My name is Andreas-Alexandros Vasilakis and I was born on October 12, 1983, in Corfu, Greece. I received my PhD on the field of Computer Graphics from the Department of Computer Science & Engineering of the University of Ioannina in Greece, under the supervision of Prof. Ioannis Fudos. My PhD studies were supported by a scholarship from the Heraclitus II grant through the operational programme "Education and Lifelong Learning" through the European Social Fund, 2010-2013. I have also received BSc and MSc Degrees from the same institution in 2006 and 2008, respectively.
We introduce a novel approach to support fast and efficient lossy compression of arbitrary animation sequences ideally suited for real-time scenarios, such as streaming and content creation applications, where input is not known a-priori and is dynamically generated. The presented method exploits temporal coherence by altering the principal component analysis (PCA) procedure from a batch- to an adaptive-basis aiming to simultaneously support three important, generally conflicting in prior art, objectives: fast compression times, reduced memory requirements and high quality reproduction results. To that end, we show how the problem of tracking subspaces via adaptive orthogonal iterations can be successfully applied to support bandwidth- as well as error-consistent encoding of sequentially processed animated data. A dynamic compression pipeline is presented that can efficiently approximate the k-largest PCA bases based on the previous iteration (frame block) at a significantly lower complexity than directly computing the singular value decomposition. To avoid under-fitting when a fixed number of basis vectors is used for all frame blocks, a flexible solution that automatically identifies the optimal subspace size for each one is also offered. An extensive experimental study is finally offered showing that our method is superior in terms of performance as compared to several direct PCA-based schemes while, at the same time, achieves plausible reconstruction output despite the constraints posed by arbitrarily complex animated scenarios.
Depth-sorted fragment determination is fundamental for a host of image-based techniques which simulates complex rendering effects. It is also a challenging task in terms of time and space required when rasterizing scenes with high depth complexity. When low graphics memory requirements are of utmost importance, k-buffer can objectively be considered as the most preferred framework which advantageously ensures the correct depth order on a subset of all generated fragments. Although various alternatives have been introduced to partially or completely alleviate the noticeable quality artifacts produced by the initial k-buffer algorithm in the expense of memory increase or performance downgrade, appropriate tools to automatically and dynamically compute the most suitable value of k are still missing. To this end, we introduce k+-buffer, a fast framework that accurately simulates the behavior of k-buffer in a single rendering pass. Two memory-bounded data structures: (i) the max-array and (ii) the max-heap are developed on the GPU to concurrently maintain the k-foremost fragments per pixel by exploring pixel synchronization and fragment culling. Memory-friendly strategies are further introduced to dynamically (a) lessen the wasteful memory allocation of individual pixels with low depth complexity frequencies, (b) minimize the allocated size of k-buffer according to different application goals and hardware limitations via a straightforward depth histogram analysis and (c) manage local GPU cache with a fixed-memory depth-sorting mechanism. Finally, an extensive experimental evaluation is provided demonstrating the advantages of our work over all prior k-buffer variants in terms of memory usage, performance cost and image quality.
Many applications require operations on multiple fragments that result from ray casting at the same pixel location. To this end, several approaches have been introduced that process for each pixel one or more fragments per rendering pass, so as to produce a multifragment effect. However, multifragment rasterization is susceptible to flickering artifacts when two or more visible fragments of the scene have identical depth values. This phenomenon is called coplanarity or Z-fighting and incurs various unpleasant and unintuitive results when rendering complex multilayer scenes. In this work, we develop depth-fighting aware algorithms for reducing, eliminating and/or detecting related flaws in scenes suffering from duplicate geometry. We adapt previously presented single and multipass rendering methods, providing alternatives for both commodity and modern graphics hardware. We report on the efficiency and robustness of all these alternatives and provide comprehensive comparison results. Finally, visual results are offered illustrating the effectiveness of our variants for a number of applications where depth accuracy and order are of critical importance.
