*1.2. Case Study*

The Torre de la Cautiva is one of the parts of the Alhambra that is not usually open to the public. In the summer of 2018, the challenge was posed to produce a pilot piece of the interior of the tower. Its goal was to explore the possibilities of 3D models obtained through

affordable passive-sensor data-acquisition techniques, not only as a simulation platform for conservation and restoration, but also as a vehicle for dissemination and engagement. Two rooms were to be covered: the main room, with profuse decoration and ambient light, and the secondary room or courtyard, with simpler decoration and little natural light (see Figure 1b). The result would be used in stereoscopic 3D in the Head Mount Display (HMD). In Milgram and Kishino's Reproduction Fidelity dimension, the aim was to achieve what they call the "'ultimate' graphic rendering", referred to as "real-time, high-fidelity 3D animation" [28]. through affordable passive-sensor data-acquisition techniques, not only as a simulation platform for conservation and restoration, but also as a vehicle for dissemination and engagement. Two rooms were to be covered: the main room, with profuse decoration and ambient light, and the secondary room or courtyard, with simpler decoration and little natural light (see Figure 1b). The result would be used in stereoscopic 3D in the Head Mount Display (HMD). In Milgram and Kishino's Reproduction Fidelity dimension, the aim was to achieve what they call the "'ultimate' graphic rendering", referred to as "realtime, high-fidelity 3D animation" [28].

The Torre de la Cautiva is one of the parts of the Alhambra that is not usually open to the public. In the summer of 2018, the challenge was posed to produce a pilot piece of the interior of the tower. Its goal was to explore the possibilities of 3D models obtained

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*1.2. Case Study* 

**Figure 1.** (**a**,**b**) Surveys with delineation techniques of the profile and floor plans of the Torre de la Cautiva. The main room has balconies on the walls, while the courtyard or secondary room has no direct lateral exits to the exterior. Compiled by the authors based on documents and plans maintained in the collection of the Patronato de la Alhambra y El Generalife (The Alhambra and Generalife Trust): P-1158\_2 and P-4615. **Figure 1.** (**a**,**b**) Surveys with delineation techniques of the profile and floor plans of the Torre de la Cautiva. The main room has balconies on the walls, while the courtyard or secondary room has no direct lateral exits to the exterior. Compiled by the authors based on documents and plans maintained in the collection of the Patronato de la Alhambra y El Generalife (The Alhambra and Generalife Trust): P-1158\_2 and P-4615.

A script was drafted to create a dramatic scene that would take place in different epochs and, therefore, different architectural and decorative moments would be shown in the same piece. The action was meant to observe a certain historical consistency. Some characters inhabited the stage, but they were not overly prominent. In this case, it was not so important to obtain data that were faultless from the architectural point of view, that is to say, the result did not have to consist of an informative HBIM model. A script was drafted to create a dramatic scene that would take place in different epochs and, therefore, different architectural and decorative moments would be shown in the same piece. The action was meant to observe a certain historical consistency. Some characters inhabited the stage, but they were not overly prominent. In this case, it was not so important to obtain data that were faultless from the architectural point of view, that is to say, the result did not have to consist of an informative HBIM model.

After becoming familiar with the first draft of the script (handling and division into temporal sequences), three strategies were combined, one for each line of work. The first consisted of the application of multi-camera photogrammetry using a vertical rig (see Figure 2) for interiors and walls in their current state of preservation. The second consisted of photogrammetry with a *giratutto* [29] to obtain the characters. Finally, in the third, conventional 3D modeling was used for the setting and assets. After becoming familiar with the first draft of the script (handling and division into temporal sequences), three strategies were combined, one for each line of work. The first consisted of the application of multi-camera photogrammetry using a vertical rig (see Figure 2) for interiors and walls in their current state of preservation. The second consisted of photogrammetry with a *giratutto* [29] to obtain the characters. Finally, in the third, conventional 3D modeling was used for the setting and assets.

Another of the project's goals, which was aimed at distinguishing this work from those that used similar approaches, was to achieve a look of veracity, or photorealism, by capturing images with low-cost DSLR cameras. The result would be experienced by the audience on a platform that allowed interaction with a freedom of observation on all three axes (pan, tilt, roll), but on principle, without the freedom of displacement. These interaction conditions are known as three degrees of freedom (3DoF) on the scale of

freedom of use [30]. It would also have a sound program to guide and complement what the viewer discovers. *Heritage* **2022**, *4* FOR PEER REVIEW 5

Another of the project's goals, which was aimed at distinguishing this work from those that used similar approaches, was to achieve a look of veracity, or photorealism, by capturing images with low-cost DSLR cameras. The result would be experienced by the audience on a platform that allowed interaction with a freedom of observation on all three An additional challenge was the time that was available for data capture, only ten hours spread over one afternoon and the following morning, which was a key factor in choosing the method with which to carry out this task. To complicate matters further, surveying methods to secure control points could not be used, nor lighting supports for photography.

