ACM CCS Research Articles

Fast Surface Modelling Using a 6th Order PDE

Abstract Although the control‐point based parametric approach is used most widely in free‐form surface modelling, complementary techniques co‐exist to meet various specialised requirements. The partial differential equation (PDE) based modelling approach is especially suitable for satisfying surface boundary constraints. They are also effective for the generation of families of free‐form surfaces, which share a common base and differ in their secondary features. In this paper, we present a fast surface modelling method using a sixth order PDE. This PDE provides enough degrees of freedom not only to accommodate tangent, but also curvature boundary conditions and offers more shape control parameters to serve as user controls for the manipulation of surface shapes. In order to achieve real‐time performance, we have constructed a surface function and developed a high‐precision approximate solution to the 6th order PDE. Unlike some existing PDE‐based techniques, this resolution method can satisfy the boundary conditions exactly, and is able to create free‐form surfaces as fast and almost as accurately as the closed‐form (analytical) solutions. Due to the fact that it has sufficient degrees of freedom to accommodate the continuity of 3‐sided and 4‐sided surface patches at their boundaries, this method is able to model complex surfaces consisting of multiple patches. Compared with existing PDE‐based modelling methods, this method is both fast and can solve a larger class of surface modelling problems.Categories and Subject Descriptors (according to ACM CCS): I.3.5 [Computer Graphics]: Curves, surfaces, solid, and object representations; physically based modelling

Balancing Considered Harmful — Faster Photon Mapping using the Voxel Volume Heuristic —

Abstract Photon mapping is one of the most important algorithms for computing global illumination. Especially for efficiently producing convincing caustics, there are no real alternatives to photon mapping. On the other hand, photon mapping is also quite costly: Each radiance lookup requires to find the k nearest neighbors in a kd‐tree, which can be more costly than shooting several rays. Therefore, the nearest‐neighbor queries often dominate the rendering time of a photon map based renderer. In this paper, we present a method that reorganizes — i.e. un balances — the kd‐tree for storing the photons in a way that allows for finding the k‐nearest neighbors much more efficiently, thereby accelerating the radiance estimates by a factor of 1.2–3.4. Most importantly, our method still finds exactly the same k‐nearest‐neighbors as the original method, without introducing any approximations or loss of accuracy. The impact of our method is demonstrated with several practical examples.Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Global Illumination I.3.7 [Computer Graphics]: Raytracing

A Biophysically‐Based Spectral Model of Light Interaction with Human Skin

Abstract Despite the notable progress in physically‐based rendering, there is still a long way to go before we can automatically generate predictable images of biological materials. In this paper, we address an open problem in this area, namely the spectral simulation of light interaction with human skin. We propose a novel biophysically based model that accounts for all components of light propagation in skin tissues, namely surface reflectance, subsurface reflectance and transmittance, and the biological mechanisms of light absorption by pigments in these tissues. The model is controlled by biologically meaningful parameters, and its formulation, based on standard Monte Carlo techniques, enables its straightforward incorporation into realistic image synthesis frameworks. Besides its biophysically‐based nature, the key difference between the proposed model and the existing skin models is its comprehensiveness, i.e., it computes both spectral (reflectance and transmittance) and scattering (bidirectional surface‐scattering distribution function) quantities for skin specimens. In order to assess the predictability of our simulations, we evaluate their accuracy by comparing results from the model with actual skin measured data. We also present computer generated images to illustrate the flexibility of the proposed model with respect to variations in the biological input data, and its applicability not only in the predictive image synthesis of different skin tones, but also in the spectral simulation of medical conditions. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism

DiFi: Fast 3D Distance Field Computation Using Graphics Hardware

Abstract We present an algorithm for fast computation of discretized 3D distance fields using graphics hardware. Given a set of primitives and a distance metric, our algorithm computes the distance field for each slice of a uniform spatial grid baly rasterizing the distance functions of the primitives. We compute bounds on the spatial extent of the Voronoi region of each primitive. These bounds are used to cull and clamp the distance functions rendered for each slice. Our algorithm is applicable to all geometric models and does not make any assumptions about connectivity or a manifold representation. We have used our algorithm to compute distance fields of large models composed of tens of thousands of primitives on high resolution grids. Moreover, we demonstrate its application to medial axis evaluation and proximity computations. As compared to earlier approaches, we are able to achieve an order of magnitude improvement in the running time. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Distance fields, Voronoi regions, graphics hardware, proximity computations

