J.-A. Beraldin

Detailed 3D reconstruction of large-scale heritage sites with integrated techniques

Many cultural heritage applications require 3D reconstruction of real-world objects and scenes. Over the past few years, it has become increasingly common to use 3D digitization and modeling for this purpose. This is mainly due to advances in laser-scanning techniques, 3D modeling software, image-based modeling techniques, computer power, and virtual reality. Our approach integrates several technologies based on our experience over more than a decade of trying to accurately and completely model large-scale heritage monuments and sites. Using both interactive and automatic techniques, we can model a highly detailed structure or site at various levels of detail. We use image-based modeling for basic shape and structural elements, and laser scanning for fine details and sculpted surfaces. To present the site in its proper context, we use image-based rendering for landscapes and surroundings. To apply this approach, we created hundreds of models from sites all over the world for documentation, walk-through movies, and interactive visualization. The results were compelling and encouraging.

Object model creation from multiple range images: acquisition, calibration, model building and verification

This paper demonstrates the accuracy of a prototype Laser Range Camera (LRC) developed at the National Research Council of Canada for the creation of models of real objects. A laser survey performed in collaboration with the Canadian Space Agency and NASA is used as a test case. The object selected for this particular test case is the Orbiter Docking System (ODS) located at the Kennedy Space Center, Florida. During the laser survey, 128 range (and registered intensity) images were acquired all around the ODS. These images were then processed in our laboratory. A full model of the top portion of the ODS was created along with an almost complete model of the ODS. The ODS has a diameter of 1.6 m and a height of 3.9 m. Targets mounted on the top portion of the ODS were used to assess the accuracy of the calibration and of the image registration process. These targets were measured with a network of theodolites a day prior to the laser survey and used as a reference. With the current calibration and range image registration techniques, an accuracy better than 0.25 mm in X and Y, and, 0.80 mm in Z was achieved. These results compare favorably with the single point accuracy obtained after calibration, i.e., about 0.25 mm in X and Y, and, 0.50 mm in Z. These figures and others should testify on the usefulness of a LRC for accurate model building.

Active optical 3D imaging for heritage applications

High-resolution 3D imaging and modeling is an important application in the heritage field. We describe several demonstration projects conducted in collaboration with museums and conservation agencies.

Comparative evaluation of the performance of passive and active 3D vision systems

Automated digital photogrammetric systems are considered to be passive three-dimensional vision systems since they obtain object coordinates from only the information contained in intensity images. Active 3-D vision systems, such as laser scanners and structured light systems obtain the object coordinates from external information such as scanning angle, time of flight, or shape of projected patterns. Passive systems provide high accuracy on well defined features, such as targets and edges however, unmarked surfaces are hard to measure. These systems may also be difficult to automate in unstructured environments since they are highly affected by the ambient light. Active systems provide their own illumination and the features to be measured so they can easily measure surfaces in most environments. However, they have difficulties with varying surface finish or sharp discontinuities such as edges. Therefore each type of sensor is more suited for a specific type of objects and features, and they are often complementary. This paper compares the measurement accuracy, on various type of features, of some technologically-different 3-D vision systems: photogrammetry-based (passive) systems, a laser scanning system (active), and a range sensor using a mask with two apertures and structured light (active).

Effective 3D modeling of heritage sites

Over the past few years, a remarkable increase has occurred in the demand for 3D models for cultural heritage applications. The techniques employed have evolved from surveying and CAD tools and/or traditional photogrammetry into laser scanning and more automated image-based techniques. However, selecting the most effective technique for a given project is not always obvious. We will discuss each technique and point out its advantages and disadvantages. We will then present our approach, which is an integration of several technologies and is based on the experience we gained over more than a decade for accurately and completely model heritage monuments and sites. It was clear from our experience that using a single technique is not an effective approach. A highly detailed structure or site is best modelled at various levels of detail. Image-based modelling is used for the basic shape and structural elements, and high-precision laser scanning for fine details and sculpted surfaces. To present the site in its proper context, image-based rendering or panorama is used for landscapes and surroundings. We demonstrate our approach on two typical heritage sites in Italy: the Abbey of Pomposa near Ferrara and the Scrovegni Chapel in Padova.

