# Situated AR Simulations of a Lantern Festival Using a Smartphone and LiDAR-Based 3D Models
## 6 Three AR Instancing Scenarios
### An Interaction with a Pre-Defined Model Layout  
The meta-relationship between elements was expected, with identified arrangements between people, installations, temporary fabrics, and permanent fabrics. The developing steps and the structural diagram of elements of the AR app can be seen in Figure 9. The markerless AR app was created from Unity(r) (2020, Unity Technologies, San Francisco, Cupertino, CA, USA) using ARKit SDK(r) (2020, Apple Inc., Cupertino, CA, USA). Visual Studio(r) (2020, Microsoft, Redmond, WA, USA) was applied to edit the display and switch the virtual models. The menu to the left of the screen could be retracted as needed. The interacted items and system interface of the app included the selection of a series of objects. All of the contents could be selected individually or accumulatively. It only required a surface, such as the ground level, to anchor the 3D models, instead of a barcode or a picture. The markerless app made the application operate more straightforwardly, without being restricted to additional marker-paste procedures or potentially causing damage to the targeted surface. Figure 8: The situated revisit and recall of an old festival with AR screenshots (**a**), screenshots in the evening and late afternoon (**b**), third-person photographs (**c**), and the stepwise operation process: launch app, scan QR code of a selected 3D model, detect plane, tap to allocate the model, and adjust the model’s scale, location, or rotation angles (**d**). An iPhone XS Max(r) (A12 Bionic chip, 64G Capacity, Apple, Cupertino, CA, USA), which was equipped with an A12 CPU with 4 GB RAM and 64 GB internal storage, was used to test the app. The final size of the app was 288.5 MB. Due to the limited storage space of a smartphone, increasing the polygon number caused a delay in the screen display, with flickering. Decimation was made by the defined polygon count or percentage. The cloud model was converted to a mesh model in OBJ format and decimated using Geomagic Studio(r) (v2014.1.0, former Raindrop Geomagic Inc., Morrisville, NC, USA, now 3D Systems Company, Rock Hill, SC, USA). For display efficiency, the original mesh model of 121 MB (0.73-62.70 MB each) was divided into seven parts along two linear and perpendicular axes (Figure 10a). Each decimated part was approximately 88,106-887,408 polygons. The standalone mesh model was 171.1 MB, comprising approximately half of the polygon numbers wrapped afterwards from the original scanned points. The axes represented one of many possible combinations that visitors perceived in different perspectives (Figure 10). The site planning, which was subjected to the programming of the festival, created a hierarchical space based on the management of two-axis spatial structures for lanterns and facilities (Figure 11). The structure constituted a semi-open arcade which consisted of the arrangement of a roof, visitors, and three groups of lanterns. One of the three groups of lanterns was selected to form another axis with two additional instances to be interacted with upon selection on the menu. The app display scenario was constructed based on the as-built spatial structure, which is similar to what occurred to the adjacent fabric in an existing 3D spatial framework in reality. Lantern installations and crowds can  
Figure 9: The development process of the AR APP (**top**), smartphone interface (**middle**), and the structural entities of the lantern festival (**bottom**). be displayed selectively to reveal the scale factor of the surrounding environment, from a smaller installation to a large canopy. An event-specific or occasion-related app should be designed to enable 3D scenes with unlimited adaption outside of the original display scenario. Our app was designed with the individual scenes separated, so that it can be carried around to project the standalone 3D model in different urban fabrics as a correspondence to different cultural contexts or as a feasibility test of different environmental settings.