AcDbCurve::extend 延长指定多义线的前后长度

int myExtendPolyline(AcDbPolyline* pl,double sOff,double eOff)
{
	ASSERT(pl);
	if (pl->numVerts() < 2)
	{
		return 0;
	}
	if (sOff > 0)
	{
		AcGePoint3d sPt,sPt2;
		pl->getStartPoint(sPt);
		pl->getPointAt(1,sPt2);
 
		double td1,td2;
		pl->getDistAtPoint(sPt,td1);
		pl->getDistAtPoint(sPt2,td2);
		double off = td2 - td1;
		
		double sParam = fabs(sOff) / off;
 
		if(pl->extend(-sParam) != Acad::eOk)
		{
			ASSERT(0);
			return 0;
		}
	}
	if (eOff > 0)
	{
		AcGePoint3d ePt,ePt2;
		pl->getEndPoint(ePt);
		pl->getPointAt(pl->numVerts() - 2,ePt2);
 
		double td1,td2;
		pl->getDistAtPoint(ePt2,td1);
		pl->getDistAtPoint(ePt,td2);
		double off = td2 - td1;
 
		double eparam = fabs(eOff) / off;
		if(pl->extend(eparam) != Acad::eOk)
		{
			ASSERT(0);
			return 0;
		}
	}
 
	return 1;
}