Jakub Krzesłowski

Archiving shape and appearance of cultural heritage objects using structured light projection and multispectral imaging

To create faithful reproduction of a cultural heritage object, it is crucial to gather information on intrinsic optical properties of the objects surface, as well as its geometry. An integrated device has been developed that performs a three-dimensional measurement using structured light projection, followed by multispectral imaging for precise color retrieval and directional illumination for estimating bidirectional reflectance distribution function (BRDF) parameters. The main advantage shown in this work is the use of only one detector during the whole acquisition process to assure ideal correspondence of multimodal surface data in the image space. A method is shown for performing the measurement using an integrated device. Methods of data organization and processing are described facilitating robust operation of the developed software. A prototype setup for the integrated system is presented together with measurement parameters and sample measurement.

Integrated three-dimensional shape and reflection properties measurement system

Creating accurate three-dimensional (3D) digitalized models of cultural heritage objects requires that information about surface geometry be integrated with measurements of other material properties like color and reflectance. Up until now, these measurements have been performed in laboratories using manually integrated (subjective) data analyses. We describe an out-of-laboratory bidirectional reflectance distribution function (BRDF) and 3D shape measurement system that implements shape and BRDF measurement in a single setup with BRDF uncertainty evaluation. The setup aligns spatial data with the angular reflectance distribution, yielding a better estimation of the surfaces reflective properties by integrating these two modality measurements into one setup using a single detector. This approach provides a better picture of an objects intrinsic material features, which in turn produces a higher-quality digitalized model reconstruction. Furthermore, this system simplifies the data processing by combining structured light projection and photometric stereo. The results of our method of data analysis describe the diffusive and specular attributes corresponding to every measured geometric point and can be used to render intricate 3D models in an arbitrarily illuminated scene.

Remote online monitoring and measuring system for civil engineering structures

In this paper a distributed intelligent system for civil engineering structures on-line measurement, remote monitoring, and data archiving is presented. The system consists of a set of optical, full-field displacement sensors connected to a controlling server. The server conducts measurements according to a list of scheduled tasks and stores the primary data or initial results in a remote centralized database. Simultaneously the server performs checks, ordered by the operator, which may in turn result with an alert or a specific action. The structure of whole system is analyzed along with the discussion on possible fields of application and the ways to provide a relevant security during data transport. Finally, a working implementation consisting of a fringe projection, geometrical moiré, digital image correlation and grating interferometry sensors and Oracle XE database is presented. The results from database utilized for on-line monitoring of a threshold value of strain for an exemplary area of interest at the engineering structure are presented and discussed.

Integrated shape, color, and reflectivity measurement method for 3D digitization of cultural heritage objects

In this paper a 3D shape measurement system with additional color and angular reflectance measurement capabilities is presented. The shape measurement system is based on structured light projection with the use of DLP projector. 3D shape measurement method is based on sinusoidal fringes and Gray codes projection. The color measurement system uses multispectral methods with a set of interference filters to separate spectral channels. Additionally the setup includes an array of compact light sources for measuring angular reflectance based on image analysis and 3D data processing. All three components of the integrated system use the same grayscale camera as a detector. The purpose of the system is to obtain complete information about shape, color and reflectance characteristic of measured surface, especially for cultural heritage objects in order to use in documentation, visualization, copying and storing. Some measurement results of real objects with a discussion of accuracy are presented along with future development plans.

Calculation methods for digital model creation based on integrated shape, color and angular reflectivity measurement

The paper presents a complete methodology for processing sets of data registered by the means of a measurement system providing integrated 3D shape, multispectral color and angular reflectance characteristic. The data comprise of clouds of points representing the shape of the measured object, a set of intensity responses as a function of wavelength of incident light used for color calculation and a set of distributions of reflected intensity as a function of illumination and observation angles. Presented approach allows to create a complete 3D model of the measured object which preserves the objects shape, color and reflectivity properties. It is developed specifically for application in the digitization of cultural heritage objects for storing and visualization purposes, as well as duplication by the means of 3D printing technology.

Integrated digitization method for cultural heritage objects

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Data processing path from multimodal 3D measurement to realistic virtual model

A set of calculation methods has been developed and tested to provide means of creating virtual copies of three dimensional (3D) historical objects with minimal user input. We present a step by step data processing path along with algorithm description required to reconstruct a realistic 3D model of a culturally significant object. The important feature for archiving historical objects is the ability to include both information about its shape and texture, allowing visualization using arbitrary conditions of illumination. Data samples used as input for the processing method chain were collected using an integrated device consisting of shape, multispectral color and simplified BRDF measurements. To confirm the usability of presented methods, it has been tested by example of real life object - statue of an ancient Greek goddess Kybele. Additional visualization methods have also been examined to render a realistic virtual representation satisfying intrinsic surface properties of the investigated specimen.

Image-based specular component estimation using structured light illumination

Color imaging depends strongly on the illumination conditions. Semi-specular surfaces impose ambiguity in colorimetric analysis and the specular reflection component has to be treated differently depending on the material. To exclude the specular component, special configuration of photometric devices is required. Such conditions cannot easily be met for images of irregular surfaces. I propose a simple method of structured light illumination for separating the specular and diffuse components of semi-specular surfaces using color camera and a calibrated flat-screen monitor.

Novel calibration procedure for SVBRDF estimation of 3D objects using directional illumination and structured light projection

Estimation of geometry and reflectance of 3D objects requires that surface geometry is registered together with photometric data. We present a method which combines geometrical camera calibration and photometric calibration into a single procedure utilizing only one calibration target. Using structured light projection and directional illumination, the surface of a 3D object can be registered with an integrated measuring device. To estimate spatial distribution of reflectance parameters, a Spatially Varying Bidirectional Reflectance Distribution Function (SVBRDF) model is used. We also show a 3D image processing method to estimate SVBRDF parameters using an arbitrary defined array of illuminators and algorithms to reconstruct this surface using specialized visualization software. This approach allows for effective measurement of geometry and visual properties of 3D objects represented by a dense point cloud model. It can become a valuable tool for documentation of digital heritage and in industrial computer vision applications.

Data acquisition enhancement in shape and multispectral color measurements of 3d objects

The paper presents application of a correction technique proposed for registering images by a CCD (Charged Coupled Device) camera. The device is installed in a 3D shape, angular reflectance distribution and multispectral color measurement system set up for digitization of cultural heritage objects. The procedure compensates for the camera noise and scene illumination non-uniformity according to previously published model. The paper describes the measurement system and provides analysis of data collected from measurements of the Color Checker reference target and uniform reference plane to evaluate enhancement of reflectance and shape accuracy after correction. Additionally a few examples of digitized objects are shown.