ToF cameras are new instruments based on CCD/CMOS sensors which measure distances instead of radi... more ToF cameras are new instruments based on CCD/CMOS sensors which measure distances instead of radiometry. The resulting point clouds show the same properties (both in terms of accuracy and resolution) of the point clouds acquired by means of traditional LiDAR devices. ToF cameras are cheap instruments (less than 10.000 €) based on video real time distance measurements and can represent an interesting alternative to the more expensive LiDAR instruments. In addition, the limited weight and dimensions of ToF cameras allow a reduction of some practical problems such as transportation and on-site management. Most of the commercial ToF cameras use the phase-shift method to measure distances. Due to the use of only one wavelength, most of them have limited range of application (usually about 5 or 10 m). After a brief description of the main characteristics of these instruments, this paper explains and comments the results of the first experimental applications of ToF cameras in Cultural Heritage 3D metric survey. The possibility to acquire more than 30 frames/s and future developments of these devices in terms of use of more than one wavelength to overcome the ambiguity problem allow to foresee new interesting applications.
The Basilica of San Pietro is a Romanic architecture located in the municipality of Tuscania in t... more The Basilica of San Pietro is a Romanic architecture located in the municipality of Tuscania in the Lazio Region about 100 km far from Rome. In 1971 the apse dome collapsed during the earthquake and the important fresco of a Christ Pantocrator was destroyed. In 1975 the dome was reconstructed using reinforced concrete. In 2010 an integrated survey of the Church has been performed using LiDAR techniques integrated with photogrammetric and topographic methodologies in order to realize a complete 2D documentation of the Basilica of San Pietro. Thanks to the acquired data a complete multi-scale 3D model of the Church and of the surroundings was realized. The aim of this work is to present different strategies in order to realize correct documentations for Cultural Heritage knowledge, using typical 3D survey methodologies (i. e. LiDAR survey and photogrammetry). After data acquisition and processing, several 2D representations were realized in order to carry out traditional supports for the different actors involved in the conservation plans; moreover, starting from the 2D drawing a simplified 3D modeling methodology has been followed in order to define the fundamental geometry of the Basilica and the surroundings: the achieved model could be useful for a small architectural scale description of the structure and for the documentation of the surroundings. For the aforementioned small architectural scale model, the 3D modeling was realized using the information derived from the 2D drawings with an approach based on the Constructive Solid Geometry. Using this approach the real shape of the object is simplified. This methodology is employed in particular when the shape of the structures is simple or to communicate new project ideas of when, as in our case, the aim is to give an idea of the complexity of an architectural Cultural Heritage. In order to follow this objective, a small architectural scale model was realized: the area of the Civita hill was modeled using the information derived from the 1:5000 scale map contours; afterwards the Basilica was modeled in a CAD software using the information derived from the 2D drawings of the Basilica. Finally, a more detailed 3D model was realized to describe the real shape of the transept. All this products were realized thanks to the data acquired during the performed survey. This research underlines that a complete 3D documentation of a Cultural Heritage during the survey phase allows the final user to derive all the products that could be necessary for a correct knowledge of the artifact.
1. Introduction
The 3D information of an object to be surveyed can be basically acquired in two w... more 1. Introduction The 3D information of an object to be surveyed can be basically acquired in two ways: using stereo image acquisitions or optical distance measurement techniques. The stereo image acquisition is already known and used for decades in the research community. The advantage of stereo vision to other range measuring devices such as LiDAR, acoustic or radar sensors is that it achieves high resolution and simultaneous acquisition of the surveyed area without energy emission or moving parts. Still, the major disadvantages are the correspondence problem, the processing time and the need of adequate illumination conditions and textured surfaces in the case of automatic matching procedures. Optical distance measurement techniques are usually classified into three main categories: triangulation, interferometry and Time-of-Flight (ToF). The triangulation normally determines an unknown point within a triangle by means of a known optical basis and the related side angles pointing to the unknown point. This often used principle is partitioned in a wealth of partly different 3D techniques, such as for instance active triangulation with structured illumination and passive triangulation (Jähne et al., 1999). Interferometry measures depth also by means of the Time-of-Flight. In this case, however, the phase of the optical wave itself is used. This requires coherent mixing and correlation of the wave-front reflected from the object with a reference wave-front. Also in this case, many variants of the optical interferometry principle have been developed, such as multi-wavelength interferometry, holographic interferometry, speckle interferometry and white light interferometry (Jähne et al., 1999). The high accuracies of distance measurements performed with interferometry mainly depend on the coherence length of the light source: interferometry is not suitable for ranges greater than few centimeters since the method is based on the evaluation of very short optical wavelength. Continuous wave and pulse ToF techniques measure the time of flight of the envelope of a modulated optical signal. These techniques usually apply incoherent optical signals. Typical examples of ToF are the optical rangefinder of total stations or classical LiDAR instruments. In this latter case, actual laser scanners allow to acquire hundreds of thousands of points per second, thanks to fast scanning mechanisms. Their measurement range can vary to a great extent for different instruments; in general it can vary between a tens of meters up to some kilometers, with an accuracy ranging from less than one millimeter to some tens of centimeters respectively. Nevertheless, the main drawbacks of LiDAR instruments are their high costs and dimensions. In the last few years a new generation of active sensors has been developed, which allows to acquire 3D point clouds without any scanning mechanism and from just one point of view at video frame rates. The working principle is the measurement of the ToF of an emitted signal by the device towards the object to be observed, with the advantage of simultaneously measuring the distance information for each pixel of the camera sensor. Many terms have been used in the literature to indicate these devices, which can be called: Time-of-Flight (ToF) cameras, Range IMaging (RIM) cameras, 3D range imagers, range cameras or a combination of the mentioned terms. In the following the term ToF cameras will be prevalently employed, which is more related to the working principle of this recent technology. Such a technology is possible because of the miniaturization of the semiconductor technology and the evolvement of the CCD/CMOS processes that can be implemented independently for each pixel. The result is the possibility to acquire distance measurements for each pixel at high speed and with accuracies up to about one centimeter. While ToF cameras based on the phase-shift measurement usually have a working range limited to ten/thirty meters, RIM cameras based on the direct ToF measurement can measure distances up to 1500 m. Moreover, ToF cameras are usually characterized by low resolution (no more than a few thousands of tens of pixels), small dimensions, costs that are an order of magnitude lower with respect to LiDAR instruments and a lower power consumption with respect to classical laser scanners. In contrast to stereo, the depth accuracy is practically independent of textural appearance, but limited to about one centimeter in the best case (actual phase-shift commercial ToF cameras). The field of real-time ToF camera-based techniques is very active and covers many areas only briefly mentioned in this thesis, which is rather focused on ToF cameras in the Geomatics field.
1.1 Motivation At present the ability to capture the surrounding area at high speed in three dimensions is one of the most challenging tasks in many fields, such as industrial automation and production, mobile mapping, monitoring, automotive safety, autonomous mobile robotics and gaming. For both dynamic and static scene there is no low-price off-the-self system that provides full range, high-resolution distance information in real time such as in the case of ToF cameras. Nevertheless, RIM cameras are usually characterized by some systematic measurement errors, which can strongly worsen the achievable distance measurement accuracy up to tens of centimeters in some cases. Therefore, suitable calibration procedures have to be developed. One of the main topic of this thesis is to propose systematic procedures for the distance calibration of commercial ToF cameras, in order to estimate and increase their measurement accuracy. The calibration procedure presented in this work belongs to the direct calibration methods, since the distance measurement accuracy of RIM cameras is directly estimated and the resulting systematic errors are modeled. The main idea is to propose a procedure which does not require additional digital cameras or cost-effective high precision measurement racks or robot-arms to calibrate ToF cameras and which can be applied to any kind of RIM camera. Suitable experimental tests are proposed in order to analyze the influence of several factors on the distance measurements, such as the camera warm-up during working time, the angle of incidence between the camera axis and the object surface, the presence of foreground objects close to the camera and the object reflectivity. The second main topic of this work is the use of ToF cameras in the Geomatics field, with the final aim of 3D object reconstruction. Since ToF cameras acquire 3D point clouds at video frame rates, this potentiality can surely be exploited for this purpose. The main problem to be faced is the registration of the point clouds acquired from different view-points with ToF cameras. For this purpose, an algorithm for ToF point cloud registration has been developed, which is called multi-frame registration algorithm. Exploiting both the amplitude images and the 3D information delivered by ToF cameras, the proposed algorithm allows to automatically perform the point cloud registration with a final accuracy which is very close to the measurement accuracy of the employed device. Such a result is obtained thanks to frame averaging (so why the term “multi-frame”) and custom-built procedures, also applying an effective filter proposed in this work, which practically removes all “mixed pixels” from the acquired data. Mixed pixels are outliers resulting from the way ToF cameras process multiple returns of the emitted signal: they can strongly affect the accuracy of the acquired point cloud especially in the case of complex scenes. Another challenging topic proposed in this thesis is the integration between ToF data and a multi-image matching approach for automatic 3D object breakline extraction, which can be very useful for speed-up the drawing production of the surveyed objects. In both cases, some improvements are proposed and discussed for future developments.
1.2 Outline This thesis is organized in six chapters. After this brief introduction, the following chapter faces with the working principle of ToF cameras, dividing them in two categories: devices based on the direct ToF principle and devices which use the indirect ToF principle (phase-shift measurement). Then, the state of the art of this technology is reported, with a complete overview about commercial ToF cameras. A description of the measurement parameters and the typical distance measurement errors of ToF cameras based on the phase-shift measurement (which are the most diffused) is reported, in order to show pros and cons of this new technology. In Chapter 3, one of the main topics of this thesis is presented, which is the distance measurement calibration of ToF cameras. First, a minimum time (warm-up time) of camera working is established for both the SR-4000 and the PMDCamCube3.0 cameras in order to achieve distance measurement stability. Then, the relation between integration time and distance measurement precision in analyzed for both devices. A distance measurement accuracy evaluation system for the SR-4000 camera is reported, which can be applied for any ToF camera model, and a distance calibration model is proposed which increases the distance measurement accuracy in a wide interval of the whole working range of the camera. Then, a procedure to evaluate the influence on distance measurements of the orientation of the acquired object surface with respect to the camera axis is proposed. The problem of “internal scattering”, which can arise when foreground objects close to the camera are present, is also faced for the SR-4000 camera; finally, some tests on object reflectivity are reported in order to test the performance of ToF cameras on real objects surfaces. In Chapter 4 a brief summary on the state of the art of applications using ToF cameras is reported, with the aim of showing how this new technology is spread...
