Is the length of the curve extension incorrect?

[1127204185]

AcGePoint3d PtO;
if (AcDbObjectId  id = Ctest0lib::SelectEnt(_T("/n选择曲线"),PtO))
{
  ExtendCurve( id , 1000);
}

AcDbObjectId SelectEnt(const TCHAR* prompt,AcGePoint3d &PtO)
{
	ads_name ent;
	ads_point pt;
	AcDbObjectId entId = AcDbObjectId::kNull;
	int ret = RTNORM;
	ret = acedEntSel(prompt, ent, pt);
	if (RTNORM != ret)	return entId;
	acdbGetObjectId(entId, ent);
	PtO = asPnt3d(pt);
	return entId;
}

static bool ExtendCurve(AcDbObjectId id , double extensionLength)
{  
	bool TF = false;
	AcDbEntity *pEnt = NULL;
	Acad::ErrorStatus es = acdbOpenObject(pEnt, id, AcDb::kForWrite);
	if (es != Acad::eOk) return TF;

	if (pEnt->isKindOf(AcDbCurve::desc()))
	{
		AcDbCurve *pCurve = AcDbCurve::cast(pEnt);
		pCurve->extend(extensionLength);
		pCurve->close();
		TF = true;
	}
	pEnt->close();
	return TF;
}

Is the length of the curve extension incorrect?

Can it be extended in both directions?

[daniel_cadext]

maybe take into consideration the param WRONG Doh!

import traceback
from pyrx_imp import Rx
from pyrx_imp import Ge
from pyrx_imp import Gi
from pyrx_imp import Db
from pyrx_imp import Ap
from pyrx_imp import Ed


def PyRxCmd_doit() -> None:
    try:

        # create a curve with a length of 100
        db = Db.curDb()
        curve = Db.Line(Ge.Point3d(0, 0, 0), Ge.Point3d(0, 100, 0))
        lineid = db.addToModelspace(curve)

        #check length
        print(curve.getDistAtParam(curve.getEndParam()))

        # extend do start first, does it matter?
        curve.extend(curve.getStartParam() - 1000)

        curve.extend(curve.getEndParam() + 1000)

        #should be 100 + 1000 + 1000
        print(curve.getDistAtParam(curve.getEndParam()))

    except Exception as err:
        traceback.print_exception(err)

output:

Command: DOIT
100.0
2100.0

[tbrammer · Accepted solution]

The parameter value of a point on an AcDbCurve is in general not the same as its distance from the starpoint! 

In case of a polyline the first vertex point has the parameter 0 the second 1 the third 2 and so on.

Unfortunately neither AcDbCurve nor AcGeCurve3d have methods to evaluate the start-/endparameter or the start-/endpoint itself of a curve that is extended by a given length. I tried AcDbCurve::getParamAtDist(dist,..)  and AcGeCurve3d::paramAtLength(dist,...). Both work only if 0<=dist<=curve length. 

I think it is best practice to calculate the new start/endpoint yourself based on your requirements and knowledge of the curve. Do you want to extent the curve with a tangential line or shall the extension follow the existing curve? 

In the latter case a different approach is to estimate the required parameter value by calculating the "parameter-velocity" at the start- or endpoint:

  curve->getFirstDeriv(parStart, vDeriv);
  velocity = vDeriv.length(); // dLength / dParam
  dParam  = addedLength / velocity;
  parStartNew = parStart-dParam;

  curve->getFirstDeriv(parEnd, vDeriv);
  velocity = vDeriv.length(); // dLength / dParam
  dParam  = addedLength / velocity;
  parEndNew = parStart + dParam;

It seems to work fine if the curve is linear at the start/endpoint. But the velocity may vary in non linear curves.

You could extend the curve multiple times until its total length is  >= old length + additionalLength. Than you can evaluate the "extension point" and cut the curve there.

[daniel_cadext]

Thanks for that, I tested on a polyline and it went wonky : |