The paper describes the first implementation of a method in which an initial CAD model is updated from a physical model. The method is based on image-mapping in which an initial CAD model is updated from images of a soft rapid prototype model (RPM) which has been sculpted in order to carry out formal developments. The RP model is made by a 3Dimensional-colour printer, has a built-in contrasting grid composed by parallel planes in the X, Y and/or Z co-ordinates and has special consistency allowing it to be easily sculpted with hand modifications. During the sculpting process changes on the surface affect the lines on the RPM, which are the external presence of the internal grid planes and are corresponding to the initial CAD construction lines. These lines (profiles) then are visually contrasted by making use of identical perspective transformations and viewpoints for the virtual model and the RP model image. The initial CAD model is then updated by modifying the surface’s
construction lines to match the lines on the RP image by moving control points, such as in the Z direction

Prieto, PA

Qin, SF

Wright, DK

English

Prieto, Pablo

Wright, David

Qin, Sheng-feng

Northumbria Research Link

A novel updating modelling methodology for free-form surface modifications in the early stages of design

Brunel University Research Archive

1INTERNATIONAL CONFERENCE ON ENGINEERING DESIGNICED 03 STOKHOLM, AUGUST 19-21, 2003A NOVEL UPDATING MODELLING METHODOLOGY FOR FREE-FORMSURFACE MODIFICATIONS IN EARLY STAGES OF DESIGNP.A.Prieto, D.K.Wright and S.F.QinAbstractThe paper describes the first implementation of a method in which an initial CAD model isupdated from a physical model. The method is based on image-mapping in which an initialCAD model is updated from images of a soft rapid prototype model (RPM) which has beensculpted in order to carry out formal developments. The RP model is made by a3Dimensional-colour printer, has a built-in contrasting grid composed by parallel planes inthe X, Y and/or Z co-ordinates and has special consistency allowing it to be easily sculptedwith hand modifications.  During the sculpting process changes on the surface affect the lineson the RPM, which are the external presence of the internal grid planes and are correspondingto the initial CAD construction lines. These lines (profiles) then are visually contrasted bymaking use of identical perspective transformations and viewpoints for the virtual model andthe RP model image. The initial CAD model is then updated by modifying the surface’sconstruction lines to match the lines on the RP image by moving control points, such as in theZ direction.Keywords: Computer aided design (CAD), Reverse Engineering (RE) updating modelling andRapid Prototyping (RP).1. IntroductionCurrently early formal developments of products are usually carried out without computertools, free nature developments are more easily carried out by hands instead computer CADtools. Sketching directly on paper or sculpting over a foam model are the most used methodsto carrying out formal development because compared with CAD design tools, they havesimpler interfaces and can provide visual and tactile real time feedback.However modern manufacturing techniques like CNC and RP are supported by CAD alongthe design process and applied at early stages advance the solution of the future problems onproduct development making the cycle shorter and efficient. Therefore formal developmentscarried out off the CAD have to be inserted into CAD systems for continuing the entireproduct development efficiently.The current process to insert early formal development into CAD uses RE techniques. 3Dscanners or 3D coordinate-measuring devices are used to obtain the geometry ofdevelopments carried out over physical model. The obtained surface data is then proceeded inorder to rebuild a new surface in CAD format. However, existing techniques are mainly2applied to reproduce 3D products or parts and there is little work done to apply it into earlystages of design, when the shape is still under development. In addition they are currentlyprohibitively expensive and inappropriate for the rapidity required in early stages which needa quick feedback.In early stages of the design process computer tools currently available for designers andmodellers have still difficulties to be included in early stages of design. Computer modellingtools have been improved considerably over the last few years. Research focused on sketchand CAD modelling [1-3], images as reference in CAD environment [4-5] and NURBSsurfaces focused on free-form surfaces modelling [6] have being carried out in order to allowsdesigners develop ideas in a user-friendly way inside CAD environment. New commercialCAD packages have powerful tools capable of building virtual products, looking at them fromdifferent points, transforming them, changing their colours, displaying them and even makinga physical representation through the RP technology link. However the modelling process isstill hard in early stages because the shape to be modelled is in this stage under developed andseveral modifications usually of free form nature are carried out away of computerenvironment.Our work is focused on