This thesis presents a generic and principled solution for optimising the visual complexity of any arbitrary computer-generated virtual environment (VE). This is performed with the ultimate goal of reducing the inherent latencies of current virtual reality (VR) technology. Effectively, we wish to remove extraneous detail from an environment which the user cannot perceive, and thus modulate the graphical complexity of a VE with little or no perceptual artifacts. The work proceeds by investigating contemporary models and theories of visual perception and then applying these to the field of real-time computer graphics. Subsequently, a technique is devised to assess the perceptual content of a computer-generated image in terms of spatial frequency (c/deg), and a model of contrast sensitivity is formulated to describe a user's ability to perceive detail under various conditions in terms of this metric. This allows us to base the level of detail (LOD) of each object in a VE on a measure of the degree of spatial detail which the user can perceive at any instant (taking into consideration the size of an object, its angular velocity, and the degree to which it exists in the peripheral field). Additionally, a generic polygon simplification framework is presented to complement the use of perceptually modulated LOD. The efficient implementation of this perceptual model is discussed and a prototype system is evaluated through a suite of experiments. These include a number of low-level psychophysical studies (to evaluate the accuracy of the model), a task performance study (to evaluate the effects of the model on the user), and an analysis of system performance gain (to evaluate the effects of the model on the system). The results show that for the test application chosen, the frame rate of the simulation was manifestly improved (by four to five-fold) with no perceivable drop in image fidelity. As a result, users were able to perform the given wayfinding task more proficiently and rapidly. Finally, conclusions are drawn on the application and utility of perceptually-based optimisations; both in reference to this work, and in the wider context.

In order to test the hypothesized existence of a powerful, thermal wind in active galactic nuclei, the hydrodynamic effects of such a wind on a model galactic interstellar medium (ISM) are investigated. The properties of several model ISMs are derived from observations of the Milky Way's ISM and those of nearby spiral and elliptical galaxies. The wind is assumed to be highly supersonic with spherical symmetry and constant mechanical luminosity, L(W). The propagation of the wind into the low density gas component of the ISM is studied using the Kompaneets approximation of a strong explosion in an exponential atmosphere. Flattened gas distributions are shown to experience "blow-out" of wind gas along the symmetry axis. The results show that for typical ISM gas pressures ( < 10⁻¹¹ ergs cm⁻³), the extent of the wind can range from 1 to 10 kpc for wind luminosities of 10⁴² to 10⁴⁶ ergs sec⁻¹ in a timescale of less than 10⁷ years. The steady state wind flow pattern and an estimate of the timescale required to reach a quasi-steady state are also determined. Next, the interaction of dense, interstellar clouds with the wind is investigated. The stability and mass loss of clouds in the wind are studied and it is proposed that clouds survive the encounter with the wind over large timescales ( >10⁷ yrs.). The physical structure and motion of the clouds are calculated, showing that large clouds (10³ solar masses) can be accelerated to velocities in excess of the galactic escape velocity. Finally, it is proposed that the Narrow Emission Line Regions (NELR) of active galaxies are the result of the interaction of active nuclei photons and a thermal wind on large, interstellar clouds. The physical state of the NELR is re-examined and shown to be compatible with this hypothesis. Arguments are presented to show that the NELR clouds must be massive and unable to be accelerated to NELR velocities by photon momentum alone. A Monte Carlo-type calculation is made to determine Narrow Emission Line profiles from an ensemble of photoionized interstellar clouds in the wind. These theoretical line profiles are shown to agree reasonably well with observed line profiles.

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