Title Participants Abstract
"On the Use of Local Ray Termination for Efficiently Constructing Qualitative BSPs, BIHs and (S)BVHs" "Matthias Moulin, Philip Dutré" "Acceleration data structures (ADSs) exploit spatial coherence by distributing a scene’s geometric primitives into spatial groups, effectively reducing the cost of ray tracing queries. The most effective ADSs are hierarchical, adaptive tree structures such as BSPs, BIHs and (S)BVHs. The de facto standard cost metric for building these structures is the Surface Area Heuristic (SAH), which assumes a scene-exterior isotropic ray distribution of non-terminating rays. Despite its restrictive assumptions, the SAH remains competitive against many fundamentally different cost metrics targeting more common ray distributions. Our goal is not to radically change and replace the SAH, but to adapt it by introducing the concept of local ray termination in the context of voxel partitioning during the ADS construction and voxel traversal order during ADS traversal. We develop heuristics to approximate local ray termination efficiently without additional preprocessing or ray (sub)sampling. Our heuristics are used for approximating the visibility probabilities in the Ray Termination Surface Area Heuristic (RTSAH) for constructing BSPs, BIHs and (S)BVHs for accelerating closest-hit ray queries and for approximating the hit probabilities in the Shadow Ray Distribution Heuristic for constructing dedicated BVHs for accelerating any-hit ray queries. The main aim of our paper is to analyze the potential of including local ray termination into the SAH. The results indicate rendering performance close to the references (SAH and NodeSATO) on average due to small and/or compensating gains in the number of ray-triangle intersection tests and ADS node traversal steps. Furthermore, prerendering build times are higher for the RTSAH due to triangle clipping."
"Natural Media Simulation and Art-Directable Simulations for Computer Animation" "Tuur Stuyck" "Computer graphics is a vast domain within computer sciences which encompasses simulation and visualization of physical effects like the behaviour of fluids, gasses and object interactions such as collisions and deformations. Such simulations are often defined by the underlying laws and equations of physics which leave little control over the outcome of these simulations. The focus of this dissertation is twofold: first of all the high-fidelity simulation of natural media such as oil paint and secondly we construct an intuitive direct-manipulation technique for easily constructing keyframes. This research enables traditional oil paint artists to truthfully practice their skills away from their art studios. As a second benefit, it enables new users to try out oil paint at no cost and a very low barrier . Additionally, it provides a new method for animators to easily construct physically plausible keyframe shapes that can be used for controlling simulations."
"A Forward Scattering Dipole Model from a Functional Integral Approximation" "Roald Frederickx, Philip Dutré" "Rendering translucent materials with physically based Monte Carlo methods tends to be computationally expensive due to the long chains of volumetric scattering interactions. In the case of strongly forward scattering materials, the problem gets compounded since each scattering interaction becomes highly anisotropic and near-specular. Various well-known approaches try to avoid the resulting sampling problem through analytical approximations based on diffusion theory. Although these methods are computationally efficient, their assumption of diffusive, isotropic scattering can lead to considerable errors when rendering forward scattering materials, even in the optically dense limit. In this paper, we present an analytical subsurface scattering model, derived with the explicit assumption of strong forward scattering. Our model is not based on diffusion theory, but follows from a connection that we identified between the functional integral formulation of radiative transport and the partition function of a worm-like chain in polymer physics. Our resulting model does not need a separate Monte Carlo solution for unscattered or single-scattered contributions, nor does it require ad-hoc regularization procedures. It has a single singularity by design, corresponding to the initial unscattered propagation, which can be accounted for by the extensive analytical importance sampling scheme that we provide. Our model captures the full behaviour of forward scattering media, ranging from unscattered straight-line propagation to the fully diffusive limit. Moreover, we derive a novel forward scattering BRDF as limiting case of our subsurface scattering model, which can be used in a level of detail hierarchy. We show how our model can be integrated in existing Monte Carlo rendering algorithms, and make comparisons to previous approaches."
"Reflection reprojection using temporal coherence" "Philip Dutré" "A powerful approach for rendering high-quality images at low cost is to exploit temporal coherence by projecting already computed images into a novel view. However, conventional temporal coherence projection methods assume pixel values remain almost unchanged from frame to frame, which does not extend well to reflection rendering. We present a novel projection method to reuse reflections from adjacent frames. A novel reflection reprojection method is introduced to establish the mapping of reflections between individual frames. By reusing the information from the reference frame, our method can reduce the overall workloads of reflection computation, which makes rendering efficiently."
