ABSTRACT Smoke simulation is a crucial element in entertainment applications, such as movies and video games. In particular, the required smoke texture varies depending on the scene, ranging from smooth textures for cigarette smoke to highly irregular ones for explosions. The texture of smoke is primarily influenced by light sources, scattering properties, and turbulence components. Light sources and scattering properties affect the appearance of smoke during rendering, influencing its color, brightness, and density. Turbulence components significantly influence the shape of smoke. While many methods have been proposed for generating turbulence components, they all require manual adjustment of turbulence parameters, which is both time‐consuming and labor‐intensive. To address this issue, we propose a method for easily generating the desired turbulence by estimating turbulence parameters from images. In our approach, users input images including the desired turbulence components, and optimal parameters representing those components are automatically estimated. This reduces the time and effort required for parameter adjustment, allowing the desired turbulence components to be represented more efficiently.

Measured bidirectional reflectance distribution functions (BRDFs) can accurately represent the measured material appearance but suffer from high storage costs and lack editability due to their high dimensionality. Recent advances in efficient acquisition techniques extend the dimensionality of measured BRDFs from 3D (isotropic) to 4D (anisotropic) and from RGB to spectra. This, however, further compounds the issues of measured BRDFs and limits their practical use. This paper proposes a non-parametric factor representation for measured anisotropic spectral BRDFs. Based on microfacet theory, our method decomposes 4D measured anisotropic BRDF per spectrum into low-dimensional, editable factors. We further compress the spectral domain of decomposed factors using principal component analysis. Experimental results show that our method can compress measured anisotropic spectral BRDFs 1/40 on average and up to 1/333. Our method also provides several editing tools for each factor to enhance the editability of measured anisotropic spectral BRDFs.

Artificial light sources make our daily life convenient, but cause a severe problem called light pollution. We propose a novel system for efficient visualization of light pollution in the night sky. Numerous methods have been proposed for rendering the sky, but most of these focus on rendering of the daytime or the sunset sky where the sun is the only, or dominant light source. For the visualization of the light pollution, however, we must consider many city light sources on the ground, resulting in excessive computational cost. We address this problem by precomputing a set of intensity distributions for the sky illuminated by city light at various locations and with different atmospheric conditions. We apply a principal component analysis and fast Fourier transform to the precomputed distributions, allowing us to efficiently visualize the extent of the light pollution. Using this method, we can achieve one to two orders of magnitudes faster computation compared to a naive approach that simply accumulates the scattered intensity for each viewing ray. Furthermore, the fast computation allows us to interactively solve the inverse problem that determines the city light intensity needed to reduce light pollution. Our system provides the user with both a forward and inverse investigation tool for the study and minimization of light pollution.

The reflection of a bright light source on a dynamic surface such as water with waves can be difficult to render well in real time due to reflection aliasing and flickering. In this paper, we propose a solution to this problem by approximating the reflection direction distribution for the water surface as an elliptical Gaussian distribution. Then we analytically integrate the reflection contribution throughout the rendering interval time. Our method can render in real time an animation of the time integrated reflection of a spherical light source on highly dynamic waves with reduced aliasing and flickering.

High-resolution fluid simulations are computationally expensive, so many post-processing methods have been proposed to add turbulent details to low-resolution flows. Guiding methods are one promising approach for adding naturalistic, detailed motions as a post-process, but can be inefficient. Thus, we propose a novel, efficient method that formulates fluid guidance as a minimization problem in stream function space. Input flows are first converted into stream functions, and a high resolution flow is then computed via optimization. The resulting problem sizes are much smaller than previous approaches, resulting in faster computation times. Additionally, our method does not require an expensive pressure projection, but still preserves mass. The method is both easy to implement and easy to control, as the user can control the degree of guiding with a single, intuitive parameter. We demonstrate the effectiveness of our method across various examples.

Abstract Recent advances in bidirectional path tracing (BPT) reveal that the use of multiple light sub‐paths and the resampling of a small number of these can improve the efficiency of BPT. By increasing the number of pre‐sampled light sub‐paths, the possibility of generating light paths that provide large contributions can be better explored and this can alleviate the correlation of light paths due to the reuse of pre‐sampled light sub‐paths by all eye sub‐paths. The increased number of pre‐sampled light subpaths, however, also incurs a high computational cost. In this paper, we propose a two‐stage resampling method for BPT to efficiently handle a large number of pre‐sampled light sub‐paths. We also derive a weighting function that can treat the changes in path probability due to the two‐stage resampling. Our method can handle a two orders of magnitude larger number of presampled light sub‐paths than previous methods in equal‐time rendering, resulting in stable and better noise reduction than state‐of‐the‐art methods.

We propose an efficient, specific method for estimating camera parameters from a single starry night image. Such an image consists of a collection of disks representing stars, so traditional estimation methods for common pictures do not work. Our method uses a database, a star catalog, that stores the positions of stars on the celestial sphere. Our method computes magnitudes (i.e., brightnesses) of stars in the input image and uses them to find the corresponding stars in the star catalog. Camera parameters can then be estimated by a simple geometric calculation. Our method is over ten times faster and more accurate than a previous method.

Bidirectional path tracing (BPT) with multiple importance sampling (MIS) is a popular technique for rendering realistic images. Recently, it has been shown that BPT can be improved by preparing multiple light sub-paths and by resampling a small number of light sub-paths from them to generate full paths with large contribution. Traditionally, for MIS weights, the balance heuristic has widely been used to minimize the upper bound of variance, where each full path is weighted in proportion to the probability of the path. Although the probability of the path can change due to the resampling process, the weighting functions used in the previous methods remain unaffected by the change in probability, resulting in less efficiency. To address this problem, we propose new weighting functions for BPT with multiple light sub-paths. Our main contribution is a precise formulation of the variance and the derivation of the weighting functions that can appropriately treat the change in probability. We demonstrate that our weighting functions significantly improve the image quality. We will release a simple version of our implementation as open source to ensure reproducibility.

In these days, we can take many pictures everyday and everywhere with mobile devices such as smartphones. After taking a picture, we often modify it by using some image enhancement tools so that the appearance of the picture becomes favorable to his/her own preference. However, since there are many parameters in the enhancement functions, it is not an easy task to find an appropriate parameter set to obtain the desired result. Some tools have a function that automatically determine the parameters but they do not take into account the user's preference. In this paper, we present a system to address this problem. Our system first estimates the user's preference by using RankNet. Next, the image enhancement parameters are optimized to maximize the estimated preference. We show some experimental results to demonstrate the usefulness of our system.

A font is an important element in designing printed materials including texts, such as documents, posters, leaflets, pamphlets, etc. Recently, many digital fonts with different styles are available for desktop publishing, but the number of Japanese-language fonts is smaller than that of European ones. This causes a problem when designing the materials including Japanese and European letters. Creating a new font is difficult and requires specialized knowledge and experience. Our research goal is to address this problem by transferring styles of the European fonts to Japanese characters by using a neural network. In this paper, we report some experimental results using the well-known deep learning framework called "pix2pix."

Recently, computer graphics is widely used in movies and games, etc., and modeling three-dimensional virtual objects is important for synthesizing realistic images. Since modeling realistic objects often requires special skills and takes long time, many methods have been developed to help the user generate models such as plants and buildings. However, little attention has been paid to the modeling of fish shapes because of the complexity of their shapes. We propose an interactive system for modeling a realistic fish shape from a single image. We also introduce a method called Direct Manipulation Blendshapes for improving the usability of our system.

Abstract We propose a hybrid smoothed particle hydrodynamics solver for efficientlysimulating incompