3D Shape Matching and Object Reconstruction

Matching of three-dimensional models and parts of fragmented objects plays an important role in data mining of three-dimensional content and the automatic or assisted assembly and comparison of 3D models. direct applications of such methods are in the areas of computational archaeology (fragment matching and artifact assembly), molecular biology (protein docking) and partial or complementary model search.

This particular research field was the subject of my PhD thesis. The main contribution of this work has been the introduction of a generic set of algorithms for the complementary matching and assembly of arbitrary three-dimensional objects based solely on geometric information. Please visit the above link for more details and a list of related publications.

Volume Generation Algorithms

Volume generation (voxelization) is an important modeling procedure where the attributes (occupancy, albedo, density etc) of arbitrary models are sampled using a regular or irregular volumetric pattern to obtain a solid (filled) or boundary representation of a set of discete values.

I have contributed to one of the first commodity hardware-accelerated voxelization algorithms that quickly generated solid regular volumes or hulls based on the depth images of orthonormal model projections (A Fast Depth Buffer Based Voxelization Algorithm). Recently, due to the applicability of volume representations to the real-time rendering of illumination effects such as ambient occlusion and diffuse global illumination, my research team is investigating novel efficient voxelization algorithms, capable of generating multi-attribute volume data in real time. One such example is the Imperfect Voxelization algorithm, targeted specifically on GI methods.

Real-time Ambient Occlusion

Ambient occlusion (AO) is a common (and relatively low-cost) alternative to indirect lighting calculation for real-time applications. One of the challenges in RT AO methods is to perform the AO estimation for arbitrary dynamic geometry while avoiding geometry- and or view-dependent problems.

One of my recent research efforts was focused on the estimation of ambient occlusion based on real-time generation of volume representation of a scene (or parts of it). The biggest issue in this type of methods is the avoidance of self occlusion and therefore a more carefull sampling solution is required. You may find more details in the volume-based AO research page on the AUEB graphics group site.

An earlier contribution of my research group to this field was the displacement field method, which involved the precalculation, discretization and encoding of a 4D distance field associated to a rigid object relative to its bounding sphere. Using the displacement field, one could trivially query the distance to the object's surface from an arbitrary location outside the object, thus considerably speeding up occlusion (visibility) collision and radiance sampling procedures.

Real-time Global Illumination

The approximation of multi-bounce light transport in a complex three-dimensional environment is a very tedious task, especially for real-time applications. One way to attack the problem is to port generic off-line GI algorithms to the modelrn GPU architectures. A different approach is to utilize image-space data such as Reflective Shadow Maps, environment maps and screen-space information in order to perform photon splatting up to some level. A third approach is to treat the effects associated with GI separately.

Diffuse inter-reflection is the most prominent indirect illumination effect and as such, a lot of effort has been concentrated on achieving credible results using GPU-based methods. Lately, me and the AUEB Graphics Group have been conducting research on real-time approximate radiance field caching and volume-based GI techniques.

Other interesting global illumination effects that can be independently approximated involve the generation of caustics and scattering of light through participating media. We have developed a real-time method for the fast calculation and display of caustics and godrays for underwater environments, using an optimized photon-splatting technique that is capable of producing high-quality results at very high framerates.

Computational Archaeology

Virtual Archaeologist was one of the first attempts to provide the archaeologist with an integrated set of tools for measuring and comparing fragments of arbitrary objects. It summarized and integrated the algorithms that I have developed during my PhD thesis for the pairwise matching of arbitrary fragments, the global fragment clustering and fragment positioning in three-dimensional space, based solely on the geometry of their surfaces.

Real-time Audio Rendering

One of my responsibilities while working at the Foundation of the Hellenic World was to develop a high fidelity event-based sound server to drive the 7.1 surround system of the FHW virtual reality dome. The system was developed back in 2006 and is still used for all audio playback in the VR shows. Due to the large number of audio channels and sound positioning and playback events present in a VR production, the audio system aggregates requests and performs lazy updates based on perceptual metrics in order to avoid messenging clutter and frequent buffer updates.

An independent sound synthesis prototype that one of my students has developed under my supervision was Kettle, a real-time kettle-drum synthesis tool that combined an intelligent on-the-fly sample-patch sequencing and an interactive user interfacing tool based on off-the-shelf graphic tablets. More on this work can be found at here.

Virtual Reality

During my empoyment as a virtual reality systems and software engineer at the Foundation of the Hellenic World, my main contribution was the design and implementation of the computing cluster API for the distributed (multi-display) rendering, the design and implementation of the multi-display configuration and update system and the design and implementation of a multi-user interaction layer for auditorium-level interactive games.