In recent years, a new generation of active cameras, based on the Time-of-Flight (ToF) principle,... more In recent years, a new generation of active cameras, based on the Time-of-Flight (ToF) principle, has been developed. The main advantages with respect to other 3D measurement techniques are the possibility to acquire data at video frame rates and to obtain 3D point clouds without scanning and from just one point of view. Several commercial ToF cameras are now available on the market, which presents some differences about sensor resolution, measurement accuracy and maximum distance between camera and object. In particular, the SR-4000 camera by Mesa and the CamCube 3.0 by PMD Technologies have good performances and are well known to researchers dealing with Range Imaging. Some experimental tests have already been performed on the SR-4000 camera, such as evaluation of the camera “warm-up”, distance accuracy, influence on the distance measurements of the “scattering artifacts” and of the camera orientation with respect to the object surface. The results of similar tests carried out using the CamCube3.0 camera will be reported in this paper, in order to give an idea of the performances of these two ToF cameras.
E’ ormai un dato di fatto che le tecniche di rilievo metrico si rivolgono oggi ad un’acquisizione... more E’ ormai un dato di fatto che le tecniche di rilievo metrico si rivolgono oggi ad un’acquisizione automatica di nuvole di punti: autocorrelazione di immagini digitali, LiDAR e le moderne camere 3D. Questo fatto fa sì che il vero rilievo metrico, ossia la definizione della forma e delle dimensioni dell’oggetto del rilievo sia oggi da eseguire a posteriori inserendo l’intelligenza del rilevatore solo dopo l’acquisizione dei punti e non prima come invece avveniva con le tradizionali tecniche fotogrammetriche di restituzione manuale o con le tecniche topografiche terrestri. E’ necessario quindi che la fase di acquisizione tenga conto di questo nuovo contesto. L’esperienza maturata e tutt’ora in corso presso il Gruppo di Ricerca del Politecnico di Torino consente di dimostrare come solo una forte integrazione tra le varie tecniche di acquisizione di nuvole di punti può consentire a posteriori di raggiungere i veri risultati del rilievo metrico che, erroneamente, molte volte vengono intesi come la semplice definizione di coordinate spaziali di una serie più o meno densa di punti. Il lavoro proposto presenta i risultati delle ricerche condotte in questi anni riguardanti lo studio e il miglioramento delle tecniche di acquisizione (fotogrammetria digitale, LiDAR e camere 3D) nonché le soluzioni originali elaborate per giungere ad una vera integrazione tra i dati primari generati da queste tecniche.
Negli ultimi anni sono stati presentati vari articoli inerenti l’integrazione tra differenti tecn... more Negli ultimi anni sono stati presentati vari articoli inerenti l’integrazione tra differenti tecniche di rilievo. L’idea che accomuna questi lavori è quella di superare gli elementi di debolezza delle singole tecniche integrandole fra loro per ottenere un risultato più completo ed affidabile. Nel presente articolo viene presentata l’integrazione tra dati acquisiti con camere basate sul tempo di volo (Time of Flight - ToF) e tecniche fotogrammetriche di matching multi-immagine per il rilievo metrico di manufatti architettonici. Le camere ToF rappresentano un sistema innovativo di acquisizione di nuvole di punti tridimensionali, paragonabili a quelle ottenibili mediante acquisizioni con strumenti LiDAR; tuttavia, il campo di misura è circoscritto a qualche decina di metri e i sensori presentano attualmente una risoluzione limitata a circa 40000 pixel. Come è noto, le tecniche di matching fotogrammetrico non sono in grado di generare nuvole di punti prive di errori grossolani in ogni condizione operativa, soprattutto in applicazioni terrestri: prese convergenti, brusche variazioni di profondità in corrispondenza di discontinuità geometriche o texture ridotte e pattern ripetitivi possono compromettere l’efficacia di questi algoritmi. Per superare tali problemi, alcuni autori hanno proposto di usare DSM approssimati ottenuti con restituzione manuale per ridurre la ricerca di punti omologhi. Il metodo proposto, invece, prevede di realizzare un modello di superficie con camera ToF, aumentando la velocità di acquisizione e riducendo il costo della strumentazione rispetto agli strumenti LiDAR. L’integrazione tra il dato ToF e la tecnica fotogrammetrica di matching multi-immagine permette di estrarre automaticamente in tre dimensioni i bordi geometrici degli oggetti rilevati. Tale risultato può essere utilizzato come dato di partenza nella produzione di elaborati grafici 2D (prospetti, piante, ecc.).
Le camere basate sul tempo di volo (Time of Flight - ToF), recentemente introdotte sul mercato, c... more Le camere basate sul tempo di volo (Time of Flight - ToF), recentemente introdotte sul mercato, consentono di acquisire nuvole di punti tridimensionali da un solo punto di vista e con velocità assimilabili a quelle di una tradizionale videocamera. Tali strumenti sono caratterizzati da piccole dimensioni e costi ridotti rispetto ai comuni strumenti LiDAR. Tuttavia, oltre a presentare un campo di misura circoscritto a qualche decina di metri e sensori di risoluzione limitata a circa 40000 pixel, le misure di distanza che tali camere forniscono sono in genere affette da errori sistematici che degradano la qualità dei dati acquisiti. Per un’attenta valutazione degli effettivi errori sistematici che si possono riscontrare utilizzando questa tecnologia e delle sue potenzialità per il rilievo metrico, sono stati eseguiti alcuni test sperimentali sulla camera SwissRanger (SR)-4000. In particolare, nel presente articolo vengono trattati due aspetti principali. Il primo riguarda la calibrazione delle distanze misurate dalla SR-4000 e più in dettaglio: stima e modellazione dell’errore sulla misura di distanza (che si attesta intorno a 1 cm) e studio dell’influenza sulla precisione delle misure di distanza della riflettività della superficie dell’oggetto ripreso. Per analizzare quest’ultimo aspetto vengono presi in considerazione alcuni materiali di comune impiego negli ambienti interni e per la realizzazione di elementi architettonici, stimando, grazie ad un opportuno sistema, l’influenza che la loro differente riflettività ha sulle misure di distanza della camera. Il secondo aspetto riguarda un primo esempio di applicazione delle camere ToF al rilievo metrico di elementi architettonici. In particolare, si riporta un confronto tra il dato acquisito con la camera SR-4000 e quello ottenuto dal LiDAR triangolatore Mensi S10, che fornisce nuvole di punti con precisione sub-millimetrica da utilizzare come dato di riferimento. Infine, per dare un’idea delle potenzialità delle camere a tempo di volo per applicazioni architettoniche, si riporta un esempio di modellazione tridimensionale relativo ad un particolare architettonico rilevato con la camera ToF.
Aerial photogrammetric surveys are usually expensive and the resolution of the acquired images is... more Aerial photogrammetric surveys are usually expensive and the resolution of the acquired images is often limited. For this reason, different innovative systems have been developed and tested in order to perform a photogrammetric survey in an inexpensive way, with high-resolution images. In this context, one of the most promising acquisition techniques is represented by the use of Unmanned Aerial Vehicles (UAVs) equipped with a digital camera. The paper deals with the acquisition and processing of low-height aerial imagery acquired by UAVs and Remote Piloted Vehicles (RPVs), in order to provide large-scale mapping to support archaeological studies: the pros and cons of these acquisition platforms are presented and discussed. These systems carry out flights that are usually very different from the manned systems as their dimensions and their light weights never allow the set course to be flown; for this reason, the acquired images are often affected by large rotations and small overlaps. Therefore, an ad hoc procedure has been implemented to overcome these limits. In this work, two remote-controlled systems (a mini-helicopter and a mini fixedwing plane) were tested over two different archaeological sites in order to provide Digital Surface Models (DSMs) and large-scale maps (numeric maps and orthophotos). Finally, an accuracy evaluation of the final products is reported.
In this work, the integration between data provided by Time-of- light cameras and a multi-image m... more In this work, the integration between data provided by Time-of- light cameras and a multi-image matching technique for metric surveys of architectural elements is presented. The main advantage is given by the quickness in the data acquisition (few minutes) and the reduced cost of the instruments. The goal of this approach is the automatic extraction of the object breaklines in a 3D environment using a photogrammetric process, which is helpful for the final user exigencies for the reduction of the time needed for the drawing production. The results of the performed tests on some architectural elements will be reported in this paper.
In the last years the Geomatic Research Group of the Politecnico di Torino faced some new researc... more In the last years the Geomatic Research Group of the Politecnico di Torino faced some new research topics about new instruments for point cloud generation (e.g. Time of Flight cameras) and strong integration between multi-image matching techniques and 3D Point Cloud information in order to solve the ambiguities of the already known matching algorithms. ToF cameras can be a good low cost alternative to LiDAR instruments for the generation of precise and accurate point clouds: up to now the application range is still limited but in a near future they will be able to satisfy the most part of the Cultural Heritage metric survey requirements. On the other hand multi-image matching techniques with a correct and deep integration of the point cloud information can give the correct solution for an “intelligent” survey of the geometric object break-lines, which are the correct starting point for a complete survey. These two research topics are strictly connected to a modern Cultural Heritage 3D survey approach. In this paper after a short analysis of the achieved results, an alternative possible scenario for the development of the metric survey approach inside the wider topic of Cultural Heritage Documentation is reported.
Le camere basate sul tempo di volo (Time of Flight - ToF), recentemente introdotte sul mercato, c... more Le camere basate sul tempo di volo (Time of Flight - ToF), recentemente introdotte sul mercato, consentono di acquisire nuvole di punti tridimensionali da un solo punto di vista e con velocità assimilabili a quelle di una tradizionale videocamera. Tuttavia, oltre a presentare un campo di misura limitato a qualche decina di metri, le misure di distanza che tali camere forniscono sono in genere affette da errori sistematici che degradano la qualità dei dati acquisiti. Per un’attenta valutazione degli effettivi errori sistematici che si possono riscontrare utilizzando questa tecnologia, sono stati eseguiti alcuni test sperimentali su una camera ToF, la SwissRanger (SR)-4000 prodotta da Mesa Imaging. In particolare, nel presente articolo vengono trattati due aspetti principali. Il primo riguarda la calibrazione delle distanze misurate dalla SR-4000 e più in dettaglio: valutazione del tempo ottimale di riscaldamento della camera per avere una certa stabilità di misura (circa 40 minuti), stima dell’errore sulla misura di distanza (che si attesta intorno a 1 cm), studio dell’influenza sulla precisione delle misure di distanza della riflettività al variare del colore e dell’angolo di incidenza rispetto alla superficie dell’oggetto ripreso. Inoltre, viene riportato un primo tentativo di modellare gli errori sistematici delle misure di distanza della SR-4000, che permette di ridurne significativamente l’entità in un intervallo di distanza compreso tra 1.5 m e 4.0 m. Il secondo aspetto riguarda invece la calibrazione fotogrammetrica delle immagini d’ampiezza fornite dalla camera grazie all’utilizzo di un poligono di calibrazione appositamente realizzato. Infine, per dare un’idea delle potenzialità di tali strumenti per applicazioni architettoniche, si riporta un primo esempio di acquisizione, elaborazione dei dati e successiva modellazione relativamente ad un particolare architettonico.
