**3.3.3 Design parameterization**

166 Reverse Engineering – Recent Advances and Applications

such as (Thompson, 1999). One promising development in recent years was the geometric feature recognition (*GFR*), which automatically recognizes solid features embedded in the Brep models. However, none of the method is able to fully automate the construction process

Based on segmented regions (with fitted surfaces), a region adjacent graph is built. This graph reflects the complete topology and serves as the basis for building the final B-rep model, also called stitched models, where the individual bounded surfaces are glued

In general, there are three steps involved in constructing B-rep models, flattening, edges and vertices calculations, and stitching (Várady et al., 1998). In flattening step, regions are extended outwards until all triangles have been classified. Note that this step is necessary to remove all gaps between regions. Sharp edges can be calculated using surface-surface intersection routines, and vertices where three surfaces meet are also determined. During the process, a complete B-rep topology tree is also constructed. A B-rep model can then be created by stitching together the faces, edges, and vertices. This operation is commonly

B-rep models are not feature-based. In order to convert a B-rep model into a feature-based solid model, the embedded solid features must be recognized, and a feature tree that

One of the most successful algorithms for geometric feature recognition has been proposed by (Venkataraman et al., 2001). The algorithm uses a simple four step process, (1) simplify imported faces, (2) analyze faces for specific feature geometry, (3) remove recognized feature and update model; and (4) return to Step 2 until all features are recognized. The process is illustrated in Fig. 6. Once all possible features are recognized, they are mapped to a new solid model of the part (Fig. 6d) that is parametric with a feature tree. This feature tree

Fig. 6. Illustration of *GFR* algorithm, (a) imported surface model with hole surface selected,

extrusions recognized, base block extrusion face selected, and (d) all features recognized and

Venkataraman's method was recently commercialized by Geometric Software Solutions, Ltd. (GSSL), and implemented in a number of CAD packages, including *SolidWorks* and

(b) hole recognized and removed, extruded face of cylinder selected, (c) cylindrical

and generate fully parametric solid models. Some level of manual work is expected.

together along their common edges to form an air-tight surface model.

**3.3.1 Boundary representation** 

supported by most solid modeling kernels.

describes the sequence of feature creation must be created.

defines the feature regeneration (or model rebuild) sequence.

mapped to solid model

**3.3.2 Solid feature recognition** 

A feature-based parametric solid model consists of two key elements: a feature tree, and fully parameterized sketches employed for protruding solid features. A fully parameterized sketch implies that the sketch profile is fully constrained and dimensioned, so that a change in dimension value yields a rebuilt in accordance with design intents as anticipated. To the author's knowledge, there is no such method proposed or offered that fully automates the process. Some capabilities are offered by commercial tools, such as *Rapidform*, that support designers to interactively create fully parameterized sketches, which accurately conform to the data points and greatly facilitates the solid modeling effort.

## **3.4 Solid model export**

Since most of the promising shape engineering capabilities are not offered in CAD packages (more details in the next section), the solid models constructed in reverse engineering software will have to be exported to mainstream CAD packages in order to support common engineering assignments. The conventional solid model exchanges via standards, such IGES or STEP AP (application protocols), are inadequate since parametric information, including solid features, feature tree, sketch constraints and dimensions, are completely lost through the exchanges. Although feature recognition capability offers some relief in recognizing geometric features embedded in B-rep models, it is still an additional step that is often labor intensive. Direct solid model export has been offered in some software, such as *liveTransfer*™ module of *Rapidform XOR3* as well as third party software, such as *TransMagic*. More will be discussed for *liveTransfer*™.

A Review on Shape Engineering and Design Parameterization in Reverse Engineering 169

data can be commonly achieved by creating section sketches from polygon meshes and following feature creating steps similar to CAD packages. Based on the survey, *Rapidform* is found the only software that supports parametric solid modeling. For hands-on evaluations,

we selected *Geomagic* and *Rapidform*, in addition to a few CAD packages.

Table 1. A summary of commercial software tools surveyed
