Multispectral Light Detection and Ranging Technology and Applications: A Review.

Light Detection and Ranging (LiDAR) is a well-established active technology for the direct acquisition of 3D data. In recent years, the geometric information collected by LiDAR sensors has been widely combined with optical images to provide supplementary spectral information to achieve more precise results in diverse remote sensing applications. The emergence of active Multispectral LiDAR (MSL) systems, which operate on different wavelengths, has recently been revolutionizing the simultaneous acquisition of height and intensity information. So far, MSL technology has been successfully applied for fine-scale mapping in various domains. However, a comprehensive review of this modern technology is currently lacking. Hence, this study presents an exhaustive overview of the current state-of-the-art in MSL systems by reviewing the latest technologies for MSL data acquisition. Moreover, the paper reports an in-depth analysis of the diverse applications of MSL, spanning across fields of "ecology and forestry", "objects and Land Use Land Cover (LULC) classification", "change detection", "bathymetry", "topographic mapping", "archaeology and geology", and "navigation". Our systematic review uncovers the potentials, opportunities, and challenges of the recently emerged MSL systems, which integrate spatial-spectral data and unlock the capability for precise multi-dimensional (nD) mapping using only a single-data source.

Close-range photogrammetry reveals morphometric changes on replicative ground stones.

The pursuit of a quantitative approach to functional analysis of stone tools is an ongoing endeavour for traceologists. Technological advancements in 3D imaging techniques, such as photogrammetry/3D scanners, CT scanning, 3D digital microscopy, confocal microscopy, AFM and FEG-SEM and micro-topographical scanning, have greatly facilitated the detailed capturing of the geometry and surface texture at multiple levels of observation, from the object-scale to the nano-scale. However, while such technological innovations have predominantly focused on flaked assemblages, ground stone tools have only recently begun to receive due attention, and a standardised protocol for their study is yet to be established. In order to comprehend the function(s) of these tools, analytical techniques that enable a 3D visualisation of the entire item and the wear affecting the used surfaces have proven to be of great support. To this end, an analytical procedure was developed and tested on slabs and pebbles in order to replicate the use-wear traces observed on Upper Palaeolithic tools. The purpose was to assemble a site-specific reference collection tailored on the artefacts from the cultural level III of the Brînzeni I cave in north-west Moldova. Experimental replicas were used to treat different plant organs during controlled sequential experiments. The present article reports on the analysis based on photogrammetric data acquired during two stages of replicative usage. We tested multiple acquisition setups and elaborations to assess the geometry modification and the surface depletion. By exploring various acquisition strategies, a critical evaluation of potential sources of bias in data collection and subsequent elaboration were performed, and the methodology was accordingly adjusted thereby enhancing the reliability and reproducibility of the results. This study highlights the importance of carefully considering the acquisition strategy in archaeological related research to ensure accurate analyses and to validate robust interpretation.

Combining Photogrammetry and Photometric Stereo to Achieve Precise and Complete 3D Reconstruction.

Image-based 3D reconstruction has been employed in industrial metrology for micro-measurements and quality control purposes. However, generating a highly-detailed and reliable 3D reconstruction of non-collaborative surfaces is still an open issue. In this paper, a method for generating an accurate 3D reconstruction of non-collaborative surfaces through a combination of photogrammetry and photometric stereo is presented. On one side, the geometric information derived with photogrammetry is used in areas where its 3D measurements are reliable. On the other hand, the high spatial resolution capability of photometric stereo is exploited to acquire a finely detailed topography of the surface. Finally, three different approaches are proposed to fuse both geometric information and high frequency details. The proposed method is tested on six different non-collaborative objects with different surface characteristics. To evaluate the accuracy of the proposed method, a comprehensive cloud-to-cloud comparison between reference data and 3D points derived from the proposed fusion methods is provided. The experiments demonstrated that, despite correcting global deformation up to an average RMSE of less than 0.1 mm, the proposed method recovers the surface topography at the same high resolution as the photometric stereo.

Colorizing the Past: Deep Learning for the Automatic Colorization of Historical Aerial Images.

The colorization of grayscale images can, nowadays, take advantage of recent progress and the automation of deep-learning techniques. From the media industry to medical or geospatial applications, image colorization is an attractive and investigated image processing practice, and it is also helpful for revitalizing historical photographs. After exploring some of the existing fully automatic learning methods, the article presents a new neural network architecture, Hyper-U-NET, which combines a U-NET-like architecture and HyperConnections to handle the colorization of historical black and white aerial images. The training dataset (about 10,000 colored aerial image patches) and the realized neural network are available on our GitHub page to boost further research investigations in this field.

Surface Reconstruction Assessment in Photogrammetric Applications.