Scalable behaviors for crowd simulation

Abstract Crowd simulation for virtual environments offers many challenges centered on the trade‐offs between rich behavior, control and computational cost. In this paper we present a new approach to controlling the behavior of agents in a crowd. Our method is scalable in the sense that increasingly complex crowd behaviors can be created without a corresponding increase in the complexity of the agents. Our approach is also more authorable; users can dynamically specify which crowd behaviors happen in various parts of an environment. Finally, the character motion produced by our system is visually convincing. We achieve our aims with a situation‐based control structure. Basic agents have very limited behaviors. As they enter new situations, additional, situation‐specific behaviors are composed on the fly to enable agents to respond appropriately. The composition is done using a probabilistic mechanism. We demonstrate our system with three environments including a city street and a theater.Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Animation

Exchanging Faces in Images

Abstract Pasting somebody's face into an existing image with traditional photo retouching and digital image processing tools has only been possible if both images show the face from the same viewpoint and with the same illumination. However, this is rarely the case for given pairs of images. We present a system that exchanges faces across large differences in viewpoint and illumination. It is based on an algorithm that estimates 3D shape and texture along with all relevant scene parameters, such as pose and lighting, from single images. Manual interaction is reduced to clicking on a set of about 7 feature points, and marking the hairline in the target image. The system can be used for image processing, virtual try‐on of hairstyles, and face recognition. By separating face identity from imaging conditions, our approach provides an abstract representation of images and a novel, high‐level tool for image manipulation. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Animation

Virtual Drypoint by a Model‐driven Strategy

Abstract In this paper we propose a method to synthesize a virtual copperplate print image based on physical model as a non‐photorealistic rendering in Computer Graphics. In particular we pick out the drypoint from many existing copperplate printing techniques. A drypoint print has a remarkable feature that ridged burrs are arising out of the copperplate and the printed lines are slightly ragged and fuzzy. Our method consists of four sub‐processes: plate making, ink applying, wiping and printing. Each process simulates the physical actions and phenomena observed in the actual copperplate printing. In this study we use a small‐scale print and an actual print created by a real copperplate artist for some experiments to indicate effects and availability of proposed method to synthesize a print image with the feature of drypoint.Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism

A Physically‐Based Model for Rendering Realistic Scratches

Abstract Individually visible scratches, also called isolated scratches, are very common in real world surfaces. Although their microgeometry is not visible, they are individually perceptible by the human eye, lying into a representation scale between BRDF and texture. In order to simulate this kind of scratches in synthetic images we need to know their position over the surface (texture scale), so we can determine where to use the specific scratch BRDF instead of the ordinary surface BRDF. Computing the BRDF of a scratch is difficult because it depends on the scratch's invisible microgeometry. In this paper, we propose a new physically based model to derive this microgeometry by simulating the formation process of scratches. We allow specifying intuitively the parameters involved in the process such as the scratching tool, the penetration forces, and the material properties of the object. From these parameters, we derive the microgeometries of the scratches by taking into account the real behaviour of the process. This behaviour has been determined by analysing existing models in the field of materials engineering and some ``scratch tests'' that we performed on metals. Our method has the advantages of easily simulating scratches with a wide range of microgeometries and taking into account the variability of their microgeometry along the scratch path. Another contribution is related to the location of the scratches over the surface. Instead of using an image of the paths as in previous work, we present a new representation based on curves defining the paths. This offers an independence on the image resolution or the distance from the observer and accurately provides the scratch direction in order to compute scratch BRDFs. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Color, shading, shadowing, and texture.