3D imaging with Time-of-Flight (ToF) cameras is a promising recent technique which allows 3D poin... more 3D imaging with Time-of-Flight (ToF) cameras is a promising recent technique which allows 3D point clouds to be acquired at video frame rates. However, the distance measurements of these devices are often affected by some systematic errors which decrease the quality of the acquired data. In order to evaluate these errors, some experimental tests on a CCD/CMOS ToF camera sensor, the SwissRanger (SR)-4000 camera, were performed and reported in this paper. In particular, two main aspects are treated: the calibration of the distance measurements of the SR-4000 camera, which deals with evaluation of the camera warm up time period, the distance measurement error evaluation and a study of the influence on distance measurements of the camera orientation with respect to the observed object; the second aspect concerns the photogrammetric calibration of the amplitude images delivered by the camera using a purpose-built multi-resolution field made of high contrast targets.
In recent years, a new generation of active cameras, based on the Time-of-Flight (ToF) principle,... more In recent years, a new generation of active cameras, based on the Time-of-Flight (ToF) principle, has been developed. The main advantages with respect to other 3D measurement techniques are the possibility to acquire data at video frame rates and to obtain 3D point clouds without scanning and from just one point of view. Some experimental tests relative to the calibration of the distance measurements delivered by a ToF camera (SwissRanger-4000 camera) were reported in our previous works (Chiabrando et al., 2009). Starting from those results, in this paper three main tests are described, which are all related to the SR-4000 distance measurements: the influence of the scattering artifacts caused by multiple internal reflections, the evaluation of influence of the angle between the camera optical axis and the normal to the object surface on the distance measurement precision and an investigation of the influence of object reflectivity on the camera distance measurement accuracy and precision. A comparison between SR-4000 data and LiDAR data on a real object is reported in this paper in order to show the potentiality of ToF cameras for metric survey purposes. Finally, our first experiences on the use of the SR-4000 camera for 3D object reconstruction are reported.
In the last few years, several papers dealing with the integration between different surveying te... more In the last few years, several papers dealing with the integration between different surveying techniques (such as photogrammetry, LiDAR, NIR image processing, 2D digital maps, etc.) have been already presented. The main idea of these works is the possibility to overcome the individual weakness of each technique through their combined use, exploiting their complementary nature. In this work, the new integration between data provided by Time-of-Flight (ToF) cameras and a multi-image matching technique is presented. In particular, this approach is considered for the cultural heritage data acquisition and processing. The main advantage is given by the quickness in the data acquisition (only few minutes are requested) and the reduced cost of the instruments, if compared to terrestrial laser scanners. According to the proposed approach, ToF camera and photogrammetric techniques continuously share information in order to extract the geometric breaklines necessary for the cultural heritage survey. One of the first experiences applying this new method on an architectural artefact is presented, in order to show its potentiality for metric survey and architectural drawing purposes.
Documentation and measurements on archaeological areas are mostly connected with close-range phot... more Documentation and measurements on archaeological areas are mostly connected with close-range photogrammetry, in order to analyze the detailed structure of the objects. Classical aerial photogrammetric surveys are expensive and limited by the image resolution. An alternative way is represented by the use of Unmanned Aerial Vehicle (UAV) equipped with photogrammetric sensors. The paper deals with the acquisition and processing of low-height aerial imagery acquired by UAVs, in order to provide large-scale mapping in support of archaeological researches. Two remote controlled systems (one mini helicopter and one mini fixed wing plane) were tested on two different archaeological sites in order to provide Digital Surface Models (DSMs) and large-scale maps (numeric maps and orthophotos). The experimental analyses underline the potentialities and the problems of these systems for photogrammetric surveys. In particular, the UAV image acquisition is far from flights performed by manned planes: their dimensions and their little weights never allow to flight the coarse previously set and, as a consequence, their images are often affected by big rotation and little overlaps. In these conditions, traditional matching algorithms don't always succeed in the generation of reliable DSMs. For this reason, a procedure for the DSM generation in these application field is proposed. It will be shown as this procedure allows satisfying the needs of the archaeological survey in the production of Solid True Ortho-Photos (STOP).
Landslide monitoring activities are of paramount importance for landslide hazard and risk assessm... more Landslide monitoring activities are of paramount importance for landslide hazard and risk assessment. They allow predictions to be made of the phenomenon evolution which can be used to define risk scenarios, especially when urban areas and infrastructures are involved. Over the last decades, spaceborne and, more recently, ground-based interferometric synthetic aperture radar (GB-InSAR) have been successfully used to identify and classify landslides. Ground-based synthetic aperture radar (GB-SAR) interferometry has already been recognized as a powerful tool, that can be considered complementary or alternative to spaceborne SAR interferometry for terrain monitoring and to detect structural changes in buildings. Terrestrial laser scanning (TLS) is a technique increasingly used in remote sensing of unstable slopes. Detailed digital models of the observed surface are generated with centimetric resolution. In particular, if multitemporal scans of the same area are performed, these scans can be compared in order to obtain displacement fields. Both GB-SAR and TLS observations were planned at the end of July 2007 and at the end of October 2007 in order to assess the hazard of a quarry that is subject to ground instabilities (the Ex-Locatelli quarry near the Baveno village in the Italian Alps on Lake Maggiore). This paper reports on these measurement campaigns and on a test site set up in Florence (Italy) and discusses the advantages of the combined use of these innovative techniques. The availability of radar and laser data at the same time has allowed a comparison to be made of the two techniques and has provided an effective validation test for both.
LIDAR data integrated with digital photogrammetry today represents one of the most attractive way... more LIDAR data integrated with digital photogrammetry today represents one of the most attractive ways of facing the metric surveying of an architectural object. Many papers have illustrated the possibility of building a complete 3D model using just these two techniques. In practice, not many restorers are used to managing 3D models and traditional graphic results, such as plans and sections, are usually required. The paper defines a correct balance between the use of traditional (manual and total station) and innovative (LIDAR and digital photogrammetry) techniques in order to satisfy the usual requirements for the metric survey of an architectural object. A project was carried out to obtain knowledge of a Church in Turin; it was necessary to prepare the survey graphic drawings that would be used for the restoration both of the decorative motives and of the structure of the church. The most important aspect of this work is the integration of traditional topographic techniques with the LIDAR technique. This integration was necessary because of the complexity of the object that was to be surveyed and of the requested short times necessary to realize the survey. The tools implemented in the new Sir-IO software (realized by a DITAG research group of the Politecnico di Torino) were of great help in this work. Thanks to this software it was possible to directly plot the details that were to be surveyed on the realized solid images and orthophotos, thus making the preparation of the survey graphic drawings considerably easier.
ToF cameras are new instruments based on CCD/CMOS sensors which measure distances instead of radi... more ToF cameras are new instruments based on CCD/CMOS sensors which measure distances instead of radiometry. The resulting point clouds show the same properties (both in terms of accuracy and resolution) of the point clouds acquired by means of traditional LiDAR devices. ToF cameras are cheap instruments (less than 10.000 €) based on video real time distance measurements and can represent an interesting alternative to the more expensive LiDAR instruments. In addition, the limited weight and dimensions of ToF cameras allow a reduction of some practical problems such as transportation and on-site management. Most of the commercial ToF cameras use the phase-shift method to measure distances. Due to the use of only one wavelength, most of them have limited range of application (usually about 5 or 10 m). After a brief description of the main characteristics of these instruments, this paper explains and comments the results of the first experimental applications of ToF cameras in Cultural Heritage 3D metric survey. The possibility to acquire more than 30 frames/s and future developments of these devices in terms of use of more than one wavelength to overcome the ambiguity problem allow to foresee new interesting applications.
The Basilica of San Pietro is a Romanic architecture located in the municipality of Tuscania in t... more The Basilica of San Pietro is a Romanic architecture located in the municipality of Tuscania in the Lazio Region about 100 km far from Rome. In 1971 the apse dome collapsed during the earthquake and the important fresco of a Christ Pantocrator was destroyed. In 1975 the dome was reconstructed using reinforced concrete. In 2010 an integrated survey of the Church has been performed using LiDAR techniques integrated with photogrammetric and topographic methodologies in order to realize a complete 2D documentation of the Basilica of San Pietro. Thanks to the acquired data a complete multi-scale 3D model of the Church and of the surroundings was realized. The aim of this work is to present different strategies in order to realize correct documentations for Cultural Heritage knowledge, using typical 3D survey methodologies (i. e. LiDAR survey and photogrammetry). After data acquisition and processing, several 2D representations were realized in order to carry out traditional supports for the different actors involved in the conservation plans; moreover, starting from the 2D drawing a simplified 3D modeling methodology has been followed in order to define the fundamental geometry of the Basilica and the surroundings: the achieved model could be useful for a small architectural scale description of the structure and for the documentation of the surroundings. For the aforementioned small architectural scale model, the 3D modeling was realized using the information derived from the 2D drawings with an approach based on the Constructive Solid Geometry. Using this approach the real shape of the object is simplified. This methodology is employed in particular when the shape of the structures is simple or to communicate new project ideas of when, as in our case, the aim is to give an idea of the complexity of an architectural Cultural Heritage. In order to follow this objective, a small architectural scale model was realized: the area of the Civita hill was modeled using the information derived from the 1:5000 scale map contours; afterwards the Basilica was modeled in a CAD software using the information derived from the 2D drawings of the Basilica. Finally, a more detailed 3D model was realized to describe the real shape of the transept. All this products were realized thanks to the data acquired during the performed survey. This research underlines that a complete 3D documentation of a Cultural Heritage during the survey phase allows the final user to derive all the products that could be necessary for a correct knowledge of the artifact.