"Real-Time Oil Painting on Mobile Hardware" "Tuur Stuyck, Philip Dutré" "This paper presents a realistic digital oil painting system, specifically targeted at the real-time performance on highly resource constrained portable hardware such as tablets and iPads. To effectively use the limited computing power, we develop an efficient adaptation of the shallow water equations that models all the characteristic properties of oil paint. The pigments are stored in a multi-layered structure to model the peculiar nature of pigment mixing in oil paint. The user experience ranges from thick shape-retaining strokes to runny diluted paint that reacts naturally to the gravity set by tablet orientation. Finally, the paint is rendered in real time using a combination of carefully chosen efficient rendering techniques. The virtual lighting adapts to the tablet orientation, or alternatively, the front-facing camera captures the lighting environment, which leads to a truly immersive user experience. Our proposed features are evaluated via a user study. In our experience, our system enables artists to quickly try out ideas and compositions anywhere when inspiration strikes, in a truly ubiquitous way. They do not need to carry expensive and messy oil paint supplies."
"Digital Painting Classroom: Learning Oil Painting Using a Tablet" "Tuur Stuyck, Philip Dutré" "We investigate how the use of a real-time physically plausible oil paint simulator implemented on mobile hardware can be used to teach novice painters the fundamentals of the art and how more experienced painters can use it as a cheap and fast experimental tool. The paint system believably recreates 3D oil painting with a number of new user-interactions. The paint reacts to the gravity vector set by the tablet and the paint is rendered using the ambient light captured by the front facing camera. The user experience ranges from thin, diluted watercolor paint to thick impasto oil paint allowing to recreate a variety of traditional painting styles digitally."
"Sculpting Fluids: A New and Intuitive Approach to Art-Directable Fluids" "Tuur Stuyck, Philip Dutré" "Fluid simulations are very useful for creating physically based water effects in computer graphics but are notoriously hard to control. In this talk we propose a novel and intuitive animation technique for fluid animations using interactive direct manipulation of the simulated fluid inspired by clay sculpting. Artists can simply shape the fluid directly into the desired visual effect whilst the fluid still adheres to its physical properties such as surface tension and volume preservation. Our approach is faster and much more intuitive compared to previous work which relies on indirect approaches such as providing reference geometry or density fields. It makes it very easy, even for novice users, to modify simulations ranging from enlarging splashes or altering droplet shapes to adjusting the flow of a large fluid body. The sculpted fluid shapes are incorporated into the simulation using guided re-simulation using control theory instead of simply using geometric deformations resulting in natural-looking animations."
"Line Sampling for Direct Illumination" "Niels Billen, Philip Dutré" "Computing direct illumination efficiently is still a problem of major significance in computer graphics. The evaluation involves an integral over the surface areas of the light sources in the scene. Because this integral typically features many discontinuities, introduced by the visibility term and complex material functions, Monte Carlo integration is one of the only general techniques that can be used to compute the integral. In this paper, we propose to evaluate the direct illumination using line samples instead of point samples. A direct consequence of line sampling is that the two-dimensional integral over the area of the light source is reduced to a one-dimensional integral. We exploit this dimensional reduction by relying on the property that commonly used sampling patterns, such as stratified sampling and low-discrepancy sequences, converge faster when the dimension of the integration domain is reduced. We show that, while line sampling is generally more computationally intensive than point sampling, the variance of a line sample is smaller than that of a point sample, resulting in a higher order of convergence."
"Thinking and Conscious Machines?" "Philip Dutré"
"Efficient Visibility Heuristics for kd-trees Using the RTSAH" "Matthias Moulin, Niels Billen, Philip Dutré" "Acceleration data structures such as kd-trees aim at reducing the per-ray cost which is crucial for rendering performance. The de-facto standard for constructing kd-trees, the Surface Area Heuristic (SAH), does not take ray termination into account and instead assumes rays never hit a geometric primitive. The Ray Termination Surface Area Heuristic (RTSAH) is a cost metric originally used for determining the traversal order of the voxels for occlusion rays that takes ray termination into account. We adapt this RTSAH to building kd-trees that aim at reducing the per-ray cost of rays. Our build procedure has the same overall computational complexity and considers the same finite set of splitting planes as the SAH. By taking ray termination into account, we favor cutting off child voxels which are not or hardly visible to each other. This results in fundamentally different and more qualitative kd-trees compared to the SAH."