The image-based 3D reconstruction pipeline aims to generate complete digital representations of the recorded scene, often in the form of 3D surfaces. These surfaces or mesh models are required to be highly detailed as well as accurate enough, especially for metric applications. Surface generation can be considered as a problem integrated in the complete 3D reconstruction workflow and thus visibility information (pixel similarity and image orientation) is leveraged in the meshing procedure contributing to an optimal photo-consistent mesh. Other methods tackle the problem as an independent and subsequent step, generating a mesh model starting from a dense 3D point cloud or even using depth maps, discarding input image information. Out of the vast number of approaches for 3D surface generation, in this study, we considered three state of the art methods. Experiments were performed on benchmark and proprietary datasets of varying nature, scale, shape, image resolution and network designs. Several evaluation metrics were introduced and considered to present qualitative and quantitative assessment of the results.

Cardiovascular interventions planning through a three-dimensional printing patient-specific approach.

: In recent years, three-dimensional modelling and printing techniques have improved diagnosis and preprocedural planning during percutaneous interventions or surgery in cardiovascular disease. The raw data for the whole process are obtained through medical imaging, where regions of interest, that is heart chambers, valves, aorta, coronary vessels etc., are segmented and converted into three-dimensional digital models, which are then reproduced in physical replica by a three-dimensional printer. In the current article, a freeware patient-specific three-dimensional modelling and printing step-by-step procedure for preprocedural planning for complex heart diseases is presented and applied on four patients. Finally, a general discussion on the potential and future developments of personalized three-dimensional modelling and rapid prototyping for preprocedural planning is also presented.

Photogrammetry of the Human Brain: A Novel Method for Three-Dimensional Quantitative Exploration of the Structural Connectivity in Neurosurgery and Neurosciences.

BACKGROUND: Anatomic awareness of the structural connectivity of the brain is mandatory for neurosurgeons, to select the most effective approaches for brain resections. Although standard microdissection is a validated technique to investigate the different white matter (WM) pathways and to verify the results of tractography, the possibility of interactive exploration of the specimens and reliable acquisition of quantitative information has not been described. Photogrammetry is a well-established technique allowing an accurate metrology on highly defined three-dimensional (3D) models. The aim of this work is to propose the application of the photogrammetric technique for supporting the 3D exploration and the quantitative analysis on the cerebral WM connectivity. METHODS: The main perisylvian pathways, including the superior longitudinal fascicle and the arcuate fascicle were exposed using the Klingler technique. The photogrammetric acquisition followed each dissection step. The point clouds were registered to a reference magnetic resonance image of the specimen. All the acquisitions were coregistered into an open-source model. RESULTS: We analyzed 5 steps, including the cortical surface, the short intergyral fibers, the indirect posterior and anterior superior longitudinal fascicle, and the arcuate fascicle. The coregistration between the magnetic resonance imaging mesh and the point clouds models was highly accurate. Multiple measures of distances between specific cortical landmarks and WM tracts were collected on the photogrammetric model. CONCLUSIONS: Photogrammetry allows an accurate 3D reproduction of WM anatomy and the acquisition of unlimited quantitative data directly on the real specimen during the postdissection analysis. These results open many new promising neuroscientific and educational perspectives and also optimize the quality of neurosurgical treatments.

Securing Color Fidelity in 3D Architectural Heritage Scenarios.

Ensuring color fidelity in image-based 3D modeling of heritage scenarios is nowadays still an open research matter. Image colors are important during the data processing as they affect algorithm outcomes, therefore their correct treatment, reduction and enhancement is fundamental. In this contribution, we present an automated solution developed to improve the radiometric quality of an image datasets and the performances of two main steps of the photogrammetric pipeline (camera orientation and dense image matching). The suggested solution aims to achieve a robust automatic color balance and exposure equalization, stability of the RGB-to-gray image conversion and faithful color appearance of a digitized artifact. The innovative aspects of the article are: complete automation, better color target detection, a MATLAB implementation of the ACR scripts created by Fraser and the use of a specific weighted polynomial regression. A series of tests are presented to demonstrate the efficiency of the developed methodology and to evaluate color accuracy ('color characterization').

Geometric and Optic Characterization of a Hemispherical Dome Port for Underwater Photogrammetry.

The popularity of automatic photogrammetric techniques has promoted many experiments in underwater scenarios leading to quite impressive visual results, even by non-experts. Despite these achievements, a deep understanding of camera and lens behaviors as well as optical phenomena involved in underwater operations is fundamental to better plan field campaigns and anticipate the achievable results. The paper presents a geometric investigation of a consumer grade underwater camera housing, manufactured by NiMAR and equipped with a 7'' dome port. After a review of flat and dome ports, the work analyzes, using simulations and real experiments, the main optical phenomena involved when operating a camera underwater. Specific aspects which deal with photogrammetric acquisitions are considered with some tests in laboratory and in a swimming pool. Results and considerations are shown and commented.