1. Introduction
The 3D information of an object to be surveyed can be basically acquired in two w... more 1. Introduction The 3D information of an object to be surveyed can be basically acquired in two ways: using stereo image acquisitions or optical distance measurement techniques. The stereo image acquisition is already known and used for decades in the research community. The advantage of stereo vision to other range measuring devices such as LiDAR, acoustic or radar sensors is that it achieves high resolution and simultaneous acquisition of the surveyed area without energy emission or moving parts. Still, the major disadvantages are the correspondence problem, the processing time and the need of adequate illumination conditions and textured surfaces in the case of automatic matching procedures. Optical distance measurement techniques are usually classified into three main categories: triangulation, interferometry and Time-of-Flight (ToF). The triangulation normally determines an unknown point within a triangle by means of a known optical basis and the related side angles pointing to the unknown point. This often used principle is partitioned in a wealth of partly different 3D techniques, such as for instance active triangulation with structured illumination and passive triangulation (Jähne et al., 1999). Interferometry measures depth also by means of the Time-of-Flight. In this case, however, the phase of the optical wave itself is used. This requires coherent mixing and correlation of the wave-front reflected from the object with a reference wave-front. Also in this case, many variants of the optical interferometry principle have been developed, such as multi-wavelength interferometry, holographic interferometry, speckle interferometry and white light interferometry (Jähne et al., 1999). The high accuracies of distance measurements performed with interferometry mainly depend on the coherence length of the light source: interferometry is not suitable for ranges greater than few centimeters since the method is based on the evaluation of very short optical wavelength. Continuous wave and pulse ToF techniques measure the time of flight of the envelope of a modulated optical signal. These techniques usually apply incoherent optical signals. Typical examples of ToF are the optical rangefinder of total stations or classical LiDAR instruments. In this latter case, actual laser scanners allow to acquire hundreds of thousands of points per second, thanks to fast scanning mechanisms. Their measurement range can vary to a great extent for different instruments; in general it can vary between a tens of meters up to some kilometers, with an accuracy ranging from less than one millimeter to some tens of centimeters respectively. Nevertheless, the main drawbacks of LiDAR instruments are their high costs and dimensions. In the last few years a new generation of active sensors has been developed, which allows to acquire 3D point clouds without any scanning mechanism and from just one point of view at video frame rates. The working principle is the measurement of the ToF of an emitted signal by the device towards the object to be observed, with the advantage of simultaneously measuring the distance information for each pixel of the camera sensor. Many terms have been used in the literature to indicate these devices, which can be called: Time-of-Flight (ToF) cameras, Range IMaging (RIM) cameras, 3D range imagers, range cameras or a combination of the mentioned terms. In the following the term ToF cameras will be prevalently employed, which is more related to the working principle of this recent technology. Such a technology is possible because of the miniaturization of the semiconductor technology and the evolvement of the CCD/CMOS processes that can be implemented independently for each pixel. The result is the possibility to acquire distance measurements for each pixel at high speed and with accuracies up to about one centimeter. While ToF cameras based on the phase-shift measurement usually have a working range limited to ten/thirty meters, RIM cameras based on the direct ToF measurement can measure distances up to 1500 m. Moreover, ToF cameras are usually characterized by low resolution (no more than a few thousands of tens of pixels), small dimensions, costs that are an order of magnitude lower with respect to LiDAR instruments and a lower power consumption with respect to classical laser scanners. In contrast to stereo, the depth accuracy is practically independent of textural appearance, but limited to about one centimeter in the best case (actual phase-shift commercial ToF cameras). The field of real-time ToF camera-based techniques is very active and covers many areas only briefly mentioned in this thesis, which is rather focused on ToF cameras in the Geomatics field.
1.1 Motivation At present the ability to capture the surrounding area at high speed in three dimensions is one of the most challenging tasks in many fields, such as industrial automation and production, mobile mapping, monitoring, automotive safety, autonomous mobile robotics and gaming. For both dynamic and static scene there is no low-price off-the-self system that provides full range, high-resolution distance information in real time such as in the case of ToF cameras. Nevertheless, RIM cameras are usually characterized by some systematic measurement errors, which can strongly worsen the achievable distance measurement accuracy up to tens of centimeters in some cases. Therefore, suitable calibration procedures have to be developed. One of the main topic of this thesis is to propose systematic procedures for the distance calibration of commercial ToF cameras, in order to estimate and increase their measurement accuracy. The calibration procedure presented in this work belongs to the direct calibration methods, since the distance measurement accuracy of RIM cameras is directly estimated and the resulting systematic errors are modeled. The main idea is to propose a procedure which does not require additional digital cameras or cost-effective high precision measurement racks or robot-arms to calibrate ToF cameras and which can be applied to any kind of RIM camera. Suitable experimental tests are proposed in order to analyze the influence of several factors on the distance measurements, such as the camera warm-up during working time, the angle of incidence between the camera axis and the object surface, the presence of foreground objects close to the camera and the object reflectivity. The second main topic of this work is the use of ToF cameras in the Geomatics field, with the final aim of 3D object reconstruction. Since ToF cameras acquire 3D point clouds at video frame rates, this potentiality can surely be exploited for this purpose. The main problem to be faced is the registration of the point clouds acquired from different view-points with ToF cameras. For this purpose, an algorithm for ToF point cloud registration has been developed, which is called multi-frame registration algorithm. Exploiting both the amplitude images and the 3D information delivered by ToF cameras, the proposed algorithm allows to automatically perform the point cloud registration with a final accuracy which is very close to the measurement accuracy of the employed device. Such a result is obtained thanks to frame averaging (so why the term “multi-frame”) and custom-built procedures, also applying an effective filter proposed in this work, which practically removes all “mixed pixels” from the acquired data. Mixed pixels are outliers resulting from the way ToF cameras process multiple returns of the emitted signal: they can strongly affect the accuracy of the acquired point cloud especially in the case of complex scenes. Another challenging topic proposed in this thesis is the integration between ToF data and a multi-image matching approach for automatic 3D object breakline extraction, which can be very useful for speed-up the drawing production of the surveyed objects. In both cases, some improvements are proposed and discussed for future developments.
1.2 Outline This thesis is organized in six chapters. After this brief introduction, the following chapter faces with the working principle of ToF cameras, dividing them in two categories: devices based on the direct ToF principle and devices which use the indirect ToF principle (phase-shift measurement). Then, the state of the art of this technology is reported, with a complete overview about commercial ToF cameras. A description of the measurement parameters and the typical distance measurement errors of ToF cameras based on the phase-shift measurement (which are the most diffused) is reported, in order to show pros and cons of this new technology. In Chapter 3, one of the main topics of this thesis is presented, which is the distance measurement calibration of ToF cameras. First, a minimum time (warm-up time) of camera working is established for both the SR-4000 and the PMDCamCube3.0 cameras in order to achieve distance measurement stability. Then, the relation between integration time and distance measurement precision in analyzed for both devices. A distance measurement accuracy evaluation system for the SR-4000 camera is reported, which can be applied for any ToF camera model, and a distance calibration model is proposed which increases the distance measurement accuracy in a wide interval of the whole working range of the camera. Then, a procedure to evaluate the influence on distance measurements of the orientation of the acquired object surface with respect to the camera axis is proposed. The problem of “internal scattering”, which can arise when foreground objects close to the camera are present, is also faced for the SR-4000 camera; finally, some tests on object reflectivity are reported in order to test the performance of ToF cameras on real objects surfaces. In Chapter 4 a brief summary on the state of the art of applications using ToF cameras is reported, with the aim of showing how this new technology is spread...
In recent years, a new generation of active cameras, based on the Time-of-Flight (ToF) principle,... more In recent years, a new generation of active cameras, based on the Time-of-Flight (ToF) principle, has been developed. The main advantages with respect to other 3D measurement techniques are the possibility to acquire data at video frame rates and to obtain 3D point clouds without scanning and from just one point of view. Several commercial ToF cameras are now available on the market, which presents some differences about sensor resolution, measurement accuracy and maximum distance between camera and object. In particular, the SR-4000 camera by Mesa and the CamCube 3.0 by PMD Technologies have good performances and are well known to researchers dealing with Range Imaging. Some experimental tests have already been performed on the SR-4000 camera, such as evaluation of the camera “warm-up”, distance accuracy, influence on the distance measurements of the “scattering artifacts” and of the camera orientation with respect to the object surface. The results of similar tests carried out using the CamCube3.0 camera will be reported in this paper, in order to give an idea of the performances of these two ToF cameras.
E’ ormai un dato di fatto che le tecniche di rilievo metrico si rivolgono oggi ad un’acquisizione... more E’ ormai un dato di fatto che le tecniche di rilievo metrico si rivolgono oggi ad un’acquisizione automatica di nuvole di punti: autocorrelazione di immagini digitali, LiDAR e le moderne camere 3D. Questo fatto fa sì che il vero rilievo metrico, ossia la definizione della forma e delle dimensioni dell’oggetto del rilievo sia oggi da eseguire a posteriori inserendo l’intelligenza del rilevatore solo dopo l’acquisizione dei punti e non prima come invece avveniva con le tradizionali tecniche fotogrammetriche di restituzione manuale o con le tecniche topografiche terrestri. E’ necessario quindi che la fase di acquisizione tenga conto di questo nuovo contesto. L’esperienza maturata e tutt’ora in corso presso il Gruppo di Ricerca del Politecnico di Torino consente di dimostrare come solo una forte integrazione tra le varie tecniche di acquisizione di nuvole di punti può consentire a posteriori di raggiungere i veri risultati del rilievo metrico che, erroneamente, molte volte vengono intesi come la semplice definizione di coordinate spaziali di una serie più o meno densa di punti. Il lavoro proposto presenta i risultati delle ricerche condotte in questi anni riguardanti lo studio e il miglioramento delle tecniche di acquisizione (fotogrammetria digitale, LiDAR e camere 3D) nonché le soluzioni originali elaborate per giungere ad una vera integrazione tra i dati primari generati da queste tecniche.
Negli ultimi anni sono stati presentati vari articoli inerenti l’integrazione tra differenti tecn... more Negli ultimi anni sono stati presentati vari articoli inerenti l’integrazione tra differenti tecniche di rilievo. L’idea che accomuna questi lavori è quella di superare gli elementi di debolezza delle singole tecniche integrandole fra loro per ottenere un risultato più completo ed affidabile. Nel presente articolo viene presentata l’integrazione tra dati acquisiti con camere basate sul tempo di volo (Time of Flight - ToF) e tecniche fotogrammetriche di matching multi-immagine per il rilievo metrico di manufatti architettonici. Le camere ToF rappresentano un sistema innovativo di acquisizione di nuvole di punti tridimensionali, paragonabili a quelle ottenibili mediante acquisizioni con strumenti LiDAR; tuttavia, il campo di misura è circoscritto a qualche decina di metri e i sensori presentano attualmente una risoluzione limitata a circa 40000 pixel. Come è noto, le tecniche di matching fotogrammetrico non sono in grado di generare nuvole di punti prive di errori grossolani in ogni condizione operativa, soprattutto in applicazioni terrestri: prese convergenti, brusche variazioni di profondità in corrispondenza di discontinuità geometriche o texture ridotte e pattern ripetitivi possono compromettere l’efficacia di questi algoritmi. Per superare tali problemi, alcuni autori hanno proposto di usare DSM approssimati ottenuti con restituzione manuale per ridurre la ricerca di punti omologhi. Il metodo proposto, invece, prevede di realizzare un modello di superficie con camera ToF, aumentando la velocità di acquisizione e riducendo il costo della strumentazione rispetto agli strumenti LiDAR. L’integrazione tra il dato ToF e la tecnica fotogrammetrica di matching multi-immagine permette di estrarre automaticamente in tre dimensioni i bordi geometrici degli oggetti rilevati. Tale risultato può essere utilizzato come dato di partenza nella produzione di elaborati grafici 2D (prospetti, piante, ecc.).
