Evolving Perspectives in Digitizing Cultural Heritage: Novel Paradigms through Self-Built and Adaptable Equipment

1. Introduction

In this paper we present a critical overview of these new systems with the aim to show a new methodological and technical scenario on which to focus research on the coming years. To fully understand problems/solutions, the evolution of each instrument created will be illustrated, to allow the reader to easily understand the rationale of each specific choice. After this Introduction, the Materials and Methods section introduces the pipelines and both custom software then hardware tools. The Results section introduces the replicable outcomes on some real case studies in different scenarios where solutions developed were employed. The Conclusions section finalizes the paper, bringing into the discussion future perspectives on the novel paradigms on fabrication and design of tools.

Figure 1. Old engraving of Andrea Palladio in the collection of the Centro Internazionale di Studi di Architettura Andrea Palladio, in Vicenza, Italy (source: Wikimedia Foundation, public domain).

Figure 1. Old engraving of Andrea Palladio in the collection of the Centro Internazionale di Studi di Architettura Andrea Palladio, in Vicenza, Italy (source: Wikimedia Foundation, public domain).

2. Materials and Methods

2.3. Developed equipment solutions

The developed custom-built equipment solutions refer to indoor acquisitions with controlled lighting and are designed to overcome the following common issues:

• lack of illuminants efficiency, with inconsistent or harmful lighting conditions leading to variations in image quality and accuracy.
• presence of erratic reflections and parasite light coming from outdoor.
• absence of planarity of the camera and the surface to be captured: without a correct planarity between the camera and the object's surface, there can be discrepancies in image resolution, affecting the clarity and details of the captured images.
• significant time required to set up the shooting stage: positioning lighting equipment, light-blocking screens, and cameras can be difficult to setup, time-consuming, increasing the overall time required for capturing images.
• too complex tools requiring specific expertise in their use or many human resources that may pose challenges for average users.
• costly and hard-to-find tools and spares: some hardware tools or setups may be expensive and not easily accessible, making it challenging to acquire them, as, e.g., in the Operation Night Watch project, by Rijksmuseum in Amsterdam [41].

The custom equipment solutions developed to minimize or solve these problems are:

• a set of very portable lights with known emissions and efficiency, to easily transport them and minimize the technical time required to set up the stage,
• a repro stand for artworks to be captured on a horizontal plane, like ancient drawings,
• a repro stand for artworks to be captured on a vertical plane, like paintings or frescoes,
• a calibrated roundtable and 3D test-field plate to capture small museum objects.

A. Portable lights - To avoid the lack of illuminants efficiency and too complex lighting systems to illuminate scenes, a new lighting system based on a series of single LED lights, characterized by limited dimensions and good portability. A custom prototype was built, consisting of sixteen or thirty-two Relio² LED lights (four or eight on each side of the repro), featuring a correlated color temperature (CCT) of 4000° K, a brightness of 40,000 lux at 0.25 m, and a Color rendering index (CRI) > 95% [42]. The Spectral Power Distribution (SPD) of these illuminators highlights high chromatic consistency across all wavelengths and excellent color rendition even during the rendering phase, with no damaging UV or IR emissions (Figure 3). In Figure 5 the photometry and chromaticity diagrams for illuminants with these features are represented, showing how their performance is close to natural light. Relio² illuminants were positioned and grouped with specifically designed and 3D printed supports to place lights at desired directions and inclinations.

These supports were crafted in ABS (acrylonitrile-butadiene-styrene) using a XYZprinting Da Vinci Pro 1.0 3D printer and 1.75 mm diameter black matte filament. The choice of ABS as preferred material, rather than other easier-to-print filaments, was driven by the need to prevent the supports from being damaged or deformed by the heat emitted by the illuminators (approximately 60°C each).

This solution is also economically affordable since it solves the problem of finding costly spare parts in case of damages during the working operations (Figure 4).

Figure 3. Comparative SPD emission charts by producers, in which the continuous spectrum of visible light for each tested light is clearly highlighted (from left to right Osram, Godox and Relio²).

Figure 3. Comparative SPD emission charts by producers, in which the continuous spectrum of visible light for each tested light is clearly highlighted (from left to right Osram, Godox and Relio²).

Figure 4. 3D printed components in ABS to support the Relio² lights.

Figure 4. 3D printed components in ABS to support the Relio² lights.

Figure 5. The photometry and chromaticity diagrams for Relio² lamps, with the histogram for CRI.

Figure 5. The photometry and chromaticity diagrams for Relio² lamps, with the histogram for CRI.

