Interfacing MATLAB With CAD - EMPossible

9/13/2018ECE 532221st Century r. Raymond C. RumpfA‐337(915) 747‐6958rcrumpf@utep.eduLecture #19Synthesizing Geometries for21st Century ElectromagneticsInterfacing MATLAB with CADLecture 191Lecture Outline STL Files– File format description– Problems and repairing MATLAB Topics––––Importing and exporting STL files in MATLABVisualizing surface meshes in MATLABGenerating faces and vertices using MATLABSurface mesh 3D grid CAD Topics–––––Lecture 19Converting flat images to STLPoint cloudsImporting custom polygons into SolidWorksSTL to CAD file conversionExporting STL from Blender with proper unitsSlide 21

9/13/2018STL File FormatDescriptionWhat is an STL File?STL – Standard Tessellation LanguageThis file format is widely used in rapidprototyping (i.e. 3D printing, additive manufacturing). Itcontains only a single triangular mesh of an objects surface.Color, texture, materials, and other attributes are notrepresented in the standard STL file format. Hackedformats exists to accommodate this type of additionalinformation.They can be text files or binary. Binary is more commonbecause they are more compact. We will look at text filesbecause that is more easily interfaced with MATLAB.Lecture 19Slide 42

9/13/2018Surface MeshDespite this sphere really being a solid object, it is represented in anSTL file by just its surface.Solid ObjectSTL RepresentationLecture 195STL File Formatsolid namefacet normal nx ny nzouter loopvertex vx,1 vy,1 vz,1vertex vx,2 vy,2 vz,2vertex vx,3 vy,3 vz,3endloopendfacetendsolid nameThis set of text is repeated for every triangle on thesurface of the object. v v v3 v1 nˆ 2 1 v2 v1 v3 v1 v2 v1 v3Bold face indicates a keyword; these must appear in lower case. Note that there is a space in “facet normal” and in“outer loop,” while there is no space in any of the keywords beginning with “end.” Indentation must be with spaces;tabs are not allowed. The notation, “{ } ,” means that the contents of the brace brackets can be repeated one ormore times. Symbols in italics are variables which are to be replaced with user-specified values. The numerical datain the facet normal and vertex lines are single precision floats, for example, 1.23456E 789. A facet normalcoordinate may have a leading minus sign; a vertex coordinate may not.Lecture 1963

9/13/2018Example STL Filesolid pyramidfacet normal -8.281842e-001 2.923717e-001 -4.781524e-001outer loopvertex 4.323172e-018 1.632799e-017 6.495190e-001vertex 3.750000e-001 7.081604e-001 4.330127e-001vertex 3.750000e-001 0.000000e 000 0.000000e 000endloopendfacetfacet normal 0.000000e 000 2.923717e-001 9.563048e-001outer loopvertex 7.500000e-001 0.000000e 000 6.495190e-001vertex 3.750000e-001 7.081604e-001 4.330127e-001vertex 0.000000e 000 0.000000e 000 6.495190e-001endloopendfacetfacet normal 8.281842e-001 2.923717e-001 -4.781524e-001outer loopvertex 3.750000e-001 -1.632799e-017 0.000000e 000vertex 3.750000e-001 7.081604e-001 4.330127e-001vertex 7.500000e-001 0.000000e 000 6.495190e-001endloopendfacetfacet normal 0.000000e 000 -1.000000e 000 0.000000e 000outer loopvertex 3.750000e-001 0.000000e 000 0.000000e 000vertex 7.500000e-001 0.000000e 000 6.495190e-001vertex 0.000000e 000 0.000000e 000 6.495190e-001endloopendfacetendsolid pyramidAn STL file is essentially just alist of all the triangles on thesurface of an object.Each triangle is defined with asurface normal and theposition of the three vertices.Lecture 19Slide 7A Single Trianglefacet normal -8.281842e-001 2.923717e-001 -4.781524e-001outer loopvertex 4.323172e-018 1.632799e-017 6.495190e-001vertex 3.750000e-001 7.081604e-001 4.330127e-001vertex 3.750000e-001 0.000000e 000 0.000000e 000endloopendfacet1. Facet normal must follow right‐handrule and point outward from object.a) Some programs set this to [0;0;0]or convey shading information.b) Don’t depend on it!2. Adjacent triangles must have twocommon vertices.3. STL files appear to be setup to handlearbitrary polygons. Don’t do this.Lecture 19Vertex 3Facet NormalVertex 1Vertex 2Slide 84