Le camere basate sul tempo di volo (Time of Flight - ToF), recentemente introdotte sul mercato, c... more Le camere basate sul tempo di volo (Time of Flight - ToF), recentemente introdotte sul mercato, consentono di acquisire nuvole di punti tridimensionali da un solo punto di vista e con velocità assimilabili a quelle di una tradizionale videocamera. Tali strumenti sono caratterizzati da piccole dimensioni e costi ridotti rispetto ai comuni strumenti LiDAR. Tuttavia, oltre a presentare un campo di misura circoscritto a qualche decina di metri e sensori di risoluzione limitata a circa 40000 pixel, le misure di distanza che tali camere forniscono sono in genere affette da errori sistematici che degradano la qualità dei dati acquisiti. Per un’attenta valutazione degli effettivi errori sistematici che si possono riscontrare utilizzando questa tecnologia e delle sue potenzialità per il rilievo metrico, sono stati eseguiti alcuni test sperimentali sulla camera SwissRanger (SR)-4000. In particolare, nel presente articolo vengono trattati due aspetti principali. Il primo riguarda la calibrazione delle distanze misurate dalla SR-4000 e più in dettaglio: stima e modellazione dell’errore sulla misura di distanza (che si attesta intorno a 1 cm) e studio dell’influenza sulla precisione delle misure di distanza della riflettività della superficie dell’oggetto ripreso. Per analizzare quest’ultimo aspetto vengono presi in considerazione alcuni materiali di comune impiego negli ambienti interni e per la realizzazione di elementi architettonici, stimando, grazie ad un opportuno sistema, l’influenza che la loro differente riflettività ha sulle misure di distanza della camera. Il secondo aspetto riguarda un primo esempio di applicazione delle camere ToF al rilievo metrico di elementi architettonici. In particolare, si riporta un confronto tra il dato acquisito con la camera SR-4000 e quello ottenuto dal LiDAR triangolatore Mensi S10, che fornisce nuvole di punti con precisione sub-millimetrica da utilizzare come dato di riferimento. Infine, per dare un’idea delle potenzialità delle camere a tempo di volo per applicazioni architettoniche, si riporta un esempio di modellazione tridimensionale relativo ad un particolare architettonico rilevato con la camera ToF.
Aerial photogrammetric surveys are usually expensive and the resolution of the acquired images is... more Aerial photogrammetric surveys are usually expensive and the resolution of the acquired images is often limited. For this reason, different innovative systems have been developed and tested in order to perform a photogrammetric survey in an inexpensive way, with high-resolution images. In this context, one of the most promising acquisition techniques is represented by the use of Unmanned Aerial Vehicles (UAVs) equipped with a digital camera. The paper deals with the acquisition and processing of low-height aerial imagery acquired by UAVs and Remote Piloted Vehicles (RPVs), in order to provide large-scale mapping to support archaeological studies: the pros and cons of these acquisition platforms are presented and discussed. These systems carry out flights that are usually very different from the manned systems as their dimensions and their light weights never allow the set course to be flown; for this reason, the acquired images are often affected by large rotations and small overlaps. Therefore, an ad hoc procedure has been implemented to overcome these limits. In this work, two remote-controlled systems (a mini-helicopter and a mini fixedwing plane) were tested over two different archaeological sites in order to provide Digital Surface Models (DSMs) and large-scale maps (numeric maps and orthophotos). Finally, an accuracy evaluation of the final products is reported.
In this work, the integration between data provided by Time-of- light cameras and a multi-image m... more In this work, the integration between data provided by Time-of- light cameras and a multi-image matching technique for metric surveys of architectural elements is presented. The main advantage is given by the quickness in the data acquisition (few minutes) and the reduced cost of the instruments. The goal of this approach is the automatic extraction of the object breaklines in a 3D environment using a photogrammetric process, which is helpful for the final user exigencies for the reduction of the time needed for the drawing production. The results of the performed tests on some architectural elements will be reported in this paper.
In the last years the Geomatic Research Group of the Politecnico di Torino faced some new researc... more In the last years the Geomatic Research Group of the Politecnico di Torino faced some new research topics about new instruments for point cloud generation (e.g. Time of Flight cameras) and strong integration between multi-image matching techniques and 3D Point Cloud information in order to solve the ambiguities of the already known matching algorithms. ToF cameras can be a good low cost alternative to LiDAR instruments for the generation of precise and accurate point clouds: up to now the application range is still limited but in a near future they will be able to satisfy the most part of the Cultural Heritage metric survey requirements. On the other hand multi-image matching techniques with a correct and deep integration of the point cloud information can give the correct solution for an “intelligent” survey of the geometric object break-lines, which are the correct starting point for a complete survey. These two research topics are strictly connected to a modern Cultural Heritage 3D survey approach. In this paper after a short analysis of the achieved results, an alternative possible scenario for the development of the metric survey approach inside the wider topic of Cultural Heritage Documentation is reported.
Le camere basate sul tempo di volo (Time of Flight - ToF), recentemente introdotte sul mercato, c... more Le camere basate sul tempo di volo (Time of Flight - ToF), recentemente introdotte sul mercato, consentono di acquisire nuvole di punti tridimensionali da un solo punto di vista e con velocità assimilabili a quelle di una tradizionale videocamera. Tuttavia, oltre a presentare un campo di misura limitato a qualche decina di metri, le misure di distanza che tali camere forniscono sono in genere affette da errori sistematici che degradano la qualità dei dati acquisiti. Per un’attenta valutazione degli effettivi errori sistematici che si possono riscontrare utilizzando questa tecnologia, sono stati eseguiti alcuni test sperimentali su una camera ToF, la SwissRanger (SR)-4000 prodotta da Mesa Imaging. In particolare, nel presente articolo vengono trattati due aspetti principali. Il primo riguarda la calibrazione delle distanze misurate dalla SR-4000 e più in dettaglio: valutazione del tempo ottimale di riscaldamento della camera per avere una certa stabilità di misura (circa 40 minuti), stima dell’errore sulla misura di distanza (che si attesta intorno a 1 cm), studio dell’influenza sulla precisione delle misure di distanza della riflettività al variare del colore e dell’angolo di incidenza rispetto alla superficie dell’oggetto ripreso. Inoltre, viene riportato un primo tentativo di modellare gli errori sistematici delle misure di distanza della SR-4000, che permette di ridurne significativamente l’entità in un intervallo di distanza compreso tra 1.5 m e 4.0 m. Il secondo aspetto riguarda invece la calibrazione fotogrammetrica delle immagini d’ampiezza fornite dalla camera grazie all’utilizzo di un poligono di calibrazione appositamente realizzato. Infine, per dare un’idea delle potenzialità di tali strumenti per applicazioni architettoniche, si riporta un primo esempio di acquisizione, elaborazione dei dati e successiva modellazione relativamente ad un particolare architettonico.
3D imaging with Time-of-Flight (ToF) cameras is a promising recent technique which allows 3D poin... more 3D imaging with Time-of-Flight (ToF) cameras is a promising recent technique which allows 3D point clouds to be acquired at video frame rates. However, the distance measurements of these devices are often affected by some systematic errors which decrease the quality of the acquired data. In order to evaluate these errors, some experimental tests on a CCD/CMOS ToF camera sensor, the SwissRanger (SR)-4000 camera, were performed and reported in this paper. In particular, two main aspects are treated: the calibration of the distance measurements of the SR-4000 camera, which deals with evaluation of the camera warm up time period, the distance measurement error evaluation and a study of the influence on distance measurements of the camera orientation with respect to the observed object; the second aspect concerns the photogrammetric calibration of the amplitude images delivered by the camera using a purpose-built multi-resolution field made of high contrast targets.
In recent years, a new generation of active cameras, based on the Time-of-Flight (ToF) principle,... more In recent years, a new generation of active cameras, based on the Time-of-Flight (ToF) principle, has been developed. The main advantages with respect to other 3D measurement techniques are the possibility to acquire data at video frame rates and to obtain 3D point clouds without scanning and from just one point of view. Some experimental tests relative to the calibration of the distance measurements delivered by a ToF camera (SwissRanger-4000 camera) were reported in our previous works (Chiabrando et al., 2009). Starting from those results, in this paper three main tests are described, which are all related to the SR-4000 distance measurements: the influence of the scattering artifacts caused by multiple internal reflections, the evaluation of influence of the angle between the camera optical axis and the normal to the object surface on the distance measurement precision and an investigation of the influence of object reflectivity on the camera distance measurement accuracy and precision. A comparison between SR-4000 data and LiDAR data on a real object is reported in this paper in order to show the potentiality of ToF cameras for metric survey purposes. Finally, our first experiences on the use of the SR-4000 camera for 3D object reconstruction are reported.
In the last few years, several papers dealing with the integration between different surveying te... more In the last few years, several papers dealing with the integration between different surveying techniques (such as photogrammetry, LiDAR, NIR image processing, 2D digital maps, etc.) have been already presented. The main idea of these works is the possibility to overcome the individual weakness of each technique through their combined use, exploiting their complementary nature. In this work, the new integration between data provided by Time-of-Flight (ToF) cameras and a multi-image matching technique is presented. In particular, this approach is considered for the cultural heritage data acquisition and processing. The main advantage is given by the quickness in the data acquisition (only few minutes are requested) and the reduced cost of the instruments, if compared to terrestrial laser scanners. According to the proposed approach, ToF camera and photogrammetric techniques continuously share information in order to extract the geometric breaklines necessary for the cultural heritage survey. One of the first experiences applying this new method on an architectural artefact is presented, in order to show its potentiality for metric survey and architectural drawing purposes.
Documentation and measurements on archaeological areas are mostly connected with close-range phot... more Documentation and measurements on archaeological areas are mostly connected with close-range photogrammetry, in order to analyze the detailed structure of the objects. Classical aerial photogrammetric surveys are expensive and limited by the image resolution. An alternative way is represented by the use of Unmanned Aerial Vehicle (UAV) equipped with photogrammetric sensors. The paper deals with the acquisition and processing of low-height aerial imagery acquired by UAVs, in order to provide large-scale mapping in support of archaeological researches. Two remote controlled systems (one mini helicopter and one mini fixed wing plane) were tested on two different archaeological sites in order to provide Digital Surface Models (DSMs) and large-scale maps (numeric maps and orthophotos). The experimental analyses underline the potentialities and the problems of these systems for photogrammetric surveys. In particular, the UAV image acquisition is far from flights performed by manned planes: their dimensions and their little weights never allow to flight the coarse previously set and, as a consequence, their images are often affected by big rotation and little overlaps. In these conditions, traditional matching algorithms don't always succeed in the generation of reliable DSMs. For this reason, a procedure for the DSM generation in these application field is proposed. It will be shown as this procedure allows satisfying the needs of the archaeological survey in the production of Solid True Ortho-Photos (STOP).