B. Repro stand for artworks to be captured on a horizontal plane –The stand was designed to express the following features:

• a stable structure to minimize blurring caused by oscillations and vibrations and lighting small movements that may cause potential non-uniformity of the light,
• a wide reproduction area capable of accommodating the open passe-partout containing the drawings to be captured ensuring a safety management of it and its planarity,
• the lighting system positioned on all four sides, equidistant from the center of the drawing to guarantee homogeneous illumination for the whole acquisition area,
• no interference between the light sources and the camera,
• easy portability within the locations where the drawings are usually stored.

The stand was designed separating the structure responsible for supporting the lighting system and housing the drawing, from the column that holds the camera, consisting in Manfrotto 809 Salon 230, 280 cm. height, equipped with a Manfrotto 410 geared head. To support four Relio² lights on each side of the shooting surface, four detachable arms made of aluminum extruded profile were prepared. They were connected to the surface using two angled brackets secured with bolts. These supports were deliberately positioned in an asymmetrical manner to enable the opening of the passe-partout and to ensure the positioning of the lights at the same distance from the center of the acquisition plane (660 mm.). In Figure 6 the prototype is illustrated while Table 1 defines the nature of the (commercial or self-built) components used to assemble it. Additionally, alignment gauges were constructed to check the perfect positioning of the arm inclination and light height once assembled at the acquisition location. In Figure 7 the bill of custom-built components and the alignment gauge are reported. In the first prototype the lighting system was made of sixteen Relio² LED positioned on all four sides, angled at 45° relative to the horizontal and equidistant from the center of the drawing to guarantee homogeneous illumination for the whole acquisition area.

Starting from a first version of the stand, to address problems like the absence of planarity of the camera and the surface to be captured and the system portability, progressive improvements were introduced over time.

Furthermore, the second prototype was built improving the safety of the artwork placed on the acquisition plane to better manage the issue that most part of the drawings to be acquire were enclosed in passe-partouts, often secured with strips of tape that allow the sheet to be flipped and viewed from both sides but not removed making difficult the acquisition of the back of the drawing due to the asymmetrical position with respect to main side of the sheet determined by these stripes. To solve this issue in the new setup the inclined arms that hold the illuminators were not directly fixed to the surface, but to an aluminum structure raised 20 mm. above the horizontal plane instead. The frame, made of 20x20 mm. aluminum profile with a thickness of 1 mm. and welded at the corners, was raised by four spacers to the four vertices. Each of them can be removed, allowing the structure to remain fixed in only three points. This guides the insertion of the drawing folder with great flexibility, positioning the drawing to be acquired directly beneath the camera. Also, a new, shorter column Lupo Repro 3, equipped with a Manfrotto 410 geared head, was adopted to disassemble it into segments no longer than two meters, making it transportable even in a medium-sized vehicle.

Another issue demanding for further solution development was the increasing of the number of the lights to better model reflections behavior. The new stand accommodated light sources inclined at two different angles, 15° and 45° relative to the acquisition surface, with the camera allowing a more accurate normal map and reflection map extraction.

To minimize the presence of erratic reflections and parasite light coming from outdoor various methods have been employed over time to darken the area around the prototypes: initially the acquisition stands required space-demanding darkening systems (Figure 8), while the latest prototype now features a guide atop the lamp-supporting arms, allowing a black curtain to slide and effectively block reflected light around the surface. This new solution has proven to be more practical both in terms of assembly time and light insulation. In Figure 9 the second acquisition prototype is illustrated while Table 2 defines the nature of the components that we used to assemble it (commercial or self-built). In Figure 10, the bill of custom-built components is illustrated.

C. Repro stand for artworks to be captured on a vertical plane - The solution for artworks to be captured vertically, such as paintings, was developed beginning from the Manfrotto 809 Salon 230, 280 cm. height, as main structure of the rig. A new structure made of square aluminum tubes measuring 20x20x1 mm. each mounted along the horizontal arm of the column is connected to the column through ABS nodes, forming a trapezoidal shape that links them to the column via aluminum and steel tension rods, to strengthen the structure (Figure 11). This new structure allows to accommodate inclined light sources at two different angles, 15° and 45° relative to the acquisition surface, with the camera now integrated into the lighting structure and positioned at its center. Due to the larger size of paintings when compared to drawings and the limited frame of each picture taken to get the needed resolution, once they need to be mosaicked together, the acquisition system had to be both vertically and horizontally movable, remaining parallel to the vertical plane of the painting to be acquired. The movement was ensured horizontally by wheels placed below the base and vertically exploiting the rack of the column. To ensure the parallelism of the arm to the painting plane and the control of the distance between the camera lens and the painting plane in different shots, two laser distance meters have been mounted on two brackets, one on the right and one on the left side.

Since the artwork cannot be often removed from the wall, and calibration panels or color checkerboard cannot be superimposed to the painting for safety reasons, in addition to the structure supporting the lights and camera it was necessary to build a separate stand to perform specifically the preliminary calibration procedures.

