From 2004 to 2007 a number of students have done projects and hands-on practice in the Laboratory with photogrammetry. The main goal was to understand pros and cons with respect to active vision: as such, 2D and 3D vision has been studied and used to perform measurments and to reconstruct real world scenes using industrial hardware.
A brief presentation of the activities carried out in the Laboratory during these years can be downloaded below.
This project was developed in the context of the National Italian Project: Low-cost 3D imaging and modeling automatic system (LIMA3D). The Laboratory aim was to design, develop and perform a metrological characterization of a low-cost optical digitizer based on the projection of a single grating of non-coherent light.
SGPS (Single Grating Phase-Shift) is a whole field profilometer based on the projection of a single pattern of Ronchi fringes: a simple slide projector can be used instead of sophysticated, very expensive devices, to match the low-cost requirement.
A novel approach to the phase demodulation of the fringes has been developed to obtain phase values monotonically increasing along the direction perpendicular to the fringe orientation. As a result, the optical head can be calibrated in an absolute way, very dense point clouds expressed in an absolute reference system are obtained, the system set-up is very easy, the device is portable and reconfigurable to the measurement problem, and multi-view acquisition is easily performed.
The objective of this research project was the reduction of the mesurement time, and the possibility of using a simple slide projector instead of those based on LCD or DLP matrices.
Fig. 1 shows the concept: the projecion of a number of fringe patterns, typical of the GCM, GCPS and PSM techniques is replaced by the projection of a single pattern, as for the SGM approach.
As depicted in Fig. 2, the optical head is simplified, and, in principle, it is possible to design the instrument in such a way that a very compact, low cost optical head is obtained.
The slide projector typically projects steady patterns as the one shown in Fig. 3.
The elaboration follows the DFGM approach and results into two phase maps whose sensitivity to height variations is proportional to the period of the two components. The pattern at higher spatial period is used to save the range of the measurement, and the pattern at the lower period is used to increase the resolution (Fig. 4).
The information from the two phase maps is used by the tringulation formula to compute the height of the object (the relative approach is used). The triangulation is performed on a relative basis, i.e., the geometrical parameters of the optical head must be accurately determined and given as input to the system. The shape map is relative with respect to a reference surface (a plane).
Fig. 5 shows an example of the quality of the obtained point clouds. Typical values of the measurement errors are in the range of 0.2 mm – 0.3 mm over an illumination area of 300mm x 230mm, and a measurement interval up to 100mm.
As shown in Fig. 1, the system is composed by the optical head and two moving stages. The optical head exploits active stereo vision. The projection device, LCD based, projects on the target object bi-dimensional patterns of non-coherent light suitable to implement the SGM, PSM GCM and GCPS techniques for light coding. Each point cloud is expressed in the reference system of the optical head (XM, YM, ZM).
The optical head is mounted on a moving stage, allowing its translation along the vertical and the horizontal directions (Tv, To); as shown in Fig. 2, areas large up to two square meters can be scanned and large surfaces can be measured by acquiring and aligning a number of separate patches. The objects presenting circular symmetry can be placed on a rotation stage: it enables the acquisition of views at different values of the rotation angle.
The system is equipped with stepped motors, to fully control the rotation and the translation of the optical head. Each view is aligned by means of rototraslation matrices in the coordinate system (XR, YR, ZR), centerd on the rotation stage. The sequence of acquisition of the range images is decided on the basis of the dimension and the shape of the object, and is performed automatically.
Fast calibration is provided, both for the optical head and for the rotation stage.
OPL-3D has been specifically designed for applications of reverse engineering and rapid prototyping, as well as for applications of measurement and quality control.
The system exploits active stereo vision (the absolute approach is implemented) using time-multiplexing based on the Gray-Code-Phase-Shifting method.
OPL-3D can host a wide variety of projectors. In the left figure in Fig. 1 the device is the ABW LCD 320: it is a microprocessor-controlled and column-driven projector, specifically intended to be used in this class of systems. Alternatively, those devices currently available for video projection can be succesfully used, as that one shown on the right figure in Fig. 1 (Kodak DP 900, based on DLP technology).
The detector is a commercial CCD video camera. In the configurations shown in Fig. 1, the camera is an inexpensive colour Hitachi KP D50, with standard resolution (752 x 582 px). However, any type of camera (black/white or colour, with single or multiple CCDs for colour separation, and with different pixel densities) can be mounted on the system, depending on the application and on the projector used. In Fig. 2, for example, a 1300 x 1030 px digital video camera (Basler model) is mounted, to acquire at the required resolution large fields of views
The projector and the camera are mounted onto a rigid bar, that can be easily moved around the scene by means of a tripod, and that holds the adjustment units for proper orientation. The mount is fully reconfigurable: all parameters can be varied according to the distance from the target, the required measurement resolution and the FoV (Fig. 3).