Landslide monitoring activities are of paramount importance for landslide hazard and risk assessm... more Landslide monitoring activities are of paramount importance for landslide hazard and risk assessment. They allow predictions to be made of the phenomenon evolution which can be used to define risk scenarios, especially when urban areas and infrastructures are involved. Over the last decades, spaceborne and, more recently, ground-based interferometric synthetic aperture radar (GB-InSAR) have been successfully used to identify and classify landslides. Ground-based synthetic aperture radar (GB-SAR) interferometry has already been recognized as a powerful tool, that can be considered complementary or alternative to spaceborne SAR interferometry for terrain monitoring and to detect structural changes in buildings. Terrestrial laser scanning (TLS) is a technique increasingly used in remote sensing of unstable slopes. Detailed digital models of the observed surface are generated with centimetric resolution. In particular, if multitemporal scans of the same area are performed, these scans can be compared in order to obtain displacement fields. Both GB-SAR and TLS observations were planned at the end of July 2007 and at the end of October 2007 in order to assess the hazard of a quarry that is subject to ground instabilities (the Ex-Locatelli quarry near the Baveno village in the Italian Alps on Lake Maggiore). This paper reports on these measurement campaigns and on a test site set up in Florence (Italy) and discusses the advantages of the combined use of these innovative techniques. The availability of radar and laser data at the same time has allowed a comparison to be made of the two techniques and has provided an effective validation test for both.
LIDAR data integrated with digital photogrammetry today represents one of the most attractive way... more LIDAR data integrated with digital photogrammetry today represents one of the most attractive ways of facing the metric surveying of an architectural object. Many papers have illustrated the possibility of building a complete 3D model using just these two techniques. In practice, not many restorers are used to managing 3D models and traditional graphic results, such as plans and sections, are usually required. The paper defines a correct balance between the use of traditional (manual and total station) and innovative (LIDAR and digital photogrammetry) techniques in order to satisfy the usual requirements for the metric survey of an architectural object. A project was carried out to obtain knowledge of a Church in Turin; it was necessary to prepare the survey graphic drawings that would be used for the restoration both of the decorative motives and of the structure of the church. The most important aspect of this work is the integration of traditional topographic techniques with the LIDAR technique. This integration was necessary because of the complexity of the object that was to be surveyed and of the requested short times necessary to realize the survey. The tools implemented in the new Sir-IO software (realized by a DITAG research group of the Politecnico di Torino) were of great help in this work. Thanks to this software it was possible to directly plot the details that were to be surveyed on the realized solid images and orthophotos, thus making the preparation of the survey graphic drawings considerably easier.
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Papers by Dario Piatti
ToF cameras are cheap instruments (less than 10.000 €) based on video real time distance measurements and can represent an interesting alternative to the more expensive LiDAR instruments. In addition, the limited weight and dimensions of ToF cameras allow a reduction of some practical problems such as transportation and on-site management.
Most of the commercial ToF cameras use the phase-shift method to measure distances. Due to the use of only one wavelength, most of them have limited range of application (usually about 5 or 10 m).
After a brief description of the main characteristics of these instruments, this paper explains and comments the results of the first experimental applications of ToF cameras in Cultural Heritage 3D metric survey.
The possibility to acquire more than 30 frames/s and future developments of these devices in terms of use of more than one wavelength to overcome the ambiguity problem allow to foresee new interesting applications.
In 2010 an integrated survey of the Church has been performed using LiDAR techniques integrated with photogrammetric and topographic methodologies in order to realize a complete 2D documentation of the Basilica of San Pietro. Thanks to the acquired data a complete multi-scale 3D model of the Church and of the surroundings was realized.
The aim of this work is to present different strategies in order to realize correct documentations for Cultural Heritage knowledge, using typical 3D survey methodologies (i. e. LiDAR survey and photogrammetry).
After data acquisition and processing, several 2D representations were realized in order to carry out traditional supports for the different actors involved in the conservation plans; moreover, starting from the 2D drawing a simplified 3D modeling methodology has been followed in order to define the fundamental geometry of the Basilica and the surroundings: the achieved model could be useful for a small architectural scale description of the structure and for the documentation of the surroundings.
For the aforementioned small architectural scale model, the 3D modeling was realized using the information derived from the 2D drawings with an approach based on the Constructive Solid Geometry. Using this approach the real shape of the object is simplified. This methodology is employed in particular when the shape of the structures is simple or to communicate new project ideas of when, as in our case, the aim is to give an idea of the complexity of an architectural Cultural Heritage.
In order to follow this objective, a small architectural scale model was realized: the area of the Civita hill was modeled using the information derived from the 1:5000 scale map contours; afterwards the Basilica was modeled in a CAD software using the information derived from the 2D drawings of the Basilica.
Finally, a more detailed 3D model was realized to describe the real shape of the transept.
All this products were realized thanks to the data acquired during the performed survey. This research underlines that a complete 3D documentation of a Cultural Heritage during the survey phase allows the final user to derive all the products that could be necessary for a correct knowledge of the artifact.
The 3D information of an object to be surveyed can be basically acquired in two ways: using stereo image acquisitions or optical distance measurement techniques.
The stereo image acquisition is already known and used for decades in the research community. The advantage of stereo vision to other range measuring devices such as LiDAR, acoustic or radar sensors is that it achieves high resolution and simultaneous acquisition of the surveyed area without energy emission or moving parts. Still, the major disadvantages are the correspondence problem, the processing time and the need of adequate illumination conditions and textured surfaces in the case of automatic matching procedures.
Optical distance measurement techniques are usually classified into three main categories: triangulation, interferometry and Time-of-Flight (ToF).
The triangulation normally determines an unknown point within a triangle by means of a known optical basis and the related side angles pointing to the unknown point. This often used principle is partitioned in a wealth of partly different 3D techniques, such as for instance active triangulation with structured illumination and passive triangulation (Jähne et al., 1999).
Interferometry measures depth also by means of the Time-of-Flight. In this case, however, the phase of the optical wave itself is used. This requires coherent mixing and correlation of the wave-front reflected from the object with a reference wave-front. Also in this case, many variants of the optical interferometry principle have been developed, such as multi-wavelength interferometry, holographic interferometry, speckle interferometry and white light interferometry (Jähne et al., 1999). The high accuracies of distance measurements performed with interferometry mainly depend on the coherence length of the light source: interferometry is not suitable for ranges greater than few centimeters since the method is based on the evaluation of very short optical wavelength.
Continuous wave and pulse ToF techniques measure the time of flight of the envelope of a modulated optical signal. These techniques usually apply incoherent optical signals. Typical examples of ToF are the optical rangefinder of total stations or classical LiDAR instruments. In this latter case, actual laser scanners allow to acquire hundreds of thousands of points per second, thanks to fast scanning mechanisms. Their measurement range can vary to a great extent for different instruments; in general it can vary between a tens of meters up to some kilometers, with an accuracy ranging from less than one millimeter to some tens of centimeters respectively. Nevertheless, the main drawbacks of LiDAR instruments are their high costs and dimensions.
In the last few years a new generation of active sensors has been developed, which allows to acquire 3D point clouds without any scanning mechanism and from just one point of view at video frame rates. The working principle is the measurement of the ToF of an emitted signal by the device towards the object to be observed, with the advantage of simultaneously measuring the distance information for each pixel of the camera sensor. Many terms have been used in the literature to indicate these devices, which can be called: Time-of-Flight (ToF) cameras, Range IMaging (RIM) cameras, 3D range imagers, range cameras or a combination of the mentioned terms. In the following the term ToF cameras will be prevalently employed, which is more related to the working principle of this recent technology.
Such a technology is possible because of the miniaturization of the semiconductor technology and the evolvement of the CCD/CMOS processes that can be implemented independently for each pixel. The result is the possibility to acquire distance measurements for each pixel at high speed and with accuracies up to about one centimeter. While ToF cameras based on the phase-shift measurement usually have a working range limited to ten/thirty meters, RIM cameras based on the direct ToF measurement can measure distances up to 1500 m. Moreover, ToF cameras are usually characterized by low resolution (no more than a few thousands of tens of pixels), small dimensions, costs that are an order of magnitude lower with respect to LiDAR instruments and a lower power consumption with respect to classical laser scanners. In contrast to stereo, the depth accuracy is practically independent of textural appearance, but limited to about one centimeter in the best case (actual phase-shift commercial ToF cameras).
The field of real-time ToF camera-based techniques is very active and covers many areas only briefly mentioned in this thesis, which is rather focused on ToF cameras in the Geomatics field.
1.1 Motivation
At present the ability to capture the surrounding area at high speed in three dimensions is one of the most challenging tasks in many fields, such as industrial automation and production, mobile mapping, monitoring, automotive safety, autonomous mobile robotics and gaming.
For both dynamic and static scene there is no low-price off-the-self system that provides full range, high-resolution distance information in real time such as in the case of ToF cameras.
Nevertheless, RIM cameras are usually characterized by some systematic measurement errors, which can strongly worsen the achievable distance measurement accuracy up to tens of centimeters in some cases. Therefore, suitable calibration procedures have to be developed.
One of the main topic of this thesis is to propose systematic procedures for the distance calibration of commercial ToF cameras, in order to estimate and increase their measurement accuracy. The calibration procedure presented in this work belongs to the direct calibration methods, since the distance measurement accuracy of RIM cameras is directly estimated and the resulting systematic errors are modeled. The main idea is to propose a procedure which does not require additional digital cameras or cost-effective high precision measurement racks or robot-arms to calibrate ToF cameras and which can be applied to any kind of RIM camera.
Suitable experimental tests are proposed in order to analyze the influence of several factors on the distance measurements, such as the camera warm-up during working time, the angle of incidence between the camera axis and the object surface, the presence of foreground objects close to the camera and the object reflectivity.
The second main topic of this work is the use of ToF cameras in the Geomatics field, with the final aim of 3D object reconstruction. Since ToF cameras acquire 3D point clouds at video frame rates, this potentiality can surely be exploited for this purpose. The main problem to be faced is the registration of the point clouds acquired from different view-points with ToF cameras. For this purpose, an algorithm for ToF point cloud registration has been developed, which is called multi-frame registration algorithm. Exploiting both the amplitude images and the 3D information delivered by ToF cameras, the proposed algorithm allows to automatically perform the point cloud registration with a final accuracy which is very close to the measurement accuracy of the employed device. Such a result is obtained thanks to frame averaging (so why the term “multi-frame”) and custom-built procedures, also applying an effective filter proposed in this work, which practically removes all “mixed pixels” from the acquired data. Mixed pixels are outliers resulting from the way ToF cameras process multiple returns of the emitted signal: they can strongly affect the accuracy of the acquired point cloud especially in the case of complex scenes. Another challenging topic proposed in this thesis is the integration between ToF data and a multi-image matching approach for automatic 3D object breakline extraction, which can be very useful for speed-up the drawing production of the surveyed objects. In both cases, some improvements are proposed and discussed for future developments.
1.2 Outline
This thesis is organized in six chapters. After this brief introduction, the following chapter faces with the working principle of ToF cameras, dividing them in two categories: devices based on the direct ToF principle and devices which use the indirect ToF principle (phase-shift measurement). Then, the state of the art of this technology is reported, with a complete overview about commercial ToF cameras. A description of the measurement parameters and the typical distance measurement errors of ToF cameras based on the phase-shift measurement (which are the most diffused) is reported, in order to show pros and cons of this new technology.
