3D Laser scanning (Architecture) Research Papers (original) (raw)

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...