Crime scene documentation and legal medicine analysis are part of a very complex process which is aimed at identifying
the offender starting from the collection of the evidences on the scene. This part of the investigation is very critical, since
the crime scene is extremely volatile, and once it is removed, it can not be precisely created again. For this reason, the
documentation process should be as complete as possible, with minimum invasiveness. The use of optical 3D imaging
sensors has been considered as a possible aid to perform the documentation step, since (i) the measurement is contactless
and (ii) the process required to editing and modeling the 3D data is quite similar to the reverse engineering procedures
originally developed for the manufacturing field. In this paper we show the most important results obtained in the
experimentation.
A whole field profilometer for the acquisition of free form shapes is presented. The system is based on the projection of
a single pattern of Ronchi fringes and on optical triangulation. A novel approach to phase measuring profilometry is
implemented to index the field of view. The system is calibrated in an absolute way, in order to obtain dense point
clouds in a global reference system. The device is reconfigurable to the measurement problem, portable and rugged and
well adapts to multi-view acquisition of free-form shapes.
A novel approach that combines optical three-dimensional imaging, reverse engineering (RE) and rapid prototyping
(RP) for mold production in the prosthetic reconstruction of facial prostheses is presented. A commercial laser-stripe
digitizer is used to perform the multiview acquisition of the patient's face; the point clouds are aligned and merged in
order to obtain a polygonal model, which is then edited to sculpture the virtual prothesis. Two physical models of both
the deformed face and the 'repaired' face are obtained: they differ only in the defect zone. Depending on the material
used for the actual prosthesis, the two prototypes can be used either to directly cast the final prosthesis or to fabricate the
positive wax pattern. Two case studies are presented, referring to prostetic reconstructions of an eye and of a nose. The
results demonstrate the advantages over conventional techniques as well as the improvements with respect to known
automated manufacturing techniques in the mold construction. The proposed method results into decreased patient's
disconfort, reduced dependence on the anaplasthologist skill, increased repeatability and efficiency of the whole
process.
KEYWORDS: 3D modeling, Head, Clouds, Solid modeling, Computer aided design, Rapid manufacturing, Cultural heritage, 3D metrology, Calibration, 3D acquisition
This paper presents the activity carried out to perform the three-dimensional acquisition of the "Vittoria Alata", a 2m-high, bronze statue, symbol of our City, located at the Civici Musei di Arte e Storia (S. Giulia) of Brescia. The acquisition of the statue has been performed by using a three-dimensional vision system based on active triangulation and on the projection of non-coherent light. This system, called OPL-3D, represents one of the research products of our Laboratory, which has been active for years in the development of techniques and systems for the contactless acquisition of free-form, complex shapes.
The study, originally motivated by the need to explore a new hypothesis on the origin of the "Vittoria Alata", led to its complete digitization and description in terms of both polygonal and NURBS-based models. A suite of copies of the whole statue has been obtained in the framework of the collaboration between the City Museum and the EOS Electro Optical Systems GmbH, located in Munich, Germany. As a first step, one 30 cm-high replica of the whole statue has been produced using a low-resolution triangle model of the statue (3.5 millions of triangles). As a second step, two 1:1 scale copies of the statue have been produced. For them, the Laboratory has provided the high resolution STL file (16 millions of triangles).
The paper discusses in detail the hardware and the software facilities used to implement the whole process, and gives a comprehensive description of the results.
KEYWORDS: Head, Clouds, 3D vision, Three dimensional sensing, Projection systems, Matrices, Data acquisition, Phase shifts, 3D acquisition, 3D metrology
This paper presents the main features of an optical instrument for 3D vision, based on the projection of structure light. The envisaged application is the non contact, fast acquisition of points clouds both for dimensional and quality control, and for reverse engineering. The components of the system are a liquid crystal projector, projecting fringe patterns on a the target, and a video-camera for the acquisition of the patterns. The measurement technique developed to elaborate the patterns, and retrieve the depth information is based on the combination of the Gray Code and the Phase Shift methods. It yields an extended measuring range at high resolution, and allows the measurement of a wide typology of objects, characterized by shape discontinuities and by fine surface details. The digitization of large objects is carried out by acquiring multiple views and by aligning them into a global reference system. To this aim, suitable rototranslation matrices are computed and used to perform the transformation. From the extensive set of experiments carried out to evaluate the measurement performance, good linearity has been observed, and an overall variability of the measurement error of +/- 35 micrometers have been estimated in correspondence with each single view. The error due to the alignment of multiple views is within 0.1 mm.