Computing Maximal Tiles and Application to Impostor‐Based Simplification

Abstract The computation of the largest planar region approximating a 3D object is an important problem with wide applications in modeling and rendering. Given a voxelization of the 3D object, we propose an efficient algorithm to solve a discrete version of this problem. The input of the algorithm is the set of grid edges connecting the interior and the exterior of the object (called sticks). Using a voting‐based approach, we compute the plane that slices the largest number of sticks and is orientation‐compatible with these sticks. The robustness and efficiency of our approach rests on the use of two different parameterizations of the planes with suitable properties. The first of these is exact and is used to retrieve precomputed local solutions of the problem. The second one is discrete and is used in a hierarchical voting scheme to compute the global maximum. This problem has diverse applications that range from finding object signatures to generating simplified models. Here we demonstrate the merits of the algorithm for efficiently computing an optimized set of textured impostors for a given polygonal model. Categories and Subject Descriptors (according to ACM CCS): I.3.5 [Computer Graphics]: Computational Geometry and Object Modeling

Point Cloud Collision Detection

AbstractIn the past few years, many efficient rendering and surface reconstruction algorithms for point clouds have been developed. However, collision detection of point clouds has not been considered until now, although this is a prerequisite to use them for interactive or animated 3D graphics.We present a novel approach for time‐critical collision detection of point clouds. Based solely on the point representation, it can detect intersections of the underlying implicit surfaces. The surfaces do not need to be closed.We construct a point hierarchy where each node stores a sufficient sample of the points plus a sphere covering of a part of the surface. These are used to derive criteria that guide our hierarchy traversal so as to increase convergence. One of them can be used to prune pairs of nodes, the other one is used to prioritize still to be visited pairs of nodes. At the leaves we efficiently determine an intersection by estimating the smallest distance.We have tested our implementation for several large point cloud models. The results show that a very fast and precise answer to collision detection queries can always be given.Categories and Subject Descriptors (according to ACM CCS): G.1.2 [Numerical Analysis]: Approximation of surfaces and contours I.3.5 [Computer Graphics]: Computational Geometry and Object‐Modeling[Geometric algorithms, languages and systems; object hierarchy; physically‐based modeling] I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism[Animation; virtual reality]

High Quality Hatching

Abstract Hatching lines are often used in line illustrations to convey tone and texture of a surface. In this paper we present methods to generate hatching lines from polygonal meshes and render them in high quality either at interactive rates for on‐screen display or for reproduction in print. Our approach is based on local curvature information that is integrated to form streamlines on the surface of the mesh. We use a new algorithm that provides an even distribution of these lines. A special processing of these streamlines ensures high quality line rendering for both intended output media later on. While the streamlines are generated in a preprocessing stage, hatching lines are rendered either for vector‐based printer output or on‐screen display, the latter allowing for interaction in terms of changing the view parameters or manipulating the entire line shading model at run‐time using a virtual machine. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Picture/Image Generation—Display algorithms; I.3.3 [Computer Graphics]: Picture/Image Generation—Line and curve generation image

Deferred Splatting

Abstract In recent years it has been shown that, above a certain complexity, points become the most efficient rendering primitives. Although the programmability of the lastest graphics hardware allows efficient implementation of high quality surface splatting algorithms, their performance remains below those obtained with simpler point based rendering algorithms when they are used for scenes of high complexity. In this paper, our goal is to apply high quality point based rendering algorithms on complex scenes. For this purpose, we show how to take advantage of temporal coherency in a very accurate hardware accelerated point selection algorithm allowing the expensive computations to be peformed only on visible points. Our algorithm is based on a multi‐pass hardware accelerated EWA splatting. It is also suitable for any rendering application since no pre‐process is needed and no assumption is made on the data structure. In addition, we briefly discuss the association of our method with other existing culling techniques and optimization for particular applications. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Viewing algorithms

