Meshlab Tutorial

1MeshLABtutorial2A.OPENINGPOINTCLOUDS(Notepad++
Texteditor)(CloudCompare
Pointcloudandmesheditor)(MeshLab
Pointcloudandmesheditor)3OPENINGPOINTCLOUDSINNOTEPAD++Letusunderstandwhatisapointcloud.Firstofall,itisacollectionofpointcoordinatesasyoucanseeinthefigurebellow.HereweseeapointcloudinPLYasciiformateditedwithNotepad++().4HereweseeapointcloudinPLYformatwithbinaryencodingeditedwithNotepad++.Binaryencodingenablessmallfilesizesbuttheircontentcan’tbeeditedmanually.Inbothcasestheheaderofthefileisreadable.Weseehowmanyvertexes,theorderforwhichcoordinatesarepresented,and,inthiscase,therearenofaces.5Thepointcloudthatcorrespondtothispicturealsopresentsnormalsforeachpoint(wemaysaythatthereisanimplicitsurface),color(inRGBspace),andaqualityparameter(lastcolumn).Again,therearenovertexes.6OPENINGPOINTCLOUDSINCLOUDCOMPARECloudComparecanbefreelydownloadedfrom’trequireinstallation.JustunzipitandlaunchcQQ.exe.BesidesPLYformatitcanalsoopenotherfileformats.Exploreit.7OPENINGPOINTCLOUDSINMESHLABMeshLabcanbefreelydownloadedfrom(ifyouassociatethisfileformattoMeshLab.HereweseethepointcloudcorrespondingtothefirstandsecondPLYfilesshownabove.Sincetherearenonormalsassociatedwiththepoints,thereisnolighteffect.8NAVIGATIONINMeshLAB1.Leftmousebutton+drag:rotatearoundtrackballcenter2.Mousewheel:moveforwardorbackward3.Centermousebutton+drag:pan4.Shift+mousewheel:changecamerafieldofview5.Doubleclickonspecificpoint:placesthatpointatthetrackballcenter6.Control+mousewheel:movesnearclippingplan7.Control+Shift+mousewheel:movesfarclippingplan8.Alt+Enter:enterfullscreenmode9.Control+Shift+leftmousebutton+drag:changeslightdirection(thisonlytakeseffectiftherearenormals)VISUALIZATIONOPTIONSINMeshLABAppearanceoptionscanbefoundunderthemenuTOOLS.OtherappearanceandinfooptionscanbefoundunderthemenuVIEW.AlsoexplorethemenuRENDERtochangelighting,shaders,rendermodeandcolor.9VISUALIZATIONOPTIONSINMeshLABLayerscanbedisplayedunderdeVIEWmenuorbyclickinginthelayericon.Thisisusefulifyouopenseveralpointcloudsormeshes.Eachpointcloudisassignedtoaspecificlayer.Thelayerdialoglooksasfollows.Youcanturnoffalayerbyclickingintheeyeicon.Youcandeletealayerbyclickingthe–icon.10SAVINGAPROJECTINMeshLABAprojectisnothingmorethananinformationstructure.Itdoesn’tcontainthepointcloudsitself.ItcanbesavedwiththeformatMLP(defaultmeshlabprojectfile)orALN(alignfile)underthemenuFILE.Boththiskindoffilescanbeeditedwithatexteditor.Aprojectfilelookslikethefollowing(editedwithNotepad++).Itstoresthepointcloudfilepathsandposes(givenbya4x4matrix).Bydefaulttheidentitymatrixisassignedtoallfilesopened.11B.EDITINGPOINTCLOUDS(MeshLab)1.Selectinganddeletingpoints2.Computingnormalsforpointsets3.Down-samplingpointclouds4.Scaling,MovingandRotation5.Combiningtransformations12SELECTINGANDDELETINGPOINTS:Pointscanbeselectedbypickingthefollowingicon.Pointsareselectedwithafence.Selectedpointsturnonred.PointscanbeaddedtoapreviousselectionbypressingCTRLwhilstselecting.PointscanberemovedfromapreviousselectionbypressingSHIFTwhilstselecting.TodeleteselectedpointspressCTRL+DELorclickontherespectiveicon(right).ExplorethemenuFILTERS/SELECTIONtoseeotherselectionmethods.Afterdeleting,ifyouwanttokeeptheresultyoumustsave.Otherwisethedeletionisnotupdated.Ifyouwanttorecoverfromaprevious(unsaved)stateyoucanclicktheRELOADicon.13COMPUTINGNORMALSFORPOINTSETS:Ifapointcollectionhasnonormalsassociatedtoit,thenvisualizationispoorandfurthereditingoptions,suchasalignorsomemeshreconstructionmethods,arenotpossible.GotoFILTERS/NORMALS,CURVATURESANDORIENTATIONS/COMPUTENORMALSFORPOINTSETS.14Thenumberofneighborsisthenumberofpointsthatareusedtoestimateatangentplan,ateachpointoftheset.15Thenormalataparticularpointisobviouslyperpendiculartothedefinedtangentplanforthatpoint.Thenormalisstoredasthecomponentsofanunitaryvectorwiththedirectiondetermined.Disambiguationcanbeobtainedbydefiningaparticularpointtowardswhichthenormalsshouldflipto.Aftertheoperationiscomplete,don’tforgettosavethepointcloudandtoselecttheNORMALoption.16Thefigurebellowshowsthefirstpossibleeffectofhavingnormalsassociatedtothepoints.ThisenableslightinclinationtobeusedtoenhancethevisualizationinMeshLab.17DOWNSAMPLINGPOINTCLOUDS:Downsamplingcanbeusefulifwehavepointdensitiesthataretohighforourpurposes.ThisisusuallythecasewithLaserScanningpointcloudswherehighdensityoccursnearthescanstation.DownsamplingcanbedonewiththePOISSONDISKSAMPLINGasshownabove.18Withthisfiltertheusercandefinetheabsolutenumberofsamples,orcandefineanaveragespacingbetweenpoints(explicitradius).TheoptionBaseMeshSubsamplingshouldbeselected.Andthepointcloudtobefilteredshouldbechosen.Attheenddon’tforgettosavethenewsamplethatwascreatedasanewlayer.19SCALING,MOVINGANDROTATIONThesetransformationscanbefoundunderFILTERS/NORMALS,CURVATURESANDORIENTATION/TRANSFORM…Atfirstsightthesearesimpletransformationsbutpleasepayattentiontotheexplanations.20SCALINGScalecanb