Given the fact that through sophisticated calibration procedures the system is able to finely estimate the operating parameters, no accurate positioning equipment (micropositioners, microrotators) is required, the only requirement being stability of the mount during the measurement procedure.
Fig. 4 shows two examples of on-site measurements of complex shapes where the full flexibility of the system was mandatory to perform the acquisition.
OPL-3D is equipped with a PC, that has the purpose of (i) driving the projector with the appropriate pattern sequence, (ii) acquiring the image sequences from the target, and (iii) elaborating the images. In addition, it contains all the features to perform sophisticated procedures for setting up and reconfiguration.
The PC is in the current configuration a Pentium III 900 MHz, 1 GB Ram, equipped with a Matrox Meteor II Frame Grabber. The Projector is operated by the PC through the Serial Connector.
OPL-3D exhibits low-measurement uncertainty (120 mm) over large measurement areas (450 x 340 mm), linearly scalable in the case of smaller areas. Special care has been devoted to flexibility of use, in-field measurement setting, reconfigurability and robustness against environmental light changes and surface colour and texture.
Fig. 5 shows the acquisition of the blue car already seen in Fig. 2. Multiview alignment and registration is performed by either purposely designed software or by means of commercially available products, depending on the complexity of the process.
OPL 3-D has been put into the market by Open Technologies s.r.l., Italy, a start-up company of the University of Brescia, under the Trade Name of 3DShape, in a manifold of versions, including sophisticated software for multi-view combination, point cloud manipulation and transformation, up to surface generation.
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.
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:
Fig. 5 shows the rendered view of the CAD model.
The purpose of this activity is the development of descriptive 3D models of the point clouds acquired by the optical digitisers developed at the Laboratory, for the implementation of the Reverse Engineering of complex shapes and in applications that priviledge the efficiency of the whole process with respect to its accuracy.
Typical fields are the production of prototypes and moulds within the collaborative design process and for copying applications, the restitution of cultural heritage, and the Virtual Reality.
The objective is also the implementation of an alternative path with respect to the traditional CAD-based process, to allow the user to model the physical shapes by means of meshes of simple geometrical elements, without requiring specialised knowledge and background, and at the same time providing total compatibility with the higher performance, higher cost, market available software environments, dedicated to CAD and copying applications.
The activity resulted in the development of a software tool called OptoSurfacer, with the following characteristics:
The flow-chart in Fig. 1 describes the tasks performed by OptoSurfacer. They are illustrated for the study case of the object shown in Fig. 2 (a roof tile). The corresponding point cloud, shown in Fig. 3 has been acquired by means of the prototype DFGM (see the Prototypes page), and is characterised by a variability of the measurement of about 200 microns.
OptoSurfacer automatically performs the ordering of the points by creating a regular reference grid and by using the surface shown in Fig. 4 as the basic geometrical element of the mesh. For the roof tile, the shapes have been modelled as shown in Fig. 5, and the resulting mesh is presented in Fig. 6. The irregularities well observable in this figure mainly depend on the roughness and the porosity of the material.
The solid model of the object has been obtained from the mesh representation of Fig. 6. OptoSurfacer generated the sections presented in Fig. 7 and, by blending them, the mathematics of the object. The final solid model is shown in Fig. 8: it is saved in the IGES format, and presents full compatibility with a wide number of CAD-CAM products market available.
Sansoni, G.; Docchio, F. “In-field performance of an optical digitizer for the reverse engineering of free-form surfaces“, The International Journal of Advanced Manufacturing Technology, Vol. 26, no. 11–12, pp. 1353–1361. 2005
OPL_Align is used to increase the quality of the alignment, especially in the case of very accurate point clouds that the performance of View-Integrator cannot align at the measurement accuracy. Also in this case, a pair-wise matching is used, based on the Iterative Closest Point approach. Fig. 1 shows an example of the performance of this tool.
The bass-rilief of Fig. 1 is chosen as the test object. The two partial views in the frames are shown in the OPL_Align environment in Fig. 2, where it is also evidenced the common region used to perform the alignment. The top figure in Fig. 3 shows the relative position between the two patches (Vista 1 and Vista 2 respectively) is shown before and after the alignment. The bottom figure in Fig. 3 shows the performance of the alignment.
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.
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.
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.
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.
This module has been developed to visualize the point clouds.
The main features of the tool are:
Fig. 1 shows an example of the facilities available in the module. The same point cloud is visualized in two independent windows, at different resolution, with the colour information (left view) and representing the only range information (right view). The visualization of multiple views is presented in Fig. 2.
