Claudio Rocchini

Metro: measuring error on simplified surfaces

This paper presents a new tool, Metro, designed to compensate for a deficiency in many simplification methods proposed in literature. Metro allows one to compare the difference between a pair of surfaces (e.g. a triangulated mesh and its simplified representation) by adopting a surface sampling approach. It has been designed as a highly general tool, and it does no assumption on the particular approach used to build the simplified representation. It returns both numerical results (meshes areas and volumes, maximum and mean error, etc.) and visual results, by coloring the input surface according to the approximation error.

A low cost 3D scanner based on structured light

Automatic 3D acquisition devices (often called 3D scanners) allow to build highly accurate models of real 3D objects in a cost‐ and time‐effective manner. We have experimented this technology in a particular application context: the acquisition of Cultural Heritage artefacts. Specific needs of this domain are: medium‐high accuracy, easy of use, affordable cost of the scanning device, self‐registered acquisition of shape and color data, and finally operational safety for both the operator and the scanned artefacts. According to these requirements, we designed a low‐cost 3D scanner based on structured light which adopts a new, versatile colored stripe pattern approach. We present the scanner architecture, the software technologies adopted, and the first results of its use in a project regarding the 3D acquisition of an archeological statue.

External memory management and simplification of huge meshes

Very large triangle meshes, i.e., meshes composed of millions of faces, are becoming common in many applications. Obviously, processing, rendering, transmission, and archiving of these meshes are not simple tasks. Mesh simplification and LOD management are a rather mature technology that, in many cases, can efficiently manage complex data. But, only a few available systems can manage meshes characterized by a huge size: RAM size is often a severe bottleneck. In this paper, we present a data structure called Octree-based External Memory Mesh (OEMM). It supports external memory management of complex meshes, loading dynamically in main memory only the selected sections and preserving data consistency during local updates. The functionalities implemented on this data structure (simplification, detail preservation, mesh editing, visualization, and inspection) can be applied to huge triangles meshes on low-cost PC platforms. The time overhead due to the external memory management is affordable. Results of the test of our system on complex meshes are presented.

Multiple textures stitching and blending on 3D objects

In this paper we propose a new approach for mapping and blending textures on 3D geometries. The system starts from a 3D mesh which represents a real object and improves this model with pictorial detail. Texture detail is acquired via a common photographic process directly from the real object. These images are then registered and stitched on the 3D mesh, by integrating them into a single standard texture map. An optimal correspondence between regions of the 3D mesh and sections of the acquired images is built. Then, a new approach is proposed to produce a smooth join between different images that map on adjacent sections of the surface, based on texture blending. For each mesh face which is on the adjacency border between different observed images, a corresponding triangular texture patch is resampled as a weighted blend of the corresponding adjacent images sections. The accuracy of the resampling and blending process is improved by computing an accurate piecewise local registration of the original images with respect to the current face vertices. Examples of the results obtained with sample Cultural Heritage objects are presented and discussed.

A general method for preserving attribute values on simplified meshes

Many sophisticated solutions have been proposed to reduce the geometric complexity of 3D meshes. A problem studied less often is how to preserve on a simplified mesh the detail (e.g., color, high frequency shape detail, scalar fields, etc.) which is encoded in the original mesh. We present a general approach for preserving detail on simplified meshes. The detail (or high frequency information) lost after simplification is encoded through texture or bump maps. The original contribution is that preservation is performed after simplification, by building set of triangular texture patches that are then packed in a single texture map. Each simplified mesh face is sampled to build the associated triangular texture patch; a new method for storing this set of texture patches into a standard rectangular texture is presented and discussed. Our detail preserving approach makes no assumptions about the simplification process adopted to reduce mesh complexity and allows highly efficient rendering. The solution is very general, allowing preservation of any attribute value defined on the high resolution mesh. We also describe an alternative application: the conversion of 3D models with 3D static procedural textures into standard 3D models with 2D textures.

Simplification of Tetrahedral meshes with accurate error evaluation

The techniques for reducing the size of a volume dataset by preserving both the geometrical/topological shape and the information encoded in an attached scalar field are attracting growing interest. Given the framework of incremental 3D mesh simplification based on edge collapse, we propose an approach for the integrated evaluation of the error introduced by both the modification of the domain and the approximation of the field of the original volume dataset. We present and compare various techniques to evaluate the approximation error or to produce a sound prediction. A flexible simplification tool has been implemented, which provides a different degree of accuracy and computational efficiency for the selection of the edge to be collapsed. Techniques for preventing a geometric or topological degeneration of the mesh are also presented.