In Chapter 3, one of the main topics of this thesis is presented, which is the distance measurement calibration of ToF cameras. First, a minimum time (warm-up time) of camera working is established for both the SR-4000 and the PMDCamCube3.0 cameras in order to achieve distance measurement stability. Then, the relation between integration time and distance measurement precision in analyzed for both devices. A distance measurement accuracy evaluation system for the SR-4000 camera is reported, which can be applied for any ToF camera model, and a distance calibration model is proposed which increases the distance measurement accuracy in a wide interval of the whole working range of the camera. Then, a procedure to evaluate the influence on distance measurements of the orientation of the acquired object surface with respect to the camera axis is proposed. The problem of “internal scattering”, which can arise when foreground objects close to the camera are present, is also faced for the SR-4000 camera; finally, some tests on object reflectivity are reported in order to test the performance of ToF cameras on real objects surfaces.
In Chapter 4 a brief summary on the state of the art of applications using ToF cameras is reported, with the aim of showing how this new technology is spread...
E’ necessario quindi che la fase di acquisizione tenga conto di questo nuovo contesto. L’esperienza maturata e tutt’ora in corso presso il Gruppo di Ricerca del Politecnico di Torino consente di dimostrare come solo una forte integrazione tra le varie tecniche di acquisizione di nuvole di punti può consentire a posteriori di raggiungere i veri risultati del rilievo metrico che, erroneamente, molte volte vengono intesi come la semplice definizione di coordinate spaziali di una serie più o meno densa di punti.
Il lavoro proposto presenta i risultati delle ricerche condotte in questi anni riguardanti lo studio e il miglioramento delle tecniche di acquisizione (fotogrammetria digitale, LiDAR e camere 3D) nonché le soluzioni originali elaborate per giungere ad una vera integrazione tra i dati primari generati da queste tecniche.
negli ambienti interni e per la realizzazione di elementi architettonici, stimando, grazie ad un opportuno sistema, l’influenza che la loro differente riflettività ha sulle misure di distanza della camera. Il secondo aspetto riguarda un primo esempio di applicazione delle camere ToF al rilievo metrico di elementi architettonici. In particolare, si riporta un confronto tra il dato acquisito con la camera SR-4000 e quello ottenuto dal LiDAR triangolatore Mensi S10, che fornisce nuvole di punti
con precisione sub-millimetrica da utilizzare come dato di riferimento. Infine, per dare un’idea delle potenzialità delle camere a tempo di volo per applicazioni architettoniche, si riporta un esempio di modellazione tridimensionale relativo ad un particolare architettonico rilevato con la camera ToF.
limited. For this reason, different innovative systems have been developed and tested in order to perform
a photogrammetric survey in an inexpensive way, with high-resolution images. In this context, one of the
most promising acquisition techniques is represented by the use of Unmanned Aerial Vehicles (UAVs)
equipped with a digital camera.
The paper deals with the acquisition and processing of low-height aerial imagery acquired by UAVs
and Remote Piloted Vehicles (RPVs), in order to provide large-scale mapping to support archaeological
studies: the pros and cons of these acquisition platforms are presented and discussed. These systems
carry out flights that are usually very different from the manned systems as their dimensions and their
light weights never allow the set course to be flown; for this reason, the acquired images are often
affected by large rotations and small overlaps. Therefore, an ad hoc procedure has been implemented to
overcome these limits. In this work, two remote-controlled systems (a mini-helicopter and a mini fixedwing
plane) were tested over two different archaeological sites in order to provide Digital Surface Models
(DSMs) and large-scale maps (numeric maps and orthophotos). Finally, an accuracy evaluation of the
final products is reported.
a good low cost alternative to LiDAR instruments for the generation of precise and accurate point clouds: up to now the application range is still limited but in a near future they will be able to satisfy the most part of the Cultural Heritage metric survey requirements. On the other hand multi-image matching techniques with a correct and deep integration of the point cloud information
can give the correct solution for an “intelligent” survey of the geometric object break-lines, which are the correct starting point for a complete survey. These two research topics are strictly connected to a modern Cultural Heritage 3D survey approach. In this paper after a short analysis of the achieved results, an alternative possible scenario for the development of the metric survey approach
inside the wider topic of Cultural Heritage Documentation is reported.
made of the phenomenon evolution which can be used to define risk scenarios, especially when urban areas and infrastructures are
involved. Over the last decades, spaceborne and, more recently, ground-based interferometric synthetic aperture radar (GB-InSAR)
have been successfully used to identify and classify landslides. Ground-based synthetic aperture radar (GB-SAR) interferometry has
already been recognized as a powerful tool, that can be considered complementary or alternative to spaceborne SAR interferometry
for terrain monitoring and to detect structural changes in buildings.
Terrestrial laser scanning (TLS) is a technique increasingly used in remote sensing of unstable slopes. Detailed digital models of the
observed surface are generated with centimetric resolution. In particular, if multitemporal scans of the same area are performed, these
scans can be compared in order to obtain displacement fields.
Both GB-SAR and TLS observations were planned at the end of July 2007 and at the end of October 2007 in order to assess the
hazard of a quarry that is subject to ground instabilities (the Ex-Locatelli quarry near the Baveno village in the Italian Alps on Lake
Maggiore). This paper reports on these measurement campaigns and on a test site set up in Florence (Italy) and discusses the
advantages of the combined use of these innovative techniques. The availability of radar and laser data at the same time has allowed
a comparison to be made of the two techniques and has provided an effective validation test for both.
of an architectural object. Many papers have illustrated the possibility of building a complete 3D model using just these two
techniques. In practice, not many restorers are used to managing 3D models and traditional graphic results, such as plans and
sections, are usually required. The paper defines a correct balance between the use of traditional (manual and total station) and
innovative (LIDAR and digital photogrammetry) techniques in order to satisfy the usual requirements for the metric survey of an
architectural object. A project was carried out to obtain knowledge of a Church in Turin; it was necessary to prepare the survey
graphic drawings that would be used for the restoration both of the decorative motives and of the structure of the church. The most
important aspect of this work is the integration of traditional topographic techniques with the LIDAR technique. This integration
was necessary because of the complexity of the object that was to be surveyed and of the requested short times necessary to realize
the survey. The tools implemented in the new Sir-IO software (realized by a DITAG research group of the Politecnico di Torino)
were of great help in this work. Thanks to this software it was possible to directly plot the details that were to be surveyed on the
realized solid images and orthophotos, thus making the preparation of the survey graphic drawings considerably easier.
ToF cameras are cheap instruments (less than 10.000 €) based on video real time distance measurements and can represent an interesting alternative to the more expensive LiDAR instruments. In addition, the limited weight and dimensions of ToF cameras allow a reduction of some practical problems such as transportation and on-site management.
Most of the commercial ToF cameras use the phase-shift method to measure distances. Due to the use of only one wavelength, most of them have limited range of application (usually about 5 or 10 m).
After a brief description of the main characteristics of these instruments, this paper explains and comments the results of the first experimental applications of ToF cameras in Cultural Heritage 3D metric survey.
The possibility to acquire more than 30 frames/s and future developments of these devices in terms of use of more than one wavelength to overcome the ambiguity problem allow to foresee new interesting applications.
In 2010 an integrated survey of the Church has been performed using LiDAR techniques integrated with photogrammetric and topographic methodologies in order to realize a complete 2D documentation of the Basilica of San Pietro. Thanks to the acquired data a complete multi-scale 3D model of the Church and of the surroundings was realized.
The aim of this work is to present different strategies in order to realize correct documentations for Cultural Heritage knowledge, using typical 3D survey methodologies (i. e. LiDAR survey and photogrammetry).
After data acquisition and processing, several 2D representations were realized in order to carry out traditional supports for the different actors involved in the conservation plans; moreover, starting from the 2D drawing a simplified 3D modeling methodology has been followed in order to define the fundamental geometry of the Basilica and the surroundings: the achieved model could be useful for a small architectural scale description of the structure and for the documentation of the surroundings.
For the aforementioned small architectural scale model, the 3D modeling was realized using the information derived from the 2D drawings with an approach based on the Constructive Solid Geometry. Using this approach the real shape of the object is simplified. This methodology is employed in particular when the shape of the structures is simple or to communicate new project ideas of when, as in our case, the aim is to give an idea of the complexity of an architectural Cultural Heritage.
In order to follow this objective, a small architectural scale model was realized: the area of the Civita hill was modeled using the information derived from the 1:5000 scale map contours; afterwards the Basilica was modeled in a CAD software using the information derived from the 2D drawings of the Basilica.
Finally, a more detailed 3D model was realized to describe the real shape of the transept.
All this products were realized thanks to the data acquired during the performed survey. This research underlines that a complete 3D documentation of a Cultural Heritage during the survey phase allows the final user to derive all the products that could be necessary for a correct knowledge of the artifact.
The 3D information of an object to be surveyed can be basically acquired in two ways: using stereo image acquisitions or optical distance measurement techniques.
The stereo image acquisition is already known and used for decades in the research community. The advantage of stereo vision to other range measuring devices such as LiDAR, acoustic or radar sensors is that it achieves high resolution and simultaneous acquisition of the surveyed area without energy emission or moving parts. Still, the major disadvantages are the correspondence problem, the processing time and the need of adequate illumination conditions and textured surfaces in the case of automatic matching procedures.
Optical distance measurement techniques are usually classified into three main categories: triangulation, interferometry and Time-of-Flight (ToF).
The triangulation normally determines an unknown point within a triangle by means of a known optical basis and the related side angles pointing to the unknown point. This often used principle is partitioned in a wealth of partly different 3D techniques, such as for instance active triangulation with structured illumination and passive triangulation (Jähne et al., 1999).
Interferometry measures depth also by means of the Time-of-Flight. In this case, however, the phase of the optical wave itself is used. This requires coherent mixing and correlation of the wave-front reflected from the object with a reference wave-front. Also in this case, many variants of the optical interferometry principle have been developed, such as multi-wavelength interferometry, holographic interferometry, speckle interferometry and white light interferometry (Jähne et al., 1999). The high accuracies of distance measurements performed with interferometry mainly depend on the coherence length of the light source: interferometry is not suitable for ranges greater than few centimeters since the method is based on the evaluation of very short optical wavelength.
Continuous wave and pulse ToF techniques measure the time of flight of the envelope of a modulated optical signal. These techniques usually apply incoherent optical signals. Typical examples of ToF are the optical rangefinder of total stations or classical LiDAR instruments. In this latter case, actual laser scanners allow to acquire hundreds of thousands of points per second, thanks to fast scanning mechanisms. Their measurement range can vary to a great extent for different instruments; in general it can vary between a tens of meters up to some kilometers, with an accuracy ranging from less than one millimeter to some tens of centimeters respectively. Nevertheless, the main drawbacks of LiDAR instruments are their high costs and dimensions.