We explore different semantics for the solid defined by a self-crossing surface (immersed sub-manifold). Specifically, we introduce rules for the interior/exterior classification of the connected components of the complement of a self-crossing surface produced through a continuous deformation process of an initial embedded manifold. We propose efficient GPU algorithms for rendering the boundary of the regularized union of the interior components, which is a subset of the initial surface and is called the trimmed boundary or simply the trim. This classification and rendering process is accomplished in realtime through a rasterization process without computing any self-intersection curve, and hence is suited to support animations of self-crossing surfaces. The solid bounded by the trim can be combined with other solids and with half-spaces using Boolean operations and hence may be capped (trimmed by a half-space) or used as a primitive in direct CSG rendering. Being able to render the trim in realtime makes it possible to adapt the tessellation of the trim in realtime by using view-dependent levels-of-details or adaptive subdivision.
In this paper, we present a skeletal rigid skinning approach. First, we describe a skeleton extraction technique that produces refined skeletons appropriate for animation from decomposed character models. Then, to avoid the artifacts generated in previous skinning approaches and the associated high training costs, we develop an efficient and robust rigid skinning technique that applies blending patches around joints. To achieve real time animation, we have adapted all steps of our rigid skinning algorithm so that they are performed efficiently on the GPU. Finally, we present an evaluation of our methods against four criteria: efficiency, quality, scope, and robustness.
Successfully predicting visual attention can significantly improve many aspects of computer graphics and games. Despite the thorough investigation in this area, selective rendering has not addressed so far fragment visibility determination problems. To this end, we present the first ''selective multi-fragment rendering'' solution that alters the classic k-buffer construction procedure from a fixed-k to a variable-k per-pixel fragment allocation guided by an importance-driven model. Given a fixed memory budget, the idea is to allocate more fragment layers in parts of the image that need them most or contribute more significantly to the visual result. An importance map, dynamically estimated per frame based on several criteria, is used for the distribution of the fragment layers across the image. We illustrate the effectiveness and quality superiority of our approach in comparison to previous methods when performing order-independent transparency rendering in various, high depth-complexity, scenarios.
✝ These authors contributed equally to this work.
In computer graphics, animation compression is essential for efficient storage, streaming and reproduction of animated meshes. Previous work has presented efficient techniques for compression using skinning transformations to derive the animated mesh from a reference pose. We present a pose-to-pose approach to skinning animated meshes by observing that only small deformation variations will normally occur between consecutive poses. The transformations are applied so that a new pose is derived by deforming the geometry of the previous pose, thus maintaining temporal coherence in the parameter space, reducing approximation error and facilitating forward propagated editing of arbitrary poses.
We introduce a novel approach to image-space ray tracing ideally suited for the photorealistic synthesis of fully dynamic environments at interactive frame rates. Our method, designed entirely on the rasterization pipeline, alters the acceleration data structure construction from a per-fragment to a per-primitive basis in order to simultaneously support three important, generally conflicting in prior art, objectives: fast construction times, analytic intersection tests and reduced memory requirements. In every frame, our algorithm operates in two stages: A compact representation of the scene geometry is built based on primitive linked-lists, followed by a traversal step that decouples the ray-primitive intersection tests from the illumination calculations; a process inspired by deferred rendering and the path integral formulation of light transport. Efficient empty space skipping is achieved by exploiting several culling optimizations both in xy- and z-space, such as pixel frustum clipping, depth subdivision and lossless buffer down-scaling. An extensive experimental study is finally offered showing that our method advances the area of image-based ray tracing under the constraints posed by arbitrarily complex and animated scenarios.
We introduce a generic method for interactive ray tracing, able to support complex and dynamic environments, without the need for precomputations or the maintenance of additional spatial data structures. Our method, which relies entirely on the rasterization pipeline, stores fragment information for the entire scene on a multiview and multilayer structure and marches through depth layers to capture both near and distant information for illumination computations. Ray tracing is efficiently achieved by concurrently traversing a novel cube-mapped A-buffer variant in image space that exploits GPU-accelerated double linked lists, decoupled storage, uniform depth subdivision and empty space skipping on a per-fragment basis. We illustrate the effectiveness and quality of our approach on path tracing and ambient occlusion implementations in scenarios, where full scene coverage is of major importance. Finally, we report on the performance and memory usage of our pipeline and compare it against GPGPU ray tracing approaches.