In Figure 12 the acquisition prototype for vertical surfaces is illustrated while Table 3 defines the nature of the components that we used to assemble it (commercial or self-built).

D. Calibrated roundtable and 3D test-field plate – The developed solution fixes the problems of the adoption of too complex tools requiring specific expertise in their use, also for technical operations such as calibration [43], as the sophisticated solutions presented in [24] (with stepper motors and controllers to drive the movement of the table). We decided to substitute complex rigs, difficult to fabricate for operators, with guidelines for a correct use instead and tools that can be fabricated directly by users, folding simple cardboard as indicated in an open-source free layout (Figure 13).

Furthermore, this simple solution better matches with the use of acquisition devices as smartphone cameras with the purpose of the effortless usage and portability, per the desired level of accuracy, and not the highest precision possible.

A rotating support was built to acquire small objects with a set of Ringed Automatically Detected (RAD) coded targets printed upon the acquisition circular flat surface, as well as on six regularly arranged cubes also textured with RAD targets to help alignment and scaling. These cubes are firmly connected to the rotating table profile by metal rods, radially placed along the circular plate’s thickness and rotating with it. For simpler calibration procedures, a 3D test-field plate hosting 150 RAD coded targets was also prepared, using 12-bit length coordinates, which can be easily plotted using a 600 dpi b/w printer on rigid paper sheet.

In Figure 13 both tools are illustrated.

Camera calibration procedure

The first step in the data processing is the camera geometric calibration, to determine both the interior and the exterior orientation parameters, as well as the additional values. The most common set of additional parameters employed to compensate for systematic errors in digital cameras is the 8-term physical model originally formulated by Brown [50]. The Brown’s model includes the 3D position of the perspective center in image space (principal distance and principal point), as well as the three coefficients for radial and two for decentering distortion. The model can be extended by two further parameters to account for affinity and shear within the sensor system. The iPhone X camera calibration is carried out using the developed 3D test-field. It consisted of taking a set of 20 images using a tripod and a photo studio illumination set. It also included a set of convergent images (some of them rotated by 90°) with good intersection angles of the rays from the camera to the test-field [51,52].

The calibration process is performed twice using different techniques:

• Self-calibration in COLMAP. By default, COLMAP tries to refine the intrinsic camera parameters (except principal point) automatically during the reconstruction. Usually, these parameters should be better estimated than the ones obtained manually with a calibration pattern. This is true only if there are enough images in the dataset and the intrinsic camera parameters between multiple images is shared. Using the OpenCV model camera the following camera calibration parameters are calculated: f focal length; cx, cy principal point coordinates; K1, K2 radial distortion coefficients; P1, P2 tangential distortion coefficients;
• b. RAD coded target based geometric calibration in Agisoft Metashape. Every centre of RAD coded target is reconstructed by 8 rays and more to enhance the accuracy allowing to calculate the Brown’s camera model parameters: fx,fy focal length coordinates; cx, cy principal point coordinates, K1, K2, K3 radial distortion coefficients; P1, P2 = tangential distortion coefficients.

The performances of the photogrammetric pipeline is evaluated by a statistical analysis that considered the following parameters:

• number of oriented images;
• Bundle Adjustment (BA) (re-projection error);
• number of points collected in the dense point cloud;
• comparison of the dense point cloud to the ground truth of the object. The photogrammetric models were compared with the a reference SLR camera models using CloudCompare.

Time - To check time, we can compare time necessary for the acquisition as recorded over the years for the drawings we digitalized and keep it as a reference.

Costs - The technology used to build the prototypes progressively became cheaper and cheaper, i.e., stands accommodate upcoming different cameras and smartphones with few modifications, reducing development costs. The progressive affordability of 3D printers with increasing quality in both materials and mechanics has effectively allowed for good adaptability to specific contexts: if specific solutions are needed, the production costs for variations to the equipment have decreased over time. An example of this is the creation of arms to support the lighting fixtures in the vertical stands, which were made modular and removable with no extra costs when specific geometries, such as the frames of paintings, could intersect with the standard system creating problems.

Usability - The know-how necessary to manage the introduced ecosystem of tools became overtime easier to use and more automatic and friendly. This was assessed offering it to museum operators who started to use it proficiently with reduced training. In fact, following the Kirkpatrick’s Four-level Training Evaluation Model [53], the learners’ reactions to training were positively influenced by the easy setup of the developed tools, which took advantage of previous knowledge by users. We could evaluate what was learned to assess the impact on results: the combination of hardware and software solutions proved to be usable with few efforts also by people not involved in IT technologies and development.

Figure 14. MTF evaluation during the calibration step: a laser-leveled orientation to check planarity.

Figure 14. MTF evaluation during the calibration step: a laser-leveled orientation to check planarity.

3. Results

4. Conclusions

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