9/13/2018Warnings About Facet Normals Since the facet normal can be calculated from thevertices using the right-hand rule, sometimes thefacet normal in the STL file contains otherinformation like shading, color, etc. Don’t depend on the right-hand rule beingfollowed. Basically, don’t depend on anything!Lecture 199STL FileProblems andRepairing5

9/13/2018Inverted NormalsAll surface normals should point mlLecture 1911Intersecting TrianglesNo faces should cut through each other. Intersections should ure 19126

9/13/2018Noise ShellsA shell is a collection of triangles that form a single independentobject. Some STL files may contain small shells that are just noise.These should be eliminated.Main shellNoise ShellLecture 1913Nonmanifold MeshesA manifold (i.e. watertight) mesh has no holes and is described by asingle continuous surface.http://http://www.autodesk.com/Lecture 19147

9/13/2018Mesh Repair Software Commercial Software– Magics– NetFabb– SpaceClaim– Autodesk Open Source Alternatives– MeshLab– NetFabb Basic– Blender– Microsoft Model RepairLecture 19Slide 15Importing andExporting STLFiles in MATLAB8

9/13/2018MATLAB Functions for STL FilesThe Mathworks website has very good functions for reading andwriting STL files in both ASCII and binary formats.STL File xchange/29906‐binary‐stl‐file‐readerSTL File scii‐stl‐fileLecture 1917How to Store the DataWe have N facets and 3N vertices to store in arrays.F(N,3) Array of triangle facetsV(?,3) Array of triangle verticesMany times, the number of vertices is 3N. Observing that many ofthe triangle facets share vertices, there will be redundant vertices.STL files can be compressed to eliminate redundant vertices, butmany times they are not.Lecture 19189

9/13/2018Lazy Array of Vertices (1 of 2) vx ,1 vx ,2V vx , Mv y ,1v y ,2 vy ,Mvz ,1 vz ,2 vz , M V is an arraycontaining theposition of all thevertices in Cartesiancoordinates.582469M is the totalnumber of1vertices.71131210Lecture 1919Lazy Array of Vertices (2 of 2)There is redundancy here. Twelve vertices are stored, but the deviceis really only composed of four.While an inefficient representation,this is probably not worth your time fixing.2,5,8SolidWorks exports a lazy array of vertices. vx ,1 vx ,2V vx , MLecture 19v y ,1v y ,2 vy,Mvz ,1 vz ,2 vz , M 4,9,111,6,123,7,102010

9/13/2018Compact Array of Vertices vx ,1 vx ,2V vx ,3 vx ,4v y ,1v y ,2v y ,3v y ,4vz ,1 vz ,2 vz ,3 vz ,4 2222Array of Vertices, V414431134313Lecture 1921Array of Faces n1,1 n2,1F nN ,1n1,2n2,2 nN ,2F is an arrayindicating the arrayindices of thevertices definingthe facet.n1,3 n2,3 nN ,3 5all integers8224694N is the totalnumber offaces.Lecture 19711311312102211

9/13/2018Example of Lazy Arrays2,5,84,9,111,6,123,7,10Lecture 1923Example of Compact Arrays2413This can make a very large difference for large and complex objects.Lecture 192412

9/13/2018Eliminating Redundant VerticesRedundant vertices are easily eliminated using MATLAB’s built inunique() command.% ELIMINATE REDUNDENT VERTICES[V,indm,indn] unique(V,'rows');F indn(F);Identifies unique rows in Vand eliminates them from V.Corrects indices in F thatreferenced eliminated vertices.indm – indices of rows in V that correspond to unique vertices.indn – Map of row indices in original V to new row indices in the reduced V.Lecture 1925STL Files Generated bySolidWorksFor some reason, Solidworks does not use the z‐axis as the verticalaxis.For convenience, STL files can be easily reoriented.% REORIENT SOLIDWORKS AXES TO MATLAB AXESY V(:,2);V(:,2) V(:,3);V(:,3) Y;Orientation in SolidWorksLecture 19Imported OrientationAdjusted Orientation2613

9/13/2018VisualizingSurface Meshesin MATLABHow to Draw the ObjectGiven the faces and vertices, the object can be drawn in MATLABusing the patch() command.% DRAW STRUCTURE USING PATCHESP h',2);Lecture 192814

9/13/2018GeneratingFaces andVertices UsingMATLABMATLAB SurfacesSurfaces composed of square facets are stored in X, Y, and Z arrays.The surface shown is constructedof arrays that are all 5 5.Lecture 193015

