Open_CASCADE_notes_-_Topology_and_Geometry.pdf

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1、OPENCASCADENOTESABLOGABOUTTHEOPENSOURCE3DMODELINGKERNEL:NOTESFROMITSFORMERDEVELOPERANDPROJECTMANAGERTopologyandGeometryinOpenCASCADE.Part1LetusconsiderthesefundamentalconceptsofOpenCASCADE.Fromtimetotimequestionsaboutthempopupontheforum,soIhopethispostwillbeusefulformanyreaders.Understand

2、ingtheseconceptsisimportantifyouhavetoworkwithmostmodelingalgorithmsandespeciallyifyouwanttodevelopyouown.Youmightalsowanttore-readvariouschaptersfromtheModelingDataUser'sGuidetorefreshyoumemory.AbstracttopologyOpenCASCADEtopologyisdesignedwithreferencetotheSTEPstandardISO-10303-42.Perhap

3、sreadingsomeitsconceptswouldbehelpful(Imyselfdidthisoncein1997).Thestructureisanorientedone-waygraph,whereparentsrefertotheirchildren,andtherearenobackreferences.AbstractstructureisimplementedasC++classesfromtheTopoDSpackage.HereisaninheritancediagramtakenfromtheDoxygen-generateddocumenta

4、tion.TopoDS_Shapeismanipulatedbyvalueandcontains3fields–location,1orientationandamyTShapehandle(oftheTopoDS_TShapetype)–seethediagrambelow(thisandothercontainonlymostimportantfields):myTShapeandLocationareusedtosharedatabetweenvariousshapesandthussavehugeamountsofmemory.Forexample,anedgeb

5、elongingtotwofaceshasequalLocationsandmyTShapefieldsbutdifferentOrientations(ForwardincontextofonefaceandReversedinoneoftheother).ConnectionwithGeometryNowlet'sconsiderhowthisabstracttopologystructureisconnectedwithgeometry.ThisisdonebyinheritingabstracttopologyclassesfromtheTopoDSpackage

6、bythoseimplementingaboundaryrepresentationmodel(fromtheBReppackage).Only3typesoftopologicalobjectshavegeometricrepresentations–vertex,edge,andface(seethediagrambelow).2Let'sconsiderthemmoreclosely.Tobecontinued...TopologyandGeometryinOpenCASCADE.Part2Continued...VertexVertexhasaprimarygeo

7、metricrepresentationwhichisapointin3Dspaceencodedasgp_Pnt.Itmayhaveotherrepresentationsbuttheyarevirtuallyneverusedinpractice.Anotherimportantattributeofavertexisitstolerancewhichindicatespossibleinaccuracyofitsplacement.GeometricalmeaningofthevertextoleranceTisasph