三维坐标系和几何学doc.docx
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三维坐标系和几何学doc.docx
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三维坐标系和几何学doc
三维坐标系和几何学
Byreadingguomuchengsongto,mostarenotgood.一ZhengBanqiao
Dcoordinatesystemsandgeometry
Type:
posted|source:
|finishingtime:
2006-06-12
Next,we'11describesomeofthetechnicalconceptsyouneedtounderstandbeforeyoucandevelopanimmediatepatternprogram・Heisnotadetaileddescriptionofthebroaderlevel,norisitanin-depthanalysisofthecomponentsoftheDirect3D.Tolearnmoreabouttheinformation,youcanlookatthetwopartsofthearchitectureoftheDirect3DimmediatepatternandtheelementsoftheDirect3Dimmediatemodel・
Ifyou,vealreadyhadsomeexperiencewitha3Dgraphicsprogram,youcanskimthroughthefollowingtolearnsomethingaboutDirect3D.
Thesecontentsaredividedintothefollowingtwoparts:
3-DCoordinateandGeometrySystemsand
Matricesandtransformations(Matrices,and,Transformations)
A.3~Dcoordinatesystemandgeometry
ToprepareDirect3Dapplications,youmusthaveagoodunderstandingofsomebasicelementsof3Dgeometry.Thisandthepartwewillintroducesomeimportantconceptsaboutthegeometryofcreating3Dscenes,whicharedividedintothe
followingsections:
3~Dcoordinatesystem(3~D,Coordinate,Systems)
3~Dprimitives(3-D,Primitives)
Triangulargratingcriterion(Triangle,Rasterization,Rules)
Shading(Shading)
1.3-Dcoordinatesystem
Ingeneral,3-DgraphicsapplicationsusetwotypesofDescartescoordinatesystems:
thelefthandcoordinatesystem(left-handed)andtheright-handedcoordinatesystem(right-handed)・Inthesetwocoordinatesystems,thex-axesarepointingstraighttotheright,andthey-axesarepointinginthesamedirection.Thedirectionofthez-axiscanbedeterminedbytheleftandrighthandrule,i.e.,holdingthefourfingeroftheleft(orright)handtorepresenttherotationfromthex-directiontothey-direction,thentheleft(orright)thumbpointingtothepositivedirectionofthez-axis・Followingchart:
Theleft-handedcoordinatesystemisusedinDirect3D・Ifyourprogramusestherighthandcoordinatesystem,youonlyneedtomaketwosmallmodificationstotheDirect3Ddata:
Changetheorderoftheverticesofthetrianglesothattheyarearrangedintheclockwisedirectionfromthefront・Inotherwords,iftheverticesareVO,VI,andV2,thenVO,V2,andVIarepassedtoDirec3D.
Usethevisualmatrix(viewmatrix)toadjusttheworldspace,andthenadjustthez-directiontoT・ThisrequireschangesintheD3DMATRIXstructure,_31,_32,_33,and_34member'splusandminusnumbers,whichareusedinthevisualmatrix.
Itisimportanttonotethattherearemanyothercoordinatesystemsfor3~Dsoftware・Thelefthandandtherighthandsystemareonlythetwomostcommonlyused.In3Dmodelingprograms,however,they-axispointstoordeviatesfromtheobserver,andthecoordinatesystemonthez-axisisoftenadopted・Inthiscase,therighthandsystemusuallyreferstoanyofthepositivecoordinatedirections(x,y,orZ)totheobserver・Thelefthandsystemusuallyrefersthenegativedirectionofanycoordinatetotheobserver・Ifyouusethez-axisinyourlefthandcoordinatesystem,pointtothetop,
Then,inadditiontowhatyousaidearlier,youalsoneedtorotateallvertexdata・
Forobjectsdefinedinathree-dimensionalcoordinatesystem,somebasicoperationsineludetranslation(translate),rotation(rotate),andscale・Youcancombinethesebasictransformationstocreateachangematrix,andtoseemoreaboutit,noticethe〃3-Dtransform,/section.
Itisimportanttorememberthattheorderinwhichtheseoperationsarecombinedisimportant,andtheresultsofthedifferentcombinationsaredifferent・
2.3-Dprimitive
A3~Dprimitiveisacollectionofverticesthatmakeupathree-dimensionalentity.Thesimplestprimitiveisacollectionofpointsinathree-dimensionalcoordinatesystem,whichwecalllistinDirec3D(point)・
Typically,3Dprimitivesarepolygons・ApolygoninDirect3Disaclosedthree-dimensionalgraphconsistingofatleastthreevertices・Thesimplestpolygonisatriangle・Direct3Dusestrianglestomakemostpolygons,becausethethreeverticesofatriangleareguaranteedtobecoplanar・Renderingnoncoplanarverticesisdifficult・Wecanusetrianglestomakeuplarger,morecomplexpolygonsandpolyhedra(mesh)・
Thebottomleftshowsacube・Eachsurfaceofacubeconsistsoftwotriangles・Thesetrianglesformacubeprimitivetogether・Youcanusetexturesandtexturesonthesurfaceofaprimitivetomakeitlookmorelikeasolidobject・Tolearnmoreaboutit,payattentiontothematerialandtexturesection.
Youcanalsousetrianglestoconstructaprimitivesurfaceconsistingofsmoothsurfaces・Theupperrightshowsaspheremadeoftriangles・Afterusingthematerial,thesurfaceofthespherelookssmoother・IfyouusetheGouraudshadingmet
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