developing a new method to update initial CAD models in the earlystages without uses of traditional RE techniques via 3D scanners or measurement devices.The new approach is an image-mapping based method in which a CAD model is updatedfrom images of a soft rapid prototype model (RPM) that has been sculpted in order to carryout formal developments. The CAD model is updated by matching the differences betweenthe initial CAD model and the modified RPM by making use of identical perspectivetransformations and viewpoints for the initial CAD model and a RPM.2. The current methodCurrently formal design attempts are first carried out off computer. These design intents areusually carried out by hand sculpting soft materials or sketching using paper and pencil; thisallows a fluent formal development without computer restrictions e.g. two-dimensionalscreen. The resulting design is then inserted in CAD by modelling it directly or using REtechniques.The process of obtaining digital models from an existing physical model by acquiring surfaceinformation using scanning or measurement devices is usually called reverse engineering. Thetraditional RE have the following different steps [7]:· Point data acquisition.· Noise filtering· Data reduction· Segmentation· Curve/surface fitting· 3D surface model generationModelling free-form shapes directly on CAD packages is extremely difficult.  Free-formsurfaces have not clear key modelling shapes such as a circle modelling a cylinder; in otherwords there are no primitives for free-form shapes. Reverse Engineering (RE) techniquescould then be used to support three-dimensional CAD modelling from a physical model. A33D scanner or a coordinate-measuring machine obtains the surface information. The outputfrom the scanning process is point cloud data that is processed and fitted with 3D surfaces in aCAD package.When a CAD model is available Rapid Prototype Models then can be used in the early stagesto check fitting, ergonomics and aesthetic issues, to share the model with other members ofthe company and even carry out physic modifications over them. The most used system is 3Dprinting because it is the cheapest and its characteristics such as accuracy, consistence, highbuilding speed and building colour capability are suitable for early models. Modifications canbe carried out directly on the surface and converted into a CAD model by applying ReverseEngineering techniques. Usually the method is recurrent and several physical models may beneeded to create a final CAD model of a satisfactory solution.Rapid prototyping is a process by means of which a physical representation of a virtual modelcan be made directly from a computer (Figure 1). The model is made layer by layer and canbe complicated three-dimensional shapes. Key aspects of accuracy and material propertieshave been improved dramatically over past years and new applications have been developed.Figure 1. The figure shows a simplified diagram to make a RP model.The 3D-printer technology has an important distinctive characteristic, which is the ability tobuild models in full colours. The standard input file for RP t is STL but for printing in coloursthe model must be painted in special package or modelled in colour and exported as a VRMLfile, which is a standard import file used by the Zcorp 3Dprinter to print colour models.The drawback of current design methodology shown in figure 2 is that it highly dependent onRE techniques for updating the models. In the past few years the accuracy and timeconsuming for the capturing surface data have been improved significantly. However, theoutput is cloud data, which must be specially processed with software functions.Figure 2. The figure shows a current design methodology using RP and traditional reverse engineering.3. The new methodIn order to update an initial CAD model in the early stages of design, we have developed animage-mapping based method in which a CAD model is updated from images of a soft rapidprototype model (RPM) which has been sculpted in order to carry out formal developments.The RPM has a built-in contrasting 3-dimensional grid composed by parallel planes in the X,Y and/or Z co-ordinates. The initial CAD model is modelled by skinning NURBS surfaceacross cross-section curves. This technique is widely used to modelling free-forms surfacesbecause of its high flexibility. The RP model is made by a 3Dimensional-colour printer andCAD model Stl file. RP sytem RP ModelCADModelStlfileRPsystemRPmodelTests Modifications REtechnique4has special consistency allowing it to be easily sculpted and formal developed with handmodifications. A single image for each modified area is then obtained, e.g., by mean of adigital camera is used. The image has to be taken from a convenient point of view in order tocapture the sculpted area of the RPM clearly. During the sculpting process changes on thesurface affect the lines on the RPM, which are the external presence of the internal gridplanes. These lines correspond with the construction curves (NURBS) of the initial CADmodel. Therefore, the differences between the initial CAD model and the modified RP modelcan be easily detected by making use of identical perspective transformations and viewpointsfor