计算机图形学computergraphVIP

1,BetterInteractivePrograms,YuanfengZhouShandongUniversity,2,Objectives,LearntobuildmoresophisticatedinteractiveprogramsusingPickingSelectobjectsfromthedisplayThreemethodsRubberbandingInteractivedrawingoflinesandrectanglesDisplayListsRetainedmodegraphics,3,Picking,Identifyauser-definedobjectonthedisplayInprinciple,itshouldbesimplebecausethemousegivesthepositionandweshouldbeabletodeterminetowhichobject(s)apositioncorrespondsPracticaldifficultiesPipelinearchitectureisfeedforward,hardtogofromscreenbacktoworldComplicatedbyscreenbeing2D,worldis3DHowclosedowehavetocometoobjecttosayweselectedit?,4,ThreeApproaches,HitlistMostgeneralapproachbutmostdifficulttoimplementUsebackorsomeotherbuffertostoreobjectidsastheobjectsarerenderedRectangularmapsEasytoimplementformanyapplicationsSeepaintprogramintext,5,RenderingModes,OpenGLcanrenderinoneofthreemodesselectedbyglRenderMode(mode)GL_RENDER:normalrenderingtotheframebuffer(default)GL_FEEDBACK:provideslistofprimitivesrenderedbutnooutputtotheframebufferGL_SELECTION:Eachprimitiveintheviewvolumegeneratesahitrecordthatisplacedinanamestackwhichcanbeexaminedlater,6,SelectionModeFunctions,glSelectBuffer():specifiesnamebufferglInitNames():initializesnamebufferglPushName(id):pushidonnamebufferglPopName():poptopofnamebufferglLoadName(id):replacetopnameonbufferidissetbyapplicationprogramtoidentifyobjects,7,UsingSelectionMode,InitializenamebufferEnterselectionmode(usingmouse)Renderscenewithuser-definedidentifiersReenternormalrendermodeThisoperationreturnsnumberofhitsExaminecontentsofnamebuffer(hitrecords)Hitrecordsincludeidanddepthinformation,8,SelectionModeandPicking,Aswejustdescribedit,selectionmodewontworkforpickingbecauseeveryprimitiveintheviewvolumewillgenerateahitChangetheviewingparameterssothatonlythoseprimitivesnearthecursorareinthealteredviewvolumeUsegluPickMatrix(seetextfordetails),9,UsingRegionsoftheScreen,ManyapplicationsuseasimplerectangulararrangementofthescreenExample:paint/CADprogramEasiertolookatmousepositionanddeterminewhichareaofscreenitisinthanusingselectionmodepicking,drawingarea,tools,menus,10,Usinganotherbufferandcolorsforpicking,Forasmallnumberofobjects,wecanassignauniquecolor(oftenincolorindexmode)toeachobjectWethenrenderthescenetoacolorbufferotherthanthefrontbuffersotheresultsoftherenderingarenotvisibleWethengetthemousepositionanduseglReadPixels()toreadthecolorinthebufferwejustwroteatthepositionofthemouseThereturnedcolorgivestheidoftheobject,11,WritingModes,framebuffer,application,bitwiselogicaloperation,12,XORwrite,Usual(default)mode:sourcereplacesdestination(d=s)Cannotwritetemporarylinesthiswaybecausewecannotrecoverwhatwas“under”thelineinafastsimplewayExclusiveORmode(XOR)(d=ds)xyx=yHence,ifweuseXORmodetowritealine,wecandrawitasecondtimeandlineiserased!,glLogicOp(mode);mode:GL_XOR,13,Rubberbanding,SwitchtoXORwritemodeDrawobjectForlinecanusefirstmouseclicktofixoneendpointandthenusemotioncallbacktocontinuouslyupdatethesecondendpointEachtimemouseismoved,redrawlinewhicherasesitandthendrawlinefromfixedfirstpositiontotonewsecondpositionAtend,switchbacktonormaldrawingmodeanddrawlineWorksforotherobjects:rectangles,circles,14,RubberbandLines,initialdisplay,drawlinewithmouseinXORmode,mousemovedtonewposition,firstpoint,secondpoint,originallineredrawnwithXOR,newlinedrawnwithXOR,15,XORinOpenGL,Thereare16possiblelogicaloperationsbetweentwobitsAllaresupportedbyOpenGLMustfirstenablelogicaloperationsglEnable(GL_COLOR_LOGIC_OP)ChooselogicaloperationglLogicOp(GL_XOR)glLogicOp(GL_COPY)(default),16,ImmediateandRetainedModes,RecallthatinastandardOpenGLprogram,onceanobjectisrenderedthereisnomemoryofitandtoredisplayit,wemustre-executethecodeforitKnownasimmediatemodegraphicsCanbeespeciallyslowiftheobjectsarecomplexandmustbesentoveranetworkAlternativeisdefineobjectsandkeeptheminsomeformthatcanberedisplayedeasilyRetainedmodegraphicsAccomplishedinOpenGLviadisplaylists,17,DisplayLists,ConceptuallysimilartoagraphicsfileMustdefine(name,create)AddcontentsCloseInclient-serverenvironment,displaylistisplacedonserverCanberedisplayedwithoutsendingprimitivesovernetworkeachtime,18,DisplayListFunctions,CreatingadisplaylistGLuintid;voidinit()id=glGenLists(1);glNewList(id,GL_COMPILE);/*otherOpenGLroutines*/glEndList();Callacreatedlistvoiddisplay()glCallList(id);,19,DisplayListsandState,MostOpenGLfunctionscanbeputindisplaylistsStatechangesmadeinsideadisplaylistpersistafterthedisplaylistisexecutedCanavoidunexpectedresultsbyusingglPushAttribandglPushMatrixuponenteringadisplaylistandglPopAttribandglPopMatrixbe