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G. Zotti:
"Computer Graphics in Historical and Modern Sky Observations";
Supervisor, Reviewer: W. Purgathofer, A Wilkie; Institut für Computergraphik und Algorithmen, TU Wien, 2007; oral examination: 2007-11-27.

This work describes work done in three areas of research where sky observations meet computer graphics. The whole topic covers several millennia of human history and posed combined challenges from fields including archaeology, astronomy, cultural heritage, digital image processing and computer graphics.

The first part presents interdisciplinary work done in the fields of archaeo-astronomy, visualisation and virtual reconstruction. A novel diagram has been developed which provides an intuitive, easy visualisation to investigate archaeological survey maps for evidence of astronomically motivated orientation of buildings. This visualisation was developed and first applied to a certain class of neolithic circular structures in Lower Austria in order to investigate the idea of solar orientation of access doorways. This diagram and its intuitive interpretation allowed the author to set up a new hypothesis about practical astronomical activities in the middle neolithic period in central Europe.

How virtual reconstructions of these buildings characteristic for a short time during the neolithic epoch can be combined with the excellent sky simulation of a modern planetarium to communicate these results to a broader audience is described thereafter.

The second part of this work describes a certain class of historical scientific instruments for sky observations and its reconstruction with methods of computer graphics. Long after the stone age, in the Middle Ages, the astrolabe was the most celebrated instrument for celestial observations and has been explained in contemporary literature, usually with the help of precomputed tables for a certain size or kind of instrument. Today, historical exhibitions frequently present one of these instruments, but its various applications are hard to explain to the general audience without hands-on demonstration. For this challenge from the cultural heritage domain, an approach using the idea of procedural modelling is presented. Here, a computer graphics model is not statically drawn but specified by parametrised plotting functions, which can then be repeatedly executed with different parameters to create the final model. This approach is demonstrated to provide a very flexible solution which can immediately be applied to specific needs just by tweaking a few parameters, instead of having to repetitively draw the whole model manually. From the two-dimensional procedural model, 3D models can be easily created, and even the production of wooden instruments on a Laser engraver/plotter is demonstrated.

The third and longest part deals with methods of sky simulation and rendering in the domain of computer graphics. In this discipline, modelling of skylight and atmospheric effects has developed tremendously over the last two decades, which is covered by an extensive survey of literature from the computer graphics and also atmosphere physics domains.

The requirements of physically correct or at least plausible rendering include realistic values for sky brightness. Measurements performed with a luminance meter on a clear sky in order to verify the currently most widely used analytic skylight model [Preetham 1999] shows however its limited applicability.

There are two classical groups of clear-sky models: numerical simulations of scattering in the atmosphere, and fast analytical models. Recently, another method for more realistic looking skylight models has been developed: digital images taken with a fisheye lens are combined into high dynamic range images which can be used for scene illumination and as sky background. These images can be calibrated by photometric measurements of absolute luminance values. Long-time exposures allow to apply this system to quantitative investigations of sky brightness, sky colours, and also nocturnal light pollution by artificial illumination. Results and other applications of the system are described, and the pipeline for creating such images is described in the appendix.

This work closes with some notes of future directions of research.