GPU‐Based Nonlinear Ray Tracing

Abstract In this paper, we present a mapping of nonlinear ray tracing to the GPU which avoids any data transfer back to main memory. The rendering process consists of the following parts: ray setup according to the camera parameters, ray integration, ray‐object intersection, and local illumination. Bent rays are approximated by polygonal lines that are represented by textures. Ray integration is based on an iterative numerical solution of ordinary differential equations whose initial values are determined during ray setup. To improve the rendering performance, we propose acceleration techniques such as early ray termination and adaptive ray integration. Finally, we discuss a variety of applications that range from the visualization of dynamical systems to the general relativistic visualization in astrophysics and the rendering of the continuous refraction in media with varying density. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Picture/Image Generation I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism

A System for View‐Dependent Animation

Abstract In this paper, we present a novel system for facilitating the creation of stylized view‐dependent 3D animation. Our system harnesses the skill and intuition of a traditionally trained animator by providing a convivial sketch based 2D to 3D interface. A base mesh model of the character can be modified to match closely to an input sketch, with minimal user interaction. To do this, we recover the best camera from the intended view direction in the sketch using robust computer vision techniques. This aligns the mesh model with the sketch. We then deform the 3D character in two stages ‐ first we reconstruct the best matching skeletal pose from the sketch and then we deform the mesh geometry. We introduce techniques to incorporate deformations in the view‐dependent setting. This allows us to set up view‐dependent models for animation. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism ‐ Animation Our system takes as input a sketch (a), and a base mesh model (b), then recovers a camera to orient the base mesh (c), then reconstructs the skeleton pose (d), and finally deforms the mesh to find the best possible match with the sketch (e). image

Rendering with Spherical Radiance Transport Maps

Abstract In this paper, we propose a real‐time method for rendering soft shadows and inter‐reflections of dynamic objects under complex illumination. In previous methods, many efforts were taken to acquire occlusion and reflection informations for dynamic scene on the fly, and the result image cannot be generated in real time. In our approach, these informations for each object are pre‐computed and stored in well‐defined Spherical Radiance Transport Maps (SRTMs). For distant complex illumination such as environment illumination and area light source, we decompose the illumination to several hundred directional lights. In rendering, we search in SRTMs for occlusion info which may cause shadows, and reflection info which may cause inter‐reflections. Finally we produce realistic soft shadows and inter‐reflections efficiently. Our method is related with but different from previous Pre‐computed Radiance Transfer techniques which are only suitable for static scene. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: shading and shadowing

Dye Advection Without the Blur: A Level‐Set Approach for Texture‐Based Visualization of Unsteady Flow

Abstract Dye advection is an intuitive and versatile technique to visualize both steady and unsteady flow. Dye can be easily combined with noise‐based dense vector field representations and is an important element in user‐centric visual exploration processes. However, fast texture‐based implementations of dye advection rely on linear interpolation operations that lead to severe diffusion artifacts. In this paper, a novel approach for dye advection is proposed to avoid this blurring and to achieve long and clearly defined streaklines or extended streak‐like patterns. The interface between dye and background is modeled as a level‐set within a signed distance field. The level‐set evolution is governed by the underlying flow field and is computed by a semi‐Lagrangian method. A reinitialization technique is used to counteract the distortions introduced by the level‐set evolution and to maintain a level‐set function that represents a local distance field. This approach works for 2D and 3D flow fields alike. It is demonstrated how the texture‐based level‐set representation lends itself to an efficient GPU implementation and therefore facilitates interactive visualization. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Picture/Image Generation I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism

Implicit Visualization and Inverse Modeling of Growing Trees

Abstract A method is proposed for photo‐realistic modeling and visualization of a growing tree. Recent visualization methods have focused on producing smoothly blending branching structures, however, these methods fail to account for the inclusion of non‐smooth features such as branch bark ridges and bud scale scars. These features constitute an important visual aspect of a living tree, and are also observed to vary over time. The proposed method incorporates these features by using an hierarchical implicit modeling system, which provides a variety of tools for combining surface components in both smooth and non smooth configurations. A procedural interface to this system supports the use of inverse modeling, which is a global‐to‐local methodology, where the local properties of plant organs are described in terms of their global position within the tree architecture. Inverse modeling is used to define both the tree structure at any time, and a continuous developmental sequence for the tree from a seedling. These techniques provide an intuitive paradigm for the definition of complex tree growth sequences and their subsequent visualization, based solely on observed phenomena. Thus, a key advantage is that they do not require any knowledge of, or simulation of, the underlying biological processes.Categories and Subject Descriptors (according to ACM CCS): I.3.5 [Computer Graphics]: Curve, surface, solid, and object representations I.3.7 [Computer Graphics]: Animation

