Beyond 3D Models: Simulation of Phased Models in Stellarium

Georg Zotti, Florian Schaukowitsch, Michael Wimmer
Beyond 3D Models: Simulation of Phased Models in Stellarium
In 25th SEAC Conference. September 2017.

Information

Abstract

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.

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BibTeX

@inproceedings{ZOTTI-2017-BM,
  title =      "Beyond 3D Models: Simulation of Phased Models in Stellarium",
  author =     "Georg Zotti and Florian Schaukowitsch and Michael Wimmer",
  year =       "2017",
  abstract =   "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. ",
  month =      sep,
  booktitle =  "25th SEAC Conference",
  location =   "Santiago de Compostela, Spain",
  note =       "25th SEAC Conference",
  URL =        "https://www.cg.tuwien.ac.at/research/publications/2017/ZOTTI-2017-BM/",
}