In the last few years a new generation of active sensors has been developed, which allows to acquire 3D point clouds without any scanning mechanism and from just one point of view at video frame rates. The working principle is the measurement of the ToF of an emitted signal by the device towards the object to be observed, with the advantage of simultaneously measuring the distance information for each pixel of the camera sensor. Many terms have been used in the literature to indicate these devices, which can be called: Time-of-Flight (ToF) cameras, Range IMaging (RIM) cameras, 3D range imagers, range cameras or a combination of the mentioned terms. In the following the term ToF cameras will be prevalently employed, which is more related to the working principle of this recent technology.
Such a technology is possible because of the miniaturization of the semiconductor technology and the evolvement of the CCD/CMOS processes that can be implemented independently for each pixel. The result is the possibility to acquire distance measurements for each pixel at high speed and with accuracies up to about one centimeter. While ToF cameras based on the phase-shift measurement usually have a working range limited to ten/thirty meters, RIM cameras based on the direct ToF measurement can measure distances up to 1500 m. Moreover, ToF cameras are usually characterized by low resolution (no more than a few thousands of tens of pixels), small dimensions, costs that are an order of magnitude lower with respect to LiDAR instruments and a lower power consumption with respect to classical laser scanners. In contrast to stereo, the depth accuracy is practically independent of textural appearance, but limited to about one centimeter in the best case (actual phase-shift commercial ToF cameras).
The field of real-time ToF camera-based techniques is very active and covers many areas only briefly mentioned in this thesis, which is rather focused on ToF cameras in the Geomatics field.
1.1 Motivation
At present the ability to capture the surrounding area at high speed in three dimensions is one of the most challenging tasks in many fields, such as industrial automation and production, mobile mapping, monitoring, automotive safety, autonomous mobile robotics and gaming.
For both dynamic and static scene there is no low-price off-the-self system that provides full range, high-resolution distance information in real time such as in the case of ToF cameras.
Nevertheless, RIM cameras are usually characterized by some systematic measurement errors, which can strongly worsen the achievable distance measurement accuracy up to tens of centimeters in some cases. Therefore, suitable calibration procedures have to be developed.
One of the main topic of this thesis is to propose systematic procedures for the distance calibration of commercial ToF cameras, in order to estimate and increase their measurement accuracy. The calibration procedure presented in this work belongs to the direct calibration methods, since the distance measurement accuracy of RIM cameras is directly estimated and the resulting systematic errors are modeled. The main idea is to propose a procedure which does not require additional digital cameras or cost-effective high precision measurement racks or robot-arms to calibrate ToF cameras and which can be applied to any kind of RIM camera.
Suitable experimental tests are proposed in order to analyze the influence of several factors on the distance measurements, such as the camera warm-up during working time, the angle of incidence between the camera axis and the object surface, the presence of foreground objects close to the camera and the object reflectivity.
The second main topic of this work is the use of ToF cameras in the Geomatics field, with the final aim of 3D object reconstruction. Since ToF cameras acquire 3D point clouds at video frame rates, this potentiality can surely be exploited for this purpose. The main problem to be faced is the registration of the point clouds acquired from different view-points with ToF cameras. For this purpose, an algorithm for ToF point cloud registration has been developed, which is called multi-frame registration algorithm. Exploiting both the amplitude images and the 3D information delivered by ToF cameras, the proposed algorithm allows to automatically perform the point cloud registration with a final accuracy which is very close to the measurement accuracy of the employed device. Such a result is obtained thanks to frame averaging (so why the term “multi-frame”) and custom-built procedures, also applying an effective filter proposed in this work, which practically removes all “mixed pixels” from the acquired data. Mixed pixels are outliers resulting from the way ToF cameras process multiple returns of the emitted signal: they can strongly affect the accuracy of the acquired point cloud especially in the case of complex scenes. Another challenging topic proposed in this thesis is the integration between ToF data and a multi-image matching approach for automatic 3D object breakline extraction, which can be very useful for speed-up the drawing production of the surveyed objects. In both cases, some improvements are proposed and discussed for future developments.
1.2 Outline
This thesis is organized in six chapters. After this brief introduction, the following chapter faces with the working principle of ToF cameras, dividing them in two categories: devices based on the direct ToF principle and devices which use the indirect ToF principle (phase-shift measurement). Then, the state of the art of this technology is reported, with a complete overview about commercial ToF cameras. A description of the measurement parameters and the typical distance measurement errors of ToF cameras based on the phase-shift measurement (which are the most diffused) is reported, in order to show pros and cons of this new technology.
In Chapter 3, one of the main topics of this thesis is presented, which is the distance measurement calibration of ToF cameras. First, a minimum time (warm-up time) of camera working is established for both the SR-4000 and the PMDCamCube3.0 cameras in order to achieve distance measurement stability. Then, the relation between integration time and distance measurement precision in analyzed for both devices. A distance measurement accuracy evaluation system for the SR-4000 camera is reported, which can be applied for any ToF camera model, and a distance calibration model is proposed which increases the distance measurement accuracy in a wide interval of the whole working range of the camera. Then, a procedure to evaluate the influence on distance measurements of the orientation of the acquired object surface with respect to the camera axis is proposed. The problem of “internal scattering”, which can arise when foreground objects close to the camera are present, is also faced for the SR-4000 camera; finally, some tests on object reflectivity are reported in order to test the performance of ToF cameras on real objects surfaces.
In Chapter 4 a brief summary on the state of the art of applications using ToF cameras is reported, with the aim of showing how this new technology is spread...
E’ necessario quindi che la fase di acquisizione tenga conto di questo nuovo contesto. L’esperienza maturata e tutt’ora in corso presso il Gruppo di Ricerca del Politecnico di Torino consente di dimostrare come solo una forte integrazione tra le varie tecniche di acquisizione di nuvole di punti può consentire a posteriori di raggiungere i veri risultati del rilievo metrico che, erroneamente, molte volte vengono intesi come la semplice definizione di coordinate spaziali di una serie più o meno densa di punti.
Il lavoro proposto presenta i risultati delle ricerche condotte in questi anni riguardanti lo studio e il miglioramento delle tecniche di acquisizione (fotogrammetria digitale, LiDAR e camere 3D) nonché le soluzioni originali elaborate per giungere ad una vera integrazione tra i dati primari generati da queste tecniche.
negli ambienti interni e per la realizzazione di elementi architettonici, stimando, grazie ad un opportuno sistema, l’influenza che la loro differente riflettività ha sulle misure di distanza della camera. Il secondo aspetto riguarda un primo esempio di applicazione delle camere ToF al rilievo metrico di elementi architettonici. In particolare, si riporta un confronto tra il dato acquisito con la camera SR-4000 e quello ottenuto dal LiDAR triangolatore Mensi S10, che fornisce nuvole di punti
con precisione sub-millimetrica da utilizzare come dato di riferimento. Infine, per dare un’idea delle potenzialità delle camere a tempo di volo per applicazioni architettoniche, si riporta un esempio di modellazione tridimensionale relativo ad un particolare architettonico rilevato con la camera ToF.
limited. For this reason, different innovative systems have been developed and tested in order to perform
a photogrammetric survey in an inexpensive way, with high-resolution images. In this context, one of the
most promising acquisition techniques is represented by the use of Unmanned Aerial Vehicles (UAVs)
equipped with a digital camera.
The paper deals with the acquisition and processing of low-height aerial imagery acquired by UAVs
and Remote Piloted Vehicles (RPVs), in order to provide large-scale mapping to support archaeological
studies: the pros and cons of these acquisition platforms are presented and discussed. These systems
carry out flights that are usually very different from the manned systems as their dimensions and their
light weights never allow the set course to be flown; for this reason, the acquired images are often
affected by large rotations and small overlaps. Therefore, an ad hoc procedure has been implemented to
overcome these limits. In this work, two remote-controlled systems (a mini-helicopter and a mini fixedwing
plane) were tested over two different archaeological sites in order to provide Digital Surface Models
(DSMs) and large-scale maps (numeric maps and orthophotos). Finally, an accuracy evaluation of the
final products is reported.
a good low cost alternative to LiDAR instruments for the generation of precise and accurate point clouds: up to now the application range is still limited but in a near future they will be able to satisfy the most part of the Cultural Heritage metric survey requirements. On the other hand multi-image matching techniques with a correct and deep integration of the point cloud information
can give the correct solution for an “intelligent” survey of the geometric object break-lines, which are the correct starting point for a complete survey. These two research topics are strictly connected to a modern Cultural Heritage 3D survey approach. In this paper after a short analysis of the achieved results, an alternative possible scenario for the development of the metric survey approach
inside the wider topic of Cultural Heritage Documentation is reported.
made of the phenomenon evolution which can be used to define risk scenarios, especially when urban areas and infrastructures are
involved. Over the last decades, spaceborne and, more recently, ground-based interferometric synthetic aperture radar (GB-InSAR)
have been successfully used to identify and classify landslides. Ground-based synthetic aperture radar (GB-SAR) interferometry has
already been recognized as a powerful tool, that can be considered complementary or alternative to spaceborne SAR interferometry
for terrain monitoring and to detect structural changes in buildings.
Terrestrial laser scanning (TLS) is a technique increasingly used in remote sensing of unstable slopes. Detailed digital models of the
observed surface are generated with centimetric resolution. In particular, if multitemporal scans of the same area are performed, these
scans can be compared in order to obtain displacement fields.
Both GB-SAR and TLS observations were planned at the end of July 2007 and at the end of October 2007 in order to assess the
hazard of a quarry that is subject to ground instabilities (the Ex-Locatelli quarry near the Baveno village in the Italian Alps on Lake
Maggiore). This paper reports on these measurement campaigns and on a test site set up in Florence (Italy) and discusses the
advantages of the combined use of these innovative techniques. The availability of radar and laser data at the same time has allowed
a comparison to be made of the two techniques and has provided an effective validation test for both.
of an architectural object. Many papers have illustrated the possibility of building a complete 3D model using just these two
techniques. In practice, not many restorers are used to managing 3D models and traditional graphic results, such as plans and
sections, are usually required. The paper defines a correct balance between the use of traditional (manual and total station) and
innovative (LIDAR and digital photogrammetry) techniques in order to satisfy the usual requirements for the metric survey of an
architectural object. A project was carried out to obtain knowledge of a Church in Turin; it was necessary to prepare the survey
graphic drawings that would be used for the restoration both of the decorative motives and of the structure of the church. The most
important aspect of this work is the integration of traditional topographic techniques with the LIDAR technique. This integration
was necessary because of the complexity of the object that was to be surveyed and of the requested short times necessary to realize
the survey. The tools implemented in the new Sir-IO software (realized by a DITAG research group of the Politecnico di Torino)
were of great help in this work. Thanks to this software it was possible to directly plot the details that were to be surveyed on the
realized solid images and orthophotos, thus making the preparation of the survey graphic drawings considerably easier.