9/13/2018Direct Construction of theSurface MeshMATLAB has a number of built‐in commands for generating surfaces.Some of these are cylinder(), sphere() and ellipsoid().% CREATE A UNIT SPHERE[X,Y,Z] sphere(41);Surfaces can be converted to triangular patches (facets and vertices)using the surf2patch() function.% CONVERT TO PATCH[F,V] surf2patch(X,Y,Z,’triangles’);The faces and vertices can be directly visualized using the patch()function.% VISUALIZE FACES AND VERTICESh .5 0.5 0.8],'EdgeColor','k');Lecture 1931Grid Surface Mesh3D objects on a grid can be converted to a surface mesh using thecommand isosurface().% CREATE ELLIPTICAL OBJECTOBJ (X/rx). 2 (Y/ry). 2 (Z/rz). 2;OBJ (OBJ 1);% CREATE SURFACE MESH[F,V] isosurface(X,Y,Z,OBJ,0.5);Object on 3D GridLecture 19Surface Mesh3216

9/13/2018May Need isocaps()When 3D objects extend to the edge of the grid, you may need touse isocaps() in addition to isosurface().isosurface()isocaps()isosurface() isocaps()% CREATE SURFACE MESH[F,V] isosurface(X,Y,Z,OBJ,0.5);[F2,V2] isocaps(X,Y,Z,OBJ,0.5);Lecture 1933Combining Faces and Verticesfrom Two ObjectsThere are no functions in MATLAB to perform Boolean operations on multiplemeshes. We can, however, combine the faces and vertices from two objects. Becareful this does not result in overlaps or gaps between objects.F1 and V1F2 and V2CorrectlyCombined% COMBINE FACES AND VERTICESF3 [ F1 ; F2 length(V1(:,1)) ]V3 [ V1 ; V2 ]Lecture 19IncorrectlyCombined% WRONG WAY TO% COMBINE FACES AND VERTICESF3 [ F1 ; F2 ]V3 [ V1 ; V2 ]3417

9/13/2018ConvertingSurface Meshesto Objects on a3D GridExample – PyramidER(nx,ny,nz)SolidWorks ModelLecture 19Import STL into MATLABConvert to Volume Object3618

9/13/2018Example – DinosaurER(nx,ny,nz)Import STL into MATLABConvert to Volume ObjectLecture 1937MATLAB Functions for“Voxelization” of STL FilesThe Mathworks website has excellent functions forconverting surface meshes to points on a 3D array.Function for /fileexchange/27390‐mesh‐voxelisationLecture 193819

9/13/2018ConvertingImages and 2DObjects to STLLoad and Resize the Image% LOAD IMAGEB imread(‘letter.jpg');% RESIZE IMAGEB imresize(B,0.2);[Nx,Ny,Nc] size(B);This will give us a coarser mesh in orderto be faster and more memory efficient.Lecture 194020

9/13/2018Flatten the ImageImages loaded from file usually contain RGBinformation making them 3D arrays. These arraysmust be converted to flat 2D arrays beforemeshing.%BBBFLATTEN COLOR IMAGE double(B); B(:,:,1) B(:,:,2) B(:,:,3); 1 - B/765;Lecture 1941Stack the ImageWe will mesh the image using isocaps(), butthat function requires a 3D array. So, we will stackthis image to be two layers thick.% STACK IMAGEB(:,:,2) B;Lecture 194221

9/13/2018Mesh the Image Usingisocaps()We only wish to mesh a single surface so we giveisocaps() the additional input argument‘zmin’ to do this.% CREATE 2D MESH[F,V] isocaps(ya,xa,[0 1],B,0.5,'zmin');Lecture 1943Save the Mesh as an STL FileWe can save this mesh as an STL file.Lecture 194422

9/13/2018Extrude Using Blender (1 of 2)1. Open Blender.exe.2. File Import Stl (.stl)3. Open the STL file you just created.Lecture 1945Extrude Using Blender (2 of 2)1. Select the object with rightmouse click.2. Press TAB to enter Edit mode.3. Press ‘e’ to extrude mesh.4. Press TAB again to exit Edit mode.5. You can now edit your objector export as a new 3D STL file.Lecture 194623

9/13/2018Point CloudsWhat is a Point Cloud?Klein Bottle (see MATLAB demos)Point Cloud Description of a Klein BottlePoint clouds represent the outside surface of object as a set of vertices defined by X, Y, and Zcoordinates. They are typically generated by 3D scanners, but can also be used to export 3Dobjects from MATLAB into SolidWorks or other CAD programs.Lecture 194824