Commercial software for Reverse Engineering

The softwares

Two commercial software suites are successfully used by the Group to carry out the reverse engineering of very complex objects. These are the Polyworks 7.0 suite and the Raindrop Geomagic Studio 3.1 suite of programs.

Polyworks is specifically designed to obtain triangle meshes from point clouds. The IM-Align module is very powerful and allows us to perform the multiview acquisition when the number of point clouds is very high (from 30 to 500). The IM-Merge, IM-Edit and IM-Compress are used to create the triangle models, depending on the level of accuracy of the original point cloud, and on the accuracy required to the polygonal mesh. The work environment allows the operator to finely adjust, smooth, fill, join, close the final model by means of a considerable number of functions.

The Geomagic environment is designed to produce, from the original point cloud, the triangle models and the NURBS models. These are obtained starting from the triangle meshes. The software privileges the automation of the whole process with respect to the fine, local adjusting of the surfaces.

In the work carried out untill now, the Polyworks Suite has been preferred when (i) the measurement targets are characterised by a high level of complexity and by the presence of small details, (ii) the acquired point clouds result into a high number of invalid points and the quality of the measurement is not optimal, and (iii) the reverse engineering process requires only the generation of triangles. This is the case of the experimental work carried out in the summer of 2001 at the Civici Musei of Brescia, dealing with the modelling of the ‘Winged Victory’.

On the other hand, the Geomagic suite is used when (i) the shapes are generally regular and are efficiently elaborated (edited, filtered, topologically controlled) in an automatic way, (ii) the process time has to be kept low, (iii) the CAD model is required. The reverse engineering of the Ferrari 250MM has been performed in spring 2002 by using this software environment.

A Reverse Engineering example

The example reported here fully documents the reverse engineering process of the object in Fig. 1 carried out by using both the mentioned software products.
It is a 1:4 scaled model of a F333 (by courtesy of Ferrari and Officine Michelotto). The following figures illustrate all the main steps of the test. These are:

  1. the optical acquisition by means of OPL-3D (Fig. 1);
  2. the alignment process to obtain the point cloud of the whole object (Fig. 2). It has been performed by using the IM-Align module;
  3. the generation of the triangle model (Fig. 3). IM-Merge has been used in this step: it allowed the creation of a number of models at different levels of detail;
  4. the generation of the CAD model (Fig. 4). It has been obtained by exporting the triangle model from the Polyworks environment to the Geomagic Environment (the STL format has been used), and by exploiting the powerful tools for the generation of the patch layout and the matematics of the surfaces available in Geomagic Studio 3.0. 

Fig. 5 shows the rendered view of the CAD model.

Fig.1 - The acquisition of the F333 by means of OPL-3D.

View_Integrator: an interactive way to align point cloud views

View_Integrator exploits the correspondence between fiduciary points (markers) in different adjacent views. The procedure requires that the user interactively selects corresponding markers in the views to be aligned. Then it estimates with sub-pixel accuracy the 3D position of the centres of each marker and minimizes the sum of all the distances between the estimated centres until a preset threshold is reached. The surface shape suggests the typology of the markers used to determine the coordinates of the fiduciary points.

Placement of ‘hard’ markers

In some cases, markers of circular shape are physically placed on the surface. This approach has the advantage that we can freely move the object with respect to the optical head, acquire all the views needed to completely acquire it, with the only constraint that the overlapping regions contain the same set of markers. However, the markers are still present on the range information, inducing additional noise. Fig. 1 and Fig. 2 illustrate this experimental case. The object under test is a mannequin head. Fig. 1 shows the marker selection; Fig. 2 presents the corresponding 3D range images and their alignment.

Fig. 1 - Marker selection on the mannequin head.
Fig. 2 - Alignment of the two views.

Placement of “soft markers”

As shown in Fig. 3, we can turn off the projection of the markers during the measurement, and turn it on for the acquisition of the color/texture information. In this way, the markers do not disturb the surface, and the alignment can be performed more accurately.
Fig. 4 illustrates the View-Integrator interface during the selection of the markers, and the result of the alignment is presented in Fig. 5.

Fig. 5 - Result of the alignment of different views in a completed mesh.

Feature based selection of the markers

The last set of figures illustrate how the alignement of the views is performed in the case that neither “hard” nor “soft” markers are used. In this situation, the selection of the fiduciary points is based on the choice of corresponding features in the images; however, this task is very time consuming and critical for the operator, especially when the number of partial views to be aligned is high and when the color information superimposed to the range does not help the operator, as is the case of the two views shown in Fig. 6.

Our approach to solve this problem is the elaboration of the range information by means of the Canny edge detector. As shown in Fig. 7, the 3D images present significant edges that are well enhanced by the filter and dramatically simplify the operator work. Fig. 8 shows the effect of the Canny edge detector and Fig. 9 the matching between the views.

Fig. 6 - Dense views of a bas-relief.
Fig. 7 - The edges of the views are quite significant and can be used to enhance the alignment.