XCLIP xrefs using ObjectARX ----- AcDbSpatialFilter 类设计用来做这个的，在AutoCAD重新生成图形（REGEN）的时候，通过 spatial filter 来决定要处理哪些对象

ACED_ARXCOMMAND_ENTRY_AUTO(CMyTestApp,
                           MyTestApp,
                           _MyClip, MyClip,
                           ACRX_CMD_TRANSPARENT |
                           ACRX_CMD_NOINTERNALLOCK,
                           NULL)

static void MyTestApp_MyClip()
{
    ads_point pt1,pt2;
    ads_name ent;
    if (acedEntSel(_T("Select xref:"),ent,pt1)!=RTNORM)
        return;
    AcDbObjectId idXref;
    if (acdbGetObjectId(idXref,ent)!=Acad::eOk)
        return;
    AcDbObjectPointer<AcDbBlockReference> pRef(idXref,AcDb::kForRead);
    if (pRef.openStatus()!=Acad::eOk)
    {
        acutPrintf(_T("Not an xref! "));
        return;
    }
    AcGePoint2dArray pts;
    if (acedGetPoint(NULL,_T("First point:"),pt1)!=RTNORM)
        return;
    //the ECS of the vertices must be defined in the
    //coordinate system of the _block_ so let's calculate
    //transform all points to that coordinate system

    AcGeMatrix3d mat(pRef->blockTransform());
    mat.invert();

    AcGePoint3d pt3d(asPnt3d(pt1));
    pt3d.transformBy(mat);
    pts.append(AcGePoint2d(pt3d.x,pt3d.y));
    while (acedGetPoint(pt1,_T("Next point:"),pt2)==RTNORM)
    {
        acedGrDraw(pt1,pt2,1,1);
        pt3d = asPnt3d(pt2);
        pt3d.transformBy(mat);
        pts.append(AcGePoint2d(pt3d.x,pt3d.y));
        memcpy(pt1,pt2,sizeof(ads_point));
    }
    acedRedraw(NULL,0);
    AcDbDatabase* pDb = acdbHostApplicationServices()->workingDatabase();
    AcGeVector3d normal;
    double elev;
    if (pDb->tilemode())
    {
        normal = pDb->ucsxdir().crossProduct(pDb->ucsydir());
        elev = pDb->elevation();
    }
    else
    {
        normal = pDb->pucsxdir().crossProduct(pDb->pucsydir());
        elev = pDb->pelevation();
    }
    normal.normalize();
    Acad::ErrorStatus es = pRef.object()->upgradeOpen();
    if (es !=Acad::eOk)
        return;
    //create the filter
    AcDbSpatialFilter* pFilter = new AcDbSpatialFilter;
    if (pFilter->setDefinition(pts,normal,elev,
        ACDB_INFINITE_XCLIP_DEPTH,-ACDB_INFINITE_XCLIP_DEPTH,true)!=Acad::eOk)
    {
        delete pFilter;
        return;
    }
    //add it to the extension dictionary of the block reference
    //the AcDbIndexFilterManger class provides convenient utility functions
    if (AcDbIndexFilterManager::addFilter(pRef.object(),pFilter)!=Acad::eOk)
        delete pFilter;
    else
    {
        acutPrintf(_T("Filter has been succesfully added! "));
        pFilter->close();
    }

}