Real‐time Rigid Body Simulation for Haptic Interactions Based on Contact Volume of Polygonal Objects

Abstract This paper proposes a new method for real‐time rigid body simulations for haptic interactions based on a penalty method regarding contact volume. Analytical methods for calculation of contact forces require too much time to maintain fast update rates for haptic controls. In addition, they prohibit direct connection of haptic interfaces. Penalty methods, which employ spring‐damper models for calculation of contact forces, offer a very rapid rate of iterations. In addition, they permit direct connection of haptic interfaces. Penalty methods are good for haptic interactions. However, previous penalty methods do not regard distribution of contact forces over the contact area. For that reason, they can't calculate normal and friction forces on face‐face contacts correctly. We propose a distributed spring‐damper model on a contact area to solve these problems. We analyze three‐dimensional geometries of the intersecting portion on the polyhedral objects. Then, we integrate forces and torques of distributed spring‐damper models. We implement a proposed simulator and compare it with a point‐based penalty method and constraint method. The comparison shows that the proposed simulator improves accuracy of the simulation of face‐face contact and friction forces and the simulation speed. In addition, we attach a six degree‐of‐freedom (6‐DOF) haptic interface to the simulator. Users can feel 6‐DOF force feedback and input 6‐DOF motions. Categories and Subject Descriptors (according to ACM CCS): I.6.5 [Model Development]: Modeling methodologies, H.5.2 [User Interfaces]: Haptic I/O, I.3.6 [Methodology and Techniques]: Interaction techniques

Procedural Texture Matching and Transformation

Abstract We present a technique for creating a smoothly varying sequence of procedural textures that interpolates between arbitrary input samples of texture. This texture transformation uses a library of procedural shaders and selects the correct shaders and associated parameters to accomplish the task. In general, selecting a procedural texture from a library, or finding the correct parameters to produce a smooth texture transition can be complex and time consuming. We propose a strategy for automating this process. While superficially this problem appears intractable for both humans and computational systems, its natural characteristics make a computational solution feasible. We present an algorithm and experimental results demonstrating this approach. Transformation between two textures can then be achieved procedurally, while enforcing perceptual similarity constraints between adjacent texture frames. We describe a technique for efficiently sampling the parameter domain of a shader based on a texture similarity function to create a smooth path through its texture range. In the case of evolving between several shaders, a method is described to obtain the best jump‐points which can be used to connect different shaders smoothly in texture space. Several examples of the technique are shown, and future directions as well as potential problems are discussed. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Texture

Single‐Strip Triangulation of Manifolds with Arbitrary Topology

Abstract Triangle strips have been widely used for efficient rendering. It is NP‐complete to test whether a given triangulated model can be represented as a single triangle strip, so many heuristics have been proposed to partition models into few long strips. In this paper, we present a new algorithm for creating a single triangle loop or strip from a triangulated model. Our method applies a dual graph matching algorithm to partition the mesh into cycles, and then merges pairs of cycles by splitting adjacent triangles when necessary. New vertices are introduced at midpoints of edges and the new triangles thus formed are coplanar with their parent triangles, hence the visual fidelity of the geometry is not changed. We prove that the increase in the number of triangles due to this splitting is 50% in the worst case, however for all models we tested the increase was less than 2%. We also prove tight bounds on the number of triangles needed for a single‐strip representation of a model with holes on its boundary. Our strips can be used not only for efficient rendering, but also for other applications including the generation of space filling curves on a manifold of any arbitrary topology. Categories and Subject Descriptors (according to ACM CCS): I.3.5 [Computer Graphics]: Geometric algorithms, Triangulation, Stripification. G.2.2 [Graph algorithms]: Hamiltonian Path, Hamiltonian Cycle, Perfect Matching.