### axes (pan, tilt, roll), but on principle, without the freedom of displacement. These interaction conditions are known as three degrees of freedom (3DoF) on the scale of *1.3. Choice of Strategy*

freedom of use [30]. It would also have a sound program to guide and complement what the viewer discovers. An additional challenge was the time that was available for data capture, only ten hours spread over one afternoon and the following morning, which was a key factor in choosing the method with which to carry out this task. To complicate matters further, surveying methods to secure control points could not be used, nor lighting supports for A wide repertoire of methods is available with which to obtain digital models [12,21], yet the unique nature of each project implies that the solutions employed in earlier strategies are not always the best approaches to adopt, and that the advances in hardware or software are not the only factors to consider [31]. Likely the only generalization that can be made for digital-model-acquisition projects is that there is not yet a methodology that is applicable to the majority of cases.

photography. *1.3. Choice of Strategy*  A wide repertoire of methods is available with which to obtain digital models [12,21], yet the unique nature of each project implies that the solutions employed in earlier strategies are not always the best approaches to adopt, and that the advances in hardware Among the known strategies, Structure from Motion (SfM) [7,11,13,29,32,33] using only digital camera photography is best adapted to the circumstances associated with the data collection in this study, and it has been used without a set order of shooting [7] or even without checkpoints. This feature constituted a good point of support, since we could not count on having active sensors or Total Stations in our work. Moreover, it was known that stereoscopically configured work reduces the need for a high number of shots [11,34].

or software are not the only factors to consider [31]. Likely the only generalization that can be made for digital-model-acquisition projects is that there is not yet a methodology that is applicable to the majority of cases. Among the known strategies, Structure from Motion (SfM) [7,11,13,29,32,33] using only digital camera photography is best adapted to the circumstances associated with the data collection in this study, and it has been used without a set order of shooting [7] or even without checkpoints. This feature constituted a good point of support, since we could not count on having active sensors or Total Stations in our work. Moreover, it was known that stereoscopically configured work reduces the need for a high number of shots [11,34]. Although positive results had been reported with mobile devices [35,36] (which is later than our data collection), much of the success of the work depended on the flexibility and reliability of the cameras in capturing, and therefore the cell-phone option was ruled out. We considered cell phones as offering insufficient and unreliable control over exposure in the conditions in which we had to work, which we would still argue today [36]. The aim was to interfere as little as possible with the physical environment, so the motto was "get in, do it and get out", just as in photogrammetry projects that involve the police or forensics, which cannot alter the environment in any way whatsoever [37]. With this remit it was clear that the background preparation for the data capture had to be as thorough as possible.

Although positive results had been reported with mobile devices [35,36] (which is later than our data collection), much of the success of the work depended on the flexibility and reliability of the cameras in capturing, and therefore the cell-phone option was ruled

## *1.4. Photorrealism 1.4. Photorrealism*

thorough as possible.

The great cinematographer Néstor Almendros used to argue that "we must learn to transcend the model, while respecting it" and that "the stylization of an image can sometimes be more important than history and logic" [38]. Obviously, this philosophy cannot be applied to obtaining data for HBIM, yet it is valid to immerse the public in the illusion of photorealism within an environment defined by a script. Similarly, it is not valid to simply confront the user with a 3D graphic model [23] via an HDM that fails to provide the user with any other immediate references to reality. Rather, it is necessary to go further, to create an impression of authenticity and accuracy in the user [20,39], but in this case one that is diachronic. It must be borne in mind that to do so implies moving with the imagination to the past, and the capacity to compare the result of a photograph of that time [40,41] with what is being experienced at that moment. The great cinematographer Néstor Almendros used to argue that "we must learn to transcend the model, while respecting it" and that "the stylization of an image can sometimes be more important than history and logic" [38]. Obviously, this philosophy cannot be applied to obtaining data for HBIM, yet it is valid to immerse the public in the illusion of photorealism within an environment defined by a script. Similarly, it is not valid to simply confront the user with a 3D graphic model [23] via an HDM that fails to provide the user with any other immediate references to reality. Rather, it is necessary to go further, to create an impression of authenticity and accuracy in the user [20,39], but in this case one that is diachronic. It must be borne in mind that to do so implies moving with the imagination to the past, and the capacity to compare the result of a photograph of that time [40,41] with what is being experienced at that moment.

[36]. The aim was to interfere as little as possible with the physical environment, so the motto was "get in, do it and get out", just as in photogrammetry projects that involve the police or forensics, which cannot alter the environment in any way whatsoever [37]. With this remit it was clear that the background preparation for the data capture had to be as

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### **2. Methodology 2. Methodology**

This project was approached in three phases that are not necessarily sequential or consecutive: pre-production, production, and post-production. Based on approaches used in the audiovisual industry, these phases were applied to a script and focused on different aspects: inside the tower, obtaining the characters, and the creation of assets. While it is clear that careful preparation is required, as reflected elsewhere in the literature [42,43], phases such as *Data Capture*, *Data Processing* and *Output Presentation* can sometimes be found, which either do not consider shot preparation as a relevant step or do not include it in their first stage [17,29–31,33]. Due to the importance of the decisions that had to be taken in the preparation of the workflow, it is established here as a singular phase, while the presentation of the final result is included in post-production as one of the processes that usually make it up [44]. This project was approached in three phases that are not necessarily sequential or consecutive: pre-production, production, and post-production. Based on approaches used in the audiovisual industry, these phases were applied to a script and focused on different aspects: inside the tower, obtaining the characters, and the creation of assets. While it is clear that careful preparation is required, as reflected elsewhere in the literature [42,43], phases such as *Data Capture*, *Data Processing* and *Output Presentation* can sometimes be found, which either do not consider shot preparation as a relevant step or do not include it in their first stage [17,29–31,33]. Due to the importance of the decisions that had to be taken in the preparation of the workflow, it is established here as a singular phase, while the presentation of the final result is included in post-production as one of the processes that usually make it up [44].

The general outline of the procedures (Figure 3) will be broken down below, with special attention to the fact that one of the objectives was specifically the development of the workflow. The general outline of the procedures (Figure 3) will be broken down below, with special attention to the fact that one of the objectives was specifically the development of the workflow.

**Figure 3.** Scheme of the processes employed in the tower project. All the preparative decisions are grouped together in the pre-production phase. Compiled by the authors.