the CAD window view and the corresponding RPM image. The initial CAD model is thenupdated by modifying the surface’s construction lines to match the lines on the RP image bymoving control points, such as in the Z direction. As a result the CAD model has now beenrefined by the new formal development carried out on the RPM.Figure 3 shows the involved steps for the new method3.1 The first implementationThe method has been evaluated using a 402c 3D-colour printer by Zcorp to produce the RPMand the environment was based on Studio from Alias Wavefront. Several examples werecarried out. Initially a cube was modified on one face with a small depression, and itscorresponding CAD representation was then updated with the new geometric information.Volumes with free-form surfaces, representing a more difficult problem for updating CADmodels were then used. Free-form shapes are difficult to reproduce in CAD packages becauseprimitives do not model them and small free formal developments carried out on physicalmodels are particularly difficult to detect and gauge for reinsertion into a CAD model withoutusing RE techniques.3.2 The inputThe initial CAD models (Fig. 4.a) were modelled in Alias using standard surface modellingtools and were then rebuilt by skinning cross-sections (Fig. 4.b). The reason for rebuilding thesurfaces was that it could reorganise the model construction lines in a standard andcorrespondent structure as the internal contrasting grid. The colour grid and the constructionlines (Fig. 4.c) are then correspondent and placed in the same scale and position to enablelater comparison between the RPM pictures and the Virtual Model’s view.InitialCADModelCross-sectionsModelGridModelVrlFileRPModel Tests SculptingDigitalImageUpdatingProcess(CAD Environment)Input Output5Figure 4. The figure shows an initial CAD model (a); The cross sections model (b); The rebuilt reference surfacemodel by the cross sections (c); The grid CAD model (d); The RP model modelled in colour Grid (e); Thesculpted RPM (f); The differences between the initial CAD model and the sculpted RPM (g); The modified crosssections (h); The new updated surface (i) and a cross section representation of the updated surface (j).3.3 Grid ModellingThe current commercial Zcorp 3d printers print a colour shell on the model, therefore thecolour is printed only on the surface model, the inside model is built without colour and everyclosed space is assumed to be solid. This points problems in the generation of an interior grid.By using Boolean operations a colour grid can be integrated in a shape combining twodifferent models. These two different models consist of a colour grid and the solid modelvolume split into cubes (figure 5).Figure 5. The scheme shows the main boolean operations for modelling the 3d printer input shape in colour grid.The figure (A) shows the grid colour, the figure (B) the volume model split in cubes and (C) shows the final gridmodel.6In the grid modelling process some little considerations must be taken. A little gap must existbetween them, grid and cubes. Small holes must go through the grid, opening every closedspace. Thus the 3D printer can recognise different objects: one inside another with differentcolours instead of a close space. The gap between grid and cubes is accomplished throughscaling all the cubes from its own central pivot points (origin). The thickness of gap has to benot more than two layers of the 3D printer slicing so as to keeping the model integrity. Finallya CAD grid model with a three-dimensional colour grid is modelled and exported as VRMLfile to the 3D printer (Fig. 4.d).3.4 3D PrintingA three-dimensional colour printer was used to print out the RPM (Fig. 4.e). The consistencyof the 3d-printed model, without any infiltration, allows modifications easily by hand. This isa tremendous advantage for sculpting models in our proposed method. Other strong point of3D printing technology is the colour printing capability that allows prints out full coloursurfaces models.The three-dimensional printers have two principal parameters to control the surface accuracyand model consistence. The thickness layer has a direct relation with the surface resolution, ifthe thickness is high the steps between the layers are more noticeable on the surface model.The second parameter is saturation that depends on the quantity of binder liquid sprayed oneach layer; if the saturation is high the model is stronger.Specials set up were used to facilitate the model as sculpting material. Through changingparameters as saturation and thickness layers, it is possible to find an appropriate modelconsistency. The parameters were modified from the default value in order to find a balancebetween resolution and an adequate model consistence and the grid resolution in order tofacilitate the sculpting process and the grid visibility. In our implementation the chosensaturation is 0.8 and (the default value is 1) and the layer thickness was 0.008 inch. (Thedefault value is 0.004). Figure 6 shows the traditional RPM and the RPM built in colour gridFigure 6. The picture shows the traditional RPM and the RPM built in colour grid.3.5 The modificationsWhen the RPM in colour grid with built, form