KEYWORDS: Demodulation, Linear filtering, Projection systems, Cameras, Information operations, Optical filters, Video, Signal processing, Electronic filtering, Imaging systems
This paper presents a system for 3D vision based on the projection of bidimensional patterns of incoherent light and on phase coding. A novel projection scheme is exploited: two gratings at different wavelengths are combined into a single pattern a d demodulated in the natural domain of the signal to retrieval the depth information. Two phase maps are determined, whose sensitivity to height variations is proportional to the wavelength of the pattern gratings: the phase unwrapping is performed by compensating the phase uncertainty of the finest gratin with the information coming from the coarse one. Thus, both high measurement resolution and extended height range are obtained. The approach requires the acquisition of only one image, shows good robustness against fine variations of the fringe period and well adapts to the measurement of free-from shape. In this paper, the phase demodulation procedure and the unwrapping algorithm are detailed, and the accuracy of the measurement is discussed.
KEYWORDS: 3D metrology, Error analysis, Projection systems, Optical sensors, Linear filtering, Video, Cameras, 3D image processing, 3D vision, Profilometers
In this paper, the analysis of the measurement errors in an optical sensor for the achievement of 3D range images is presented together with the procedures developed for their compensation. The optical sensor performs the 3D measurement by means of the active stereo vision approach in which the correspondence problem is solved by means of the projection of structured light, basically consisting of patterns of fringes with rectangular profile. Two measurement errors have been identified: the former, thereafter called 'waviness,' is intrinsic to the rectangular profile of the fringes. This error has been dramatically reduced by means of a filtering block, based on a cascade of two Butterworth low-pass IIR filters. The latter, thereafter called 'slope,' is due to the crossed axes optical geometry of the system, which results into a stripe broadening of the projected fringes. The compensation of this error has been achieved by taking into account the orientation of the optical devices of the system and by mapping light directions into measurement coordinates. In this paper, the main features of the procedures developed to minimize these errors are presented and some interesting experimental results are shown.
In this paper, a 3D measurement procedure is presented, which combines the gray code light projection technique and the phase shift method. The aim is the improvement of the performance of a 3D imaging system based on structured light projection. The measuring procedure based on gray code light projection demonstrates the ability of measuring objects presenting marked discontinuities of shape, such as steep slopes, grooves and holes: the experimental tests performed show that the measurement accuracy is up to 0.42% and the precision is 0.3% of the measuring range. The accuracy of the measurement is mainly limited by its resolution. To increase the system performances, the ability of the phase shift method to achieve higher resolution has been used: the height values given by these two methods are combined in a procedure which shows increased resolution and accuracy on an extended measuring range. The basic aspects of the two basic techniques for 3D imaging and profiling are here discussed. The combined procedure which integrates them is detailed, and some relevant experimental results are reported.
In this paper, two measurement procedures, aimed at the improvement of the performance of an optical whole field profilometer based on grating projection, are presented. The first procedure is based on the Gray code method. It performs a space encoding of the measurement area and yields 3-D range images in which even sharp discontinuities of the object shape can be measured. The second procedure performs the calibration of the optical components of the system, in order to describe the 3-D profile in a global coordinate system, and limiting the use of a reference surface only during the calibration of the profilometer. In the paper, the two procedures are detailed and some interesting experimental results are reported.
A nonlinear filtering technique for the preprocessing of very low contrast images has been applied to optical profilometry, as an attempt to improve the accuracy of the measurement of objects in harsh conditions. The technique is based on the application of a nonlinear architecture composed of linear Laplacian filters followed by quadratic filters which detect correlated elements. The above sequence of operators results in efficient highpass filtering, keeping at the same time the signal-to-noise ratio within acceptable limits. When applied to highly transparent or weakly diffusive surfaces, the preelaboration technique has largely improved the accuracy of the profilometer. In this paper the preelaboration technique is presented. In particular, the influence of the nonlinear image elaboration on the overall system performance is discussed.
In this work, the authors present a prototype of an adaptive whole-field profilometer based on grating projection. Adaptiveness is achieved by performing the projection of the gratings by an LCD projector. The main features of the hardware and software components of the system are detailed. Profile evaluation, system calibration and certification are also covered. The performances of the instrument, as it emerges from the experimental tests carried out on the prototype version of the system, are discussed.
We did a comprehensive characterization of liquid crystal displays. The aim was to select the most proper for the development of an adaptive light pattern projector for use in industrial profilometry. Super twisted nematic displays have been chosen for this application. Active matrix displays have proven to be superior to passive matrix displays in terms of contrast, spatial characteristics of transmission, transient behavior of the transmission, and dependence of the transmission curves upon temperature. The choice of a suitable liquid crystal display made it possible to define the project criteria for the development of the projector unit. The main features of this projector and its use with profilometry for automatic quality control in industrial framework are presented.
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