**3. Results and Discussion**

12a–c.

*3.1. Multi-Camera Techniques with Rigs*

Using a multi-camera data-capture setup ensures the overlapping of images so that each point appears in more than one, which is known since stereoscopy has been used for the same processes [3,11,32,46]. But it allows us to go a step further, covering a larger area in the same amount of time, relative to the number of cameras at each shooting position. A simple technique to prepare the multi-camera shot is to imagine the space in slices (which would be equivalent to the geographical longitude, the vertical sector to be covered), which are represented by each shooting position of a vertical rig, and segments (the geographical latitude, the horizontal sector to be covered), which are represented by each camera anchor on the tripod.

There is concern about the minimum cost that needs to be addressed for a photogrammetry project [7,11,29,33,45]. While increasing the number of cameras clearly increases the budget, at current prices, it is still much less than covering the costs of using more photographers, a laser scanner or a Total Station [36]. In this project, mid-range DSLR cameras with about 18 Mpix resolutions and conventional optics were used, but mirrorless, medium-format cameras with higher resolution (thus providing more detail from the same number of shots) and faster speeds, because each shot does not involve mechanical displacement, are becoming more affordable. When a large number of shots is involved, time savings are multiplied, especially if bracketing is used to ensure correct exposure, which reduces the possibility of occlusion due to under- or over-exposure. In addition, access to better optics also reduces the exposure time, but raises the budget. Nevertheless, this project was less expensive overall as the costs associated with the number of cameras used were much lower than those that would have been imposed by increasing the number of photographers [45].

Another important issue to foresee is the shooting path. It is often necessary to resort to complex algorithms, such as those of the Traveling Salesman Problem for active sensors [47]. In this case, the chosen route was more or less simple and always parallel to the walls with sufficient shots. To generalize the optimization of the route, one should perhaps resort to artificial intelligence (AI), which considers factors such as the geometry of the object, the available equipment, the environmental conditions and the demands of the script [17]. Any software used must be able to interpret camera positions and locate sufficient homologous points, thereby avoiding occlusions. In our case, it was important to cover all possible angulations in order to obtain a sufficient number of overlapping points, but equal care must be taken not to oversize the shots, as it makes the project unmanageable. On the other hand, when using a multi-camera setup, coordination errors that may exist are multiplied, so extreme care must be taken in this regard. Other factors are focus and exposure, which are explained below.

Some 1900 photographs were taken in the tower (7 cameras × 4 passes × 2 heights × 29–35 rig positions), and some 800 without a tripod. Among them, about 2500 were used for the photogrammetric models. For the characters, about 2150 pictures were taken (6 models × 2 poses × 12 shots × 15 cameras) and about 720 were used.

The time required to make adjustments before each shot increases when using the multi-camera technique. The exposure and focus must be set manually, even if some parameters are automatic. Again, affordable technology facilitates the operation, as nowadays it is common to control camera viewfinders remotely (both the image displayed and the overlays) as they are the main operations for exposure, focus and shooting.

Another problem was camera shaking; in order to gain height and ensure angles that might otherwise cause occlusions, the cameras had to be raised as high as the pole would allow without swaying. Maintaining stability with light equipment three meters off the ground was difficult, but essential when the shots exceeded 1/30th of a second. This circumstance arose in virtually all of the cases in our work. However, the results were equally dramatic, enabling views from the coffered ceiling with the viewer positioned four meters above the floor to be included in the final experience.

There is no doubt that a robotic system that incorporates AI, which is capable of making some decisions regarding settings and routes and also of detecting and solving possible occlusions, would ultimately simplify and reduce the cost of projects, leading to optimal results.
