實例都是以值的方式處理而不是引用。當複製這種對象時，是對象本體。改變一個實例不會影響到其他的實例。

可用的幾何實體如下所示：

創建幾何對象之前，根據你是將幾何對象用於二維還是三維來確定。若你需要一個幾何對象集而不是單一的幾何對象，即用來處理一類幾何元素，包TColgp就是提供這種功能的。

Package TColgp提供類如：XY, XYZ, Pnt, Pnt2d, Vec, Vec2d, Lin, Lin2D, Circ, Circ2d的TCollection的實例。包中的類簡單列舉如下：

個人意見，若使用標準C++的容器類(The STL Template Container)，就不需要創建這麼多類了。

有幾個庫提供了曲線和曲面的基本計算功能。若要處理由包gp創建的幾何對象，初等曲線曲面的有用算法庫在包：ElCLib和ElSLib中。包Precision提供兩個數字比較的功能。

Package ElCLib; ElCLib代表：Elementary Curves Library. 提供初等曲線曲面的基本幾何計算功能；
Package ElSLib; ElSLib代表：Elementary Surfaces Library. 提供初等曲面的基本幾何計算。
Package Bnd；提供二維和三維空間中幾何元素包圍盒的計算功能；
Package Precision; 由於浮點數在計算機內實際上是一個近似表示，在手工計算看來爲正確的結果，在計算機中運算未必得出正確的結果。所以，我們得到一個重要的經驗：使用浮點數進行相等（==）和不等（！=）比較的操作通常是有問題的。浮點數的相等比較，一般總是使用兩者相減的值是否落在0的鄰域中來判斷。這就是鄰域比較技術。在OpenCASCADE中專門提供包Precision來處理兩個數值的比較問題。

//------------------------------------------------------------------------------

//    Copyright (c) 2012 eryar All Rights Reserved.

//

//        File    : Main.cpp

//        Author  : [email protected]

//        Date    : 2012-6-23 21:30

//        Version : 1.0v

//

//    Description : Test primitive Geometric Types in OpenCASCADE.

//

//      The Geometric Processor package, called gp.

//

//      The pg package offers classes for both 2D and 3D objects which

//      are handled by value rather than by reference. When this sort of object

//      is copied, it is copied entirely. Changes in one instance will not be

//      reflected in another.

//

//==============================================================================

// Use Toolkit TKernel.

#pragma comment(lib,"TKernel.lib")

// Use Toolkit TKMath.

#pragma comment(lib, "TKMath.lib")

#include <gp.hxx>

#include <gp_Pnt.hxx>

#include <gp_Trsf.hxx>

#include <Precision.hxx>

void DumpPoint(const gp_Pnt& p);

int main(int argc, char* argv[])

    gp_Pnt  aPoint(0, 0, 0);

    // 1. Translate a point in a direction.

    // The direction determined by a gp_Vec or two gp_Pnt.

    cout<<"Before translated:";

    DumpPoint(aPoint);

    aPoint.Translate(gp_Pnt(2, 2, 3), gp_Pnt(10, 10, 0));

    cout<<"After translated:";

    DumpPoint(aPoint);

    // 2. Rotate a point.

    // Rotate a point by an axis and the rotate angle.

    cout<<"Before rotated:";

    DumpPoint(aPoint);

    // Roate 45 degree about Z axis.

    // Positive angle value will be rotated counterclockwise.

    aPoint.Rotate(gp::OZ(), PI/4);

    cout<<"After rotated 45 degree about Z axis:";

    DumpPoint(aPoint);

    // 2.1 Test Package Precision.

    if (aPoint.X() < Precision::Confusion() && aPoint.X() > -Precision::Confusion())

        cout<<"Point X value:"<<aPoint.X()<<endl;

        cout<<"Precision::Confusion() value:"<<Precision::Confusion()<<endl;

    aPoint.Rotate(gp::OZ(), PI/4);

    cout<<"After rotate 45 degree about Z axis:";

    DumpPoint(aPoint);

    // 3. Transform a point by gp_Trsf.

    gp_Trsf transform;

    transform.SetMirror(gp::OX());

    cout<<"Before gp_Trsf:";

    DumpPoint(aPoint);

    aPoint.Transform(transform);

    cout<<"After gp_Trsf:";

    DumpPoint(aPoint);

    // 4. Mirror a point.

    // 4.1 Performs the symmetrical transformation of

    // a point with respect to an axis placement which

    // is the axis of the symmetry.

    cout<<"Before mirrored with a symmetric axis:";

    DumpPoint(aPoint);

    aPoint.Mirror(gp::OY());

    cout<<"After mirrored with a symmetric axis:";

    DumpPoint(aPoint);

    // 4.2 Performs the symmetrical transformation of

    // a point with respect to a plane.

    cout<<"Before mirrored with a symmetric plane:";

    DumpPoint(aPoint);

    aPoint.Mirror(gp::XOY());

    cout<<"After mirrored with a symmetric plane";

    DumpPoint(aPoint);

    // 5. Scale a point.

    aPoint.SetCoord(1, 2, 1);

    cout<<"Before Scaled:";

    DumpPoint(aPoint);

/*

    // Scale point source code...

    inline void gp_Pnt::Scale (const gp_Pnt& P,

    const Standard_Real S)

    gp_XYZ XYZ = P.coord;

    XYZ.Multiply (1.0 - S);

    coord.Multiply (S);

    coord.Add      (XYZ);

*/

    aPoint.Scale(gp_Pnt(1, 2, 2), 2);

    cout<<"After Scaled:";

    DumpPoint(aPoint);

    return 0;

/**

* Description: Dump point information.

*/

void DumpPoint( const gp_Pnt& p )

    cout<<"("<<p.X()<<","<<p.Y()<<","<<p.Z()<<")"<<endl;

輸出結果如下：

   1:  Before translated:(0,0,0)

   2:  After translated:(8,8,-3)

   3:  Before rotated:(8,8,-3)

   4:  After rotated 45 degree about Z axis:(8.88178e-016,11.3137,-3)

   5:  Point X value:8.88178e-016

   6:  Precision::Confusion() value:1e-007

   7:  After rotate 45 degree about Z axis:(-8,8,-3)

   8:  Before gp_Trsf:(-8,8,-3)

   9:  After gp_Trsf:(-8,-8,3)

  10:  Before mirrored with a symmetric axis:(-8,-8,3)

  11:  After mirrored with a symmetric axis:(8,-8,-3)

  12:  Before mirrored with a symmetric plane:(8,-8,-3)

  13:  After mirrored with a symmetric plane(8,-8,3)

  14:  Before Scaled:(1,2,1)

  15:  After Scaled:(1,2,0)

  16:  Press any key to continue . . .

包gp提供了基本的幾何元素表示及初等解析幾何計算功能。對於幾何元素的集合也有自己的類庫。對於兩個數值的比較採用了鄰域比較技術。

原文鏈接：

http://www.cnblogs.com/opencascade/p/3527417.html