Acquiring, stitching and blending diffuse appearance attributes on 3D models

A new system for the construction of highly realistic models of real free-form 3D objects is proposed, based on the integration of several techniques (automatic 3D scanning, inverse illumination, inverse texture-mapping and textured 3D graphics). Our system improves the quality of a 3D model (e.g. acquired with a range scanning device) by adding color detail and, if required, high-frequency shape detail. Detail is obtained by processing a set of digital photographs of the object. This is carried out by performing several subtasks: to compute camera calibration and position, to remove illumination effects obtaining both illumination-invariant reflectance properties and a high-resolution surface normal field, and finally to blend and stitch the acquired detail on the triangle mesh via standard texture mapping. In particular, the smooth join between different images that map on adjacent sections of the surface is obtained by applying an accurate piecewise local registration of the original images and by blending textures. For each mesh face which is on the adjacency border between different observed images, a corresponding triangular texture patch can also be resampled as a weighted blend of the corresponding adjacent image sections. Examples of the results obtained with sample works of art are presented and discussed.

Preserving attribute values on simplified meshes by re-sampling detail textures

Many sophisticated solutions have been proposed to reduce the geometric complexity of 3D meshes. A less studied problem is how to preserve on a simplified mesh the detail (e.g. color, high frequency shape detail, scalar fields, etc.) which is encoded in the original mesh. We present a general approach for preserving detail on simpli-fied meshes which is completely independent on the simplification technique adopted to reduce mesh size. The detail (or high frequency information) lost after simplification is encoded through texture or bump maps. The original contribution is that preservation is performed after simplification, by building set of triangular texture patches that are then packed in a single texture map. Each simplified mesh face is sampled to build the associ-ated triangular texture patch; different methods for storing this set of texture patches into a standard rectangular texture are presented and discussed. The solution is very general, because it can be applied to the output of any simplification code and because it allows preservation of any attribute value defined on the high resolution mesh. Moreover, the produced mesh de-couples shape from detail (the latter is encoded with texture maps) and allows highly efficient rendering. We also describe an alternative application: the conversion of 3D models with 3D procedural textures (which generally force the use of a software renderers) into standard 3D models with 2D bitmap textures, which con-versely can be rendered on any standard HW/SWgraphics subsystem. EMAIL:: r.scopigno@cnuce.cnr.it

Three-dimensional modelling of statues: the Minerva of Arezzo

Abstract The Minerva of Arezzo is an ancient bronze statue located at the Museo Archeologico in Florence and currently under repair at the Restoration Centre of the Soprintendenza Archeologica of the Tuscany Region. We assembled a complete three-dimensional (3D) digital model of the Minerva before the restoration started. More 3D models will be produced to keep track of the variations that occurred during the restoration process, up to the final acquisition of the form of the restored artwork. The modelling of the Minerva will be the focal point of an ambitious “Minerva Project” that involves the integration of data from other sources in a 3D digital model of the object. Besides this, the project is aimed at showing how 3D techniques can be used to design useful and easily manageable new tools for the diagnostics of archaeological objects. 3D measurements have been realized by means of a high-resolution laser scanner developed at National Institute for Applied Optics (INOA). The instrument is composed of commercial low-cost components in order to be competitive with the very expensive commercial devices. Besides this, our scanner is supported by an efficient and flexible software developed by Consiglio Nazionale delle Ricerche (CNR) that supports all the post-processing phases of a 3D scanning session (range data alignment, merge and simplification).

The Marching Intersections algorithm for merging range images

AbstractA new algorithm for the integration of partially overlapping range images into a triangular mesh is presented. The algorithm consists of three main steps: it locates the intersections between the range surfaces and a reference grid chosen by the user, then merges all nearly coincident and redundant intersections according to a proximity criterion, and, finally, reconstructs the merged surface(s) from the filtered intersection set. Compared with previous methods, which adopt a volumetric approach, our algorithm shows lower computational costs and improves the accuracy of the surfaces produced. It takes into account the quality of the input measurements and is able to patch small holes corresponding to the parts of the 3D scanned object that were not observed by the acquisition device. The algorithm has been tested on several datasets of range maps; graphical and numeric results are reported.