In this work, we investigate an efficient approach to treat fragment racing when computing k-nearest fragments. Based on the observation that knowing the depth position of the k-th fragment we can optimally find the k-closest fragments, we introduce a novel fragment culling component by employing occupancy maps. Without any software redesign, the proposed scheme can easily be attached at any k-buffer pipeline to efficiently perform early-z culling. Finally, we report on the efficiency, memory space, and robustness of the upgraded k-buffer alternatives providing comprehensive comparison results.
In this work, we investigate an efficient approach to treat fragment racing when computing k-nearest fragments. Based on the observation that knowing the depth position of the k-th fragment we can optimally find the k-closest ones, we introduce a novel orderindependent fragment culling component, easily attached to the k+ buffer pipeline. An additional rendering pass of the scene’s geometry is initially employed to construct a per pixel binary fragment occupancy discretization. Then, the nearest depth of the k-th per pixel fragment is concurrently computed by performing bit counting operations and subsequently utilized to perform early-z rejection for the k+ buffer construction process that follows. Any fragment with depth larger than this value will fail the depth test, avoiding the cost of its pixel shading execution. Note that no software modifications are required to the actual k+ buffer implementation.
We report on the development of a novel interactive augmented reality app called AR-TagBrowse, built on Unity 3D that enables users to tag and browse 3D objects. Users upload 3D objects (polygonal representation and diffuse maps) through a web server. 3D objects are then linked to real world information such as images and GPS location. Users may optionally segment the objects into areas of interest. Such objects will subsequently pop up in the AR-TagBrowse app when one of these events is detected (visible location or image). The user is then capable of interactively viewing the 3D object, browsing tags or entering new tags providing comments or information for specific parts of the object.
k-buffer facilitates novel approaches to multi-fragment rendering and visualization for developing interactive applications on the GPU. Various alternatives have been proposed to alleviate its memory hazards and to avoid completely or partially the necessity of geometry pre-sorting. However, that came with the burden of excessive memory allocation and depth precision artifacts. We introduce k+-buffer, a fast and accurate framework that simulates the k-buffer behavior by exploiting fragment culling and pixel synchronization. Two GPU-accelerated data structures have been developed: (i) the max-array and (ii) the max-heap. These memory-bounded data structures accurately maintain the k-foremost fragments per pixel in a single geometry pass. The choice of the data structure depends on the size k (application-dependent). Without any software-redesign, the proposed scheme can be adapted to perform as a Z-buffer or an A-buffer capturing a single or all generated fragments, respectively. A memory-friendly strategy is also proposed, extending the proposed pipeline to dynamically lessen the potential wasteful memory allocation. Finally, an extensive experimental evaluation is provided demonstrating the advantages of k+-buffer over all prior k-buffer variants in terms of memory usage, performance cost and image quality.
This work introduces S-buffer, an efficient and memory-friendly gpu-accelerated A-buffer architecture for multi-fragment rendering. Memory is organized into variable contiguous regions for each pixel, thus avoiding limitations set in linked-lists and fixed-array techniques. S-buffer exploits fragment distribution for precise allocation of the needed storage and pixel sparsity (empty pixel ratio) for computing the memory offsets for each pixel in a parallel fashion. An experimental comparative evaluation of our technique over previous multi-fragment rendering approaches in terms of memory and performance is provided.
Efficient capturing of the entire topological and geometric information of a 3D scene is an important feature in many graphics applications for rendering multi-fragment effects. Example applications include order independent transparency, volume rendering, CSG rendering, trimming, and shadow mapping all of which require operations on more than one fragment per pixel location. An influential multi-pass technique is front-to-back (F2B) depth peeling which works by peeling off a single fragment per pass and by exploiting the GPU capabilities to accumulate the final result. The major drawback of this peeling algorithm is that fragment layers with depth identical to the fragment depth detected in the previous pass are discarded and so not peeled. Stencil Routed A-buffer (SRAB) treats z-fighting for sorted fragments. However, SRAB is limited by the resolution of the stencil buffer and is incompatible with hardware supported multisample antialiasing. k-buffer processes k fragments in a single pass, thus performing up to k times faster than F2B. k-buffer suffers from read-modify-write hazards and needs a small fixed amount of additional memory which is allocated in the form of multi render target buffers. Similarly to SRAB, k-buffer requires a pre-sorting of the primitives of the scene to treat correctly up to k Z-fighting fragments. In this work, we introduce a novel technique for commodity graphics hardware that completely treats Z-fighting by extending F2B depth peeling with the overhead of one extra geometry pass. To speed up depth peeling at scenes with large number of layers with same depth values, we also propose an approximate z-fighting free depth peeling technique that combines the F2B and the k-buffer algorithms.
