计算机图形学教学ppt课件.ppt

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1、ClippinginarasterworldCanbedoneAnalyticallyOntheflyduringscanconversionAspartofacopyPixelfromacanvasstoringunclippedprimitivestothedestinationcanvasBedoneasrapidlyaspossible!9/18/2021preparation9/18/2021vectorsDefinition:9/18/2021Cont.矢量的模长(VectorMode

2、)矢量的方向角非零矢量与三个坐标轴的夹角称为该矢量的方向角矢量的单位化9/18/2021Cont.矢量的方向余弦：非零矢量的方向角的余弦称为该矢量的方向余弦9/18/2021Cont.两矢量的夹角9/18/2021Cont.两矢量的数积(点积)(VectorDotMetrix)9/18/2021Cont.两矢量的矢积(叉积)(VectorProduct)9/18/2021Cont.9/18/2021Parametricequationsoflines(a,b)9/18/2021Cont.9/18/

3、2021ClippingissueDistinguishingtheinterior/exteriorpartofprimitivesagainstclipwindowsDecisiontheinterior/exteriorpartPrimitivesinteractingclipwindowsClipwindows:rectanglesXY9/18/2021裁剪的实质:就是决定图形中哪些点、线段、文字、以及多边形在窗口之内.裁剪的基本方法:将图形元素(如线段）与窗口边界求交点,交点连接起来在窗

4、口内的部分就是裁剪后的显示图形(即求交).2DCLIPPING9/18/2021CLIPPINGEFGIJHDCBAD’G’H’J’I’EFCliprectangle(a)GIJHDCBAJ’I’G’H’D’(b)Casesforclippinglines.9/18/2021Clippingendpoints(x,y)necessaryandsufficientconditiononwhichaendpoint(x,y)isinwindo

5、w:9/18/2021ytxlxryb21345CLIPPINGLINES9/18/2021Line-ClippingAlgorithmsClippingLinesbySolvingSimultaneousEquationsTheCohen—SutherlandLine-ClippingAlgorithm中点分割法Nicholl-Lee-NillconLine-ClippingAlgorithmTheLiang-Barskyalgorithm9/18/2021Cohen-SutherlandLin

6、e-ClippingAlgorithmFirst,endpointpairsarecheckedfortrivialacceptance（显然完全可见）.Ifthelinecannotbetriviallyaccepted,trivialrejection（显然完全不可见）aredone9/18/2021Second,endpointpairsarecheckedfortrivialrejection.Wecantriviallyrejectlineswithbothendpointsinregi

7、onstotheleftofxmin,rightofxmax,belowymin,andaboveymax.Ifthelinecannotbetriviallyrejected,regionchecks（区域检测）aredoneCohen—SutherlandLine-ClippingAlgorithm9/18/2021Ifthelinesegmentcanbeneithertriviallyacceptednorrejected,itisdividedintotwosegmentsataclip

8、edge,sothatonesegmentcanbetriviallyrejected.Asegmentisiterativelyclippedbytestingfortrivialacceptanceorrejection,andisthensubdividedifneithertestissuccessful,untilwhatremainsiscompletelyinsidethecliprectangleorcanbetriviallyrejected.Cohen—Suth