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G. Zotti, M. Wimmer et al.:
"Beyond 3D Models: Simulation of Phased Models in Stellarium";
Talk: 25th SEAC Conference, Santiago de Compostela, Spain (invited); 2017-09-18 - 2017-09-22; in: "Proceedings of 25th SEAC Conference", (2017), ISBN: 978-84-697-5608-9; 206 - 207.

In recent years, the interactive visual exploration and demonstration of three-dimensional virtual models of buildings or natural structures of archaeoastronomical interest under a simulated sky has become available for users of the open-source desktop planetarium program Stellarium [Zotti, 2015, 2016]. Users can load an architectural model in the well-known OBJ format and walk around to explore sight lines or light-and-shadow interaction in present and past times [Frischer et al., 2016].

However, until now, the model itself did not change in time, and loading models for various building phases (e.g., the assumed order of building the various standing stones, timber circles and stone circles of Stonehenge) always required a break in simulation and user interaction to load a model for the next phase. On the other hand, displaying a model under the sky of the wrong time may lead to inappropriate conclusions. Large-area models required considerable time to load, and loading caused a reset of location, so the user interested in changes in a certain viewing axis had to recreate that view again. Given that Stellarium is an "astronomical time machine", nowadays capable of replaying sky vistas thousands of years ago with increasing accuracy [Zotti et al., submitted] and also for models with several million triangular faces, it seemed worth to explore possibilities to also show changes over time in the simulated buildings. The Scenery3D plugin of Stellarium is, however, not a complete game engine, and replicating the infrastructure found in such game engines like Unity3D - for example to interactively move game objects, or load small sub-components like standing stones and place them at arbitrary coordinates - seemed overkill. The solution introduced here is remarkably simple and should be easily adoptable for the casual model-making researcher: the MTL material description for the model, a simple plain-text file that describes colour, reflection behaviour, photo-texture or transparency of the various parts of the object, can be extended for our rendering system. Newly introduced values describe dates where parts of the model can appear and disappear (with transitional transparency to allow for archaeological dating uncertainties). The model parts with these enhanced, time-aware materials appear to fade in during the indicated time, will be fully visible in their "active" time, and will fade out again when Stellarium is set to simulate the sky when the real-world structures most likely have vanished. The only requirement for the model creator is now to separate objects so that they receive unique materials that can then be identified and augmented with these entries in the MTL text file.

The advantages of this new feature should be clear: an observer can remain in a certain location in the virtual model and let the land- and skyscape change over decades or centuries, without the need to load new models. This allows the simulation of construction and reconstruction phases while still always keeping particularly interesting viewpoints unchanged, and will always show the matching sky for the most appropriate reconstruction phase of the model.

http://publik.tuwien.ac.at/files/publik_261586.pdf