WebGL Points, Lines, and Triangles

As mentioned in the first article WebGL draws points, lines, and triangles. It does this when we call gl.drawArrays or gl.drawElements . English Français 日本語 한국어 Polski Portuguese Русский 简体中文 TableofContents WebGLFundamentals.org Fix,Fork,Contribute WebGLPoints,Lines,andTriangles Themajorityofthissitedrawseverything withtriangles.Thisisarguablythenormalthing that99%ofWebGLprogramsdo.But,forthesake ofcompletenesslet'sgooverafewothercases. Asmentionedinthefirstarticle WebGLdrawspoints,lines,andtriangles.Itdoesthis whenwecallgl.drawArraysorgl.drawElements. Weprovideavertexshaderwhatoutputsclipspace coordinatesandthen,basedonthefirstargument togl.drawArraysorgl.drawElementsWebGLwill drawpoints,lines,ortriangles. Thevalidvaluesforthefirstargumenttogl.drawArrays andgl.drawElementsare POINTS Foreachclipspacevertexoutputbythevertexshaderdrawasquare centeredoverthatpoint.Thesizeofthesquareis specifiedbysettingaspecialvariablegl_PointSize insidethevertexshadertothesizewewantforthissquareinpixels. Note:Themaximum(andminimum)sizethatsquarecanbe isimplementationdependentwhichyoucanquerywith const[minSize,maxSize]=gl.getParameter(gl.ALIASED_POINT_SIZE_RANGE); Alsoseeanotherissuehere. LINES Foreach2clipspaceverticesoutputbythevertexshader drawalineconnectingthe2points.IfwehadpointsA,B,C,D,E,Fthen we'dget3lines. Thespecsayswecansetthethicknessofthisline bycallinggl.lineWidthandspecifyingawidthinpixels. Inrealitythoughthemaximum widthisimplementationdependentandforthemajority ofimplementationsthemaximumwidthis1. const[minSize,maxSize]=gl.getParameter(gl.ALIASED_LINE_WIDTH_RANGE); Thisismostlybecausevalues>1havebeendeprecated incoreDesktopOpenGL. LINE_STRIP Foreachclipspacevertexoutputbythevertexshader drawalinefromthepreviouspointoutputbythevertex shader. So,ifyououtputclipspaceverticesA,B,C,D,E,Fyou'llget5lines. LINE_LOOP ThisisthesameasLINE_STRIPexampleonemoreline isdrawnfromthelastpointtothefirstpoint. TRIANGLES Forevery3clipspaceverticesoutputbythevertexshader drawatrianglefromthe3points.Thisisthemostusedmode. TRIANGLE_STRIP Foreachclipspacevertexoutputbythevertexshader drawatrianglefromthelast3vertices.Inotherwords Ifyououtput6pointsA,B,C,D,E,Fthen4triangleswillbe drawn.A,B,CthenB,C,DthenC,D,EthenD,E,F TRIANGLE_FAN Foreachclipspacevertexoutputbythevertexshader drawatrianglefromthefirstvertexandthelast2 vertices.Inotherwordsifyououtput6pointsA,B,C,D,E,F then4triangleswillbedrawn.A,B,CthenA,C,Dthen A,D,EandfinallyA,E,F I'msuresomeotherswilldisagreebutinmyexperience TRIANGLE_FANandTRIANGLE_STRIParebestavoided. Theyfitonlyafewexceptionalcasesandtheextracode forhandlingthosecasesisnotworthjustdoingeverything intrianglesinthefirstplace.Inparticularmaybeyou havetoolstobuildnormalsorgeneratetexturecoordinates ordoanyothernumberofthingswithvertexdata.By stickingtojustTRIANGLESyourfunctionswilljustwork. AssoonasyoustartaddinginTRIANGLE_FANandTRIANGLE_STRIP youneedmorefunctionstohandlemorecases. You'refreetodisagreeanddowhateveryouwant. I'mjustsayingthat'smyexperienceandtheexperienceof afewAAAgamedevsI'veasked. SimilarlyLINE_LOOPandLINE_STRIParenotsouseful andhavesimilarissues. LikeTRIANGLE_FANandTRIANGLE_STRIPthesituations tousethemarerare.Forexampleyoumightthinkyou wanttodraw4connectedlineseachmadefrom4points. IfyouuseLINE_STRIPyou'dneedtomake4callstogl.drawArrays andmorecallstosetuptheattributesforeachlinewhereasifyou justuseLINESthenyoucaninsertallthepointsneededtodraw all4setsoflineswithasinglecalltogl.drawArrays.Thatwill bemuchmuchfaster. Further,LINEScanbegreattousefordebuggingorsimple effectsbutgiventheir1pixelwidthlimitonmostplatforms it'softenthewrongsolution.Ifyouwanttodrawagridforagraphor showtheoutlinesofpolygonsina3dmodelingprogramusingLINES mightbegreatbutifyouwanttodrawstructuredgraphicslike SVGorAdobeIllustratorthenitwon'tworkandyouhave torenderyourlinessomeotherway,usuallyfromtriangles. 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