In computer animation, key-frame compression is essential for efficient storage, processing and reproduction of animation sequences. Previous work has presented efficient techniques for compression using affine or rigid transformations to derive the skin from the initial pose using a relatively small number of control joints. We present a novel pose-to-pose approach to skinning animated meshes by observing that only small deformation variations will normally occur between sequential poses. The transformations are applied so as a new pose is derived by transforming the vertices of the previous pose, thus maintaining temporal coherence in the parameter space, reducing error and enabling a novel forward propagated editing of arbitrary animation frames.
Skeleton-based skinning is widely used for realistic animation of complex characters defining mesh movement as a function of the underlying skeleton. In this paper, we propose a new robust skeletal animation framework for 3D articulated models. The contribution of this work is twofold. First, we present refinement techniques for improving skeletal representation based on local characteristics which are extracted using centroids and principal axes of the character’s components. Then, we use rigid skinning deformations to achieve realistic motion avoiding vertex weights. A novel method eliminates the artifacts caused by self-intersections, providing sufficiently smooth skin deformation.
This report presents a thorough investigation of the complex and active research area in both interactive global illumination (GI) and inverse lighting (IL) problems, with a focus on interactive applications and dynamic environments.
This thesis studies the problem of direct rendering skinned approximations of arbitrary deformable objects which may also self-intersect on the graphics hardware, which is an important topic in computer animation and visualization. First, we provide efficient methodologies for editable segmentation and skinning representations of arbitrary animated mesh sequences that exploit temporal coherence from a pose-to-pose perspective. Second, we develop rendering algorithms for efficient detection and trimming of (self)-crossing surfaces in the image-space, realized through novel multi-fragment rasterization, without computing any intersections. Since capturing multiple fragments efficiently on the GPU is a challenging task in terms of time, memory and robustness, we study several aspects of the multi-fragment rendering problem from various perspectives and present alternatives for reducing fragment-contention, eliminating z-fighting and avoiding fragment-overflow.
In this dissertation, we propose a novel robust skeleton-based animation framework of articulated modular solid objects. The contribution of this work is twofold. First, we present refinement techniques for improving the skeletal representation based on local characteristics which are extracted using centroids and principal axes of the components of the character. Skeleton-based animation is then performed using forward kinematics and quaternions. The components position varies over time, guided by an animation controller. Then, we use rigid skinning deformations by assigning each skin vertex one driver bone to achieve realistic skin motion avoiding vertex weights. A novel method eliminates the artifacts caused by self-intersections, especially in areas near joints, providing sufficiently smooth skin deformation. Finally, we have implemented all the above steps and we perform an extensive experimental evaluation of our suite of techniques with respect to efficiency, robustness and quality of the final animation outcome.
This document contains a comprehensive list of (currently active) annual international events and premier publishers of science and technology resources: link.
A collection of different 3D levels for Gravity Ball; a marker-based Augmented Reality game developed in Frailsafe project, targeted for mobile devices. The goal is to guide a virtual sphere into the level’s hole, the finishing point, as fast and as steadily as possible by moving the tangible handheld marker (virtual textured terrain) accordingly.
3D digitization of a Belem Tower souvenir; a delicate cultural heritage object bought from Lisboa, Portugal. This task included the digital recording via a 3D handhold laser scanner as well as the data processing of the digitized object, which mainly involves geometric data repairing & fairing.
A point cloud animation that consists of three disjointed subsequences. Each one is generated by moving (morphing) from one number into another with the aim of forming the word ''2017''. Each number represents a keyed particle system that randomly place points inside its volume. This animation was created via Blender Modeling Software.
Morphing 2017 5000 Vertices 600 Frames 30 MB
A highly complex animation genereted after applying a number of concurrent local self-crossing deformation operations (free-form deformations in conjunction with Laplacian smoothing) on a jug object.
Self-intersecting Jug 7478 Vertices 500 Frames 101 MB