PCurve这个概念，字面上来理解就是参数曲线（Parametric Curve）。参数空间曲线是在参数曲面的双参数空间中的二维样条曲线。
二维曲线定义的目的只有一个：pCurve，参数曲线。OCC采用参数法构建几何结构，所有的三维曲线（面的边界）都对应于二维参数曲线pCurve。二维参数曲线pCurve上的二维点对应实际曲线上的三维点。如下代码，我们遍历一个Topo_Face的Edge，然后求得Edge对应的pCurve。并显示Face的二维参数面。
// OpenCascade library.
#define WNT
#include <TopoDS.hxx>
#include <TopExp.hxx>
#include <TopExp_Explorer.hxx>
#include <BRepPrimAPI_MakeSphere.hxx>
#include <BRepBuilderAPI_MakeWire.hxx>
#include <BRep_Tool.hxx>
#include <BRepBuilderAPI_MakeEdge2d.hxx>
#pragma comment(lib, "TKernel.lib")
#pragma comment(lib, "TKMath.lib")
#pragma comment(lib, "TKBRep.lib")
#pragma comment(lib, "TKTopAlgo.lib")
#pragma comment(lib, "TKPrim.lib")
int main(int argc, char* argv[])
{
TopoDS_Face face = BRepPrimAPI_MakeSphere(1.0);
TopExp_Explorer aWireExp(face, TopAbs_WIRE);
for (aWireExp; aWireExp.More(); aWireExp.Next())
{
//获取每条wire
BRepBuilderAPI_MakeWire wire2d;
TopoDS_Wire aTempWire = TopoDS::Wire(aWireExp.Current());
//获取Edge
TopExp_Explorer aEdgeExp(aTempWire, TopAbs_EDGE);
for (aEdgeExp; aEdgeExp.More(); aEdgeExp.Next()) {
TopoDS_Edge aTempEdge = TopoDS::Edge(aEdgeExp.Current());
//获取PCurve
Standard_Real aPFirst, aPLast;
Handle(Geom2d_Curve) aPCurve = BRep_Tool::CurveOnSurface(aTempEdge, face, aPFirst, aPLast);
//获取此时PCurve的端点坐标，即UV值。
gp_Pnt2d pStart, pEnd;
aPCurve->D0(aPFirst, pStart);
aPCurve->D0(aPLast, pEnd);
std::cout << "\n";
std::cout << " 起始点坐标：[" << pStart.X() << "," << pStart.Y() << "]\n";
std::cout << " 终止点坐标：[" << pEnd.X() << "," << pEnd.Y() << "]\n";
TopoDS_Edge aEdge2d = BRepBuilderAPI_MakeEdge2d(aPCurve, aPFirst, aPLast);
wire2d.Add(aEdge2d);
}
}
return 0;
}起始点坐标：[0,1.5708]
终止点坐标：[6.28319,1.5708]
起始点坐标：[6.28319,-1.5708]
终止点坐标：[6.28319,1.5708]
起始点坐标：[0,-1.5708]
终止点坐标：[6.28319,-1.5708]
起始点坐标：[0,-1.5708]
终止点坐标：[0,1.5708]