System for indoor 3D mapping and virtual environments

The key to navigate in a 3D environment or designing autonomous vehicles that can successfully maneuver and manipulate objects in their environment is the ability to create, maintain, and use effectively a 3D digital model that accurately represents its physical counterpart. Virtual exploring of real places and environments, either for leisure, engineering design, training and simulation, or tasks in remote or hazardous environments, is more effective and useful if geometrical relationships and dimensions in the virtual model are accurate. A system which can rapidly, reliably, remotely and accurately perform measurements in the 3D space for the mapping of indoor environments is needed for many applications. In this paper we present a mobile mapping system that is designed to generate a geometrically precise 3D model of an unknown indoor environment. The same general design concept can be used for environments ranging from simple office hallways to long winding underground mine tunnels. Surfaces and features can be accurately mapped from images acquired by a unique configuration of different types of optical imaging sensor and dead reckoning positioning device. This configuration guarantees that all the information required to create the 3D model of the environment is included in the collected data. Sufficient overlaps between 2D intensity images, in combination with information from 3D range images, insure that the complete environment can be accurately reconstructed when all the data is simultaneously processed. The system, the data collection and processing procedure, test results, the modeling and display at our virtual environment facility are described.

Optimized position sensors for flying-spot active triangulation systems

A description of the integrated sensors developed for flying-spot active triangulation is given. All the sensors have been fabricated using standard CMOS technology that allows the monolithic integration of photo-sensors, together with readout circuits, and digital signal processors. Position sensors are classified into two classes that allow a better understanding of the pros and cons of each one. A description of the proposed position sensor that is optimized for accurate and fast 3D acquisition is given alongside some experimental results.

Eye-safe digital 3-D sensing for space applications

This paper focuses on the characteristics and performance of an eye-safe laser range scanner (LARS) with short- and medium-range 3-D sensing capabilities for space applications. This versatile LARS is a precision measurement tool that will complement the current Canadian Space Vision System. The major advantages of the LARS over conventional video-based imaging are its ability to operate with sunlight shining directly into the scanner and its immunity to spurious reflections and shadows, which occur frequently in space. Because the LARS is equipped with two high-speed galvanometers to steer the laser beam, any spatial location within the field of view of the camera can be addressed. This versatility enables the LARS to operate in two basic scan pattern modes: (1) variable-scan-resolution mode and (2) raster-scan mode. In the variable-resolution mode, the LARS can search and track targets and geometrical features on objects located within a field of view of 30 by 30 deg and with corresponding range from about 0.5 to 2000 m. The tracking mode can reach a refresh rate of up to 130 Hz. The raster mode is used primarily for the measurement of registered range and intensity information on large stationary objects. It allows, among other things, target-based measurements, feature-based measurements, and surface-reflectance monitoring. The digitizing and modeling of human subjects, cargo payloads, and environments are also possible with the LARS. Examples illustrating its capabilities are presented.

Digital 3D imaging system for rapid response on remote sites

A compact digital 3D imaging system based on laser triangulation was designed for applications requiring a rapid response on remote sites. Heritage, forensic and industrial applications are among the best fields to benefit from this new technology. This paper focuses on such aspects as the acquisition, calibration, verification, and model creation. These aspects were all optimized to create a versatile system that is compact, i.e. hand portable to a remote site. Emphasis is placed on accuracy verification and monitoring which are critical factors for obtaining high-quality reconstruction of 3D models from multiple range images. A summary of the experimental results acquired in 1997 and 1998 at a number of sites in Italy is presented in this paper.

Sensor based creation of indoor virtual environment models

A system that can rapidly, reliably, remotely and accurately create a photo-realistic 3D model of indoor environments is needed for many applications. In this paper, we present a mobile mapping system that is designed to generate such models automatically from various sensors. Surfaces and features can be accurately mapped from images acquired by a unique configuration of different types of optical imaging sensor and dead-reckoning positioning device. This configuration guarantees that all the information required to create the 3D model of the environment is included in the collected data. The same general design concept can be used for environments ranging from simple office hallways to long, winding underground mine tunnels. This paper describes the hardware systems and processing procedures. We also present test results and describe our virtual environment facility to display such models.