fitting, ergonomic and/or aesthetic tests arecarried out. During this process, modifications are carried out directly on the model surface(Fig. 4.f), which change the grid information shown on the RPM surface.3.6 Comparison & updatingA single picture is located on the CAD package background and visually compared with thevirtual model (Fig. 4.g). The chosen point of view has to show clearly the modified area.Model cross sections have been located in the same place with the RP model. It is importantto make use of identical perspective transformations and viewpoints for the initial CADmodel and the corresponding RPM.7To fit the CAD model to the RPM picture some basic geometric characteristics must beconsidered. The X and Y co-ordinates are the same because the cross sections (constructionsurface lines) and the correspondent line on the RPM surface are in the same cross plane.Therefore, the surface is fitted to the RP picture by moving control points of the cross sectionsonly along the Z-axis. When the cross sections are fitted to its correspondent picture section(Fig.4.h and 4.i), a new surface is rebuilt automatically (Fig.4.j).Finally the new modified CAD model can be received and if necessary it can be used as initialinput for next recurrent design cycle.4. Results & DiscussionCompared with typical reverse engineering processes, our approach is simpler and easier. It isnot necessary to use 3D scanning or coordinate measuring devices for updating existent initialgeometrical CAD models with data obtained from modified physical models. Moreover, thereis in addition no substantial modification to the mathematical design of surface model.The method bridges gaps between different techniques and technologies involved in earlystages. Gaps between RP and sculpting, sculpting and Virtual Modelling and virtualmodelling and RPT are connected through an understandable method. In this way theapproach gives to designers a more integrated, natural and intuitive way to work the earlystages of design process using the advantages of the current technologies.The approach has been demonstrated as a new interface to improve the interaction in earlydesign environments. It provides a more interactive and natural way to work through thedesign process, especially in the early stages. Our method improves the interaction betweencomputer and designer through increasing the tactile and visual fed-back, taking advantage ofbasic motor and perceptual designer’s skills.5. Future workThe method has proved to be a good routine for early stages of design and the focus will be ondeveloping a supporting tool capable to update the a CAD model from the RP images inautomatic way. This tool should improve the method considerably because the user will befocused in the creative process instead of computer interface problems.Another work being considered is using laser devices to create lines (profiles) on big surfacesfor some applications as in car industry were big RP models are not possible or too expensive.AcknowledgementsThe authors are grateful to the Interaction Design Group for valuables discussions especiallyto Matthew Benham and Paul Gammans.This work is partially supported by the EPSRC Research Council Grant (GR/S01702/01).Pablo Prieto would also like to thank the Chilean government for financial support.8References[1] Casper G.C and Amnon A.C., “Sketch input for conceptual surface design”, Computersin Industry, Vol. 34, 1997, pp. 125-137.[2] Lynn Eggli, Chiang-yao Hsu, Beat D Brüderlin and Gershon Elber., “Inferring 3Dmodels from freehand sketches and constraints”, Computer-Aided Design, Vol. 29,1996, pp. 101-112.[3] S.F.Qin, D.K.Wright and I.N.Jordanov., “From on-line sketching to 2D and 3Dgeometry: a system based on fuzzy knowledge”, Computer-Aided Design, Vol. 32,2000, pp. 851-866.[4] Henri Veldhuis and George Vosselman., “The 3D reconstruction of straight and curvedpipes using digital line photogrammetry”, Photogrammetry & Remote sensing, Vol 53,1998, pp. 6-16.[5] A.R François and G.G. Medioni., “Interctive 3D model extraction from a single image”,Image and Vision Computing, Vol. 19, 2001, pp. 317-328.[6] E. Dimas and d. Briassoulis., “ 3D geometric modelling based on NURBS: a review”,Advances in Engineering Software, Vol. 30, 1999, pp. 741-751.[7] Tamás Váraldy, Ralph R. Martin and Jordan Cox., “Reverse engineering of geometricmodels, an introduction”, Computer-Aided Design, Vol. 29, 1997, pp. 255-268.Pablo Prieto     Department of Design, Brunel University, Egham, Surrey TW20 0JZ, United KingdomTel: +44 (0) 1784 431 341    Fax: +44 (0) 1784 472 879     E-mail: pablo.prieto@brunel.ac.uk

3D geometric modelling based on NURBS: a review”,

From on-line sketching to 2D and 3D geometry: a system based on fuzzy knowledge”,

Inferring 3D models from freehand sketches and constraints”,

Interctive 3D model extraction from a single image”,

Reverse engineering of geometric models, an introduction”,

Sketch input for conceptual surface design”,

The 3D reconstruction of straight and curved pipes using digital line photogrammetry”,

A novel updating modelling methodology for free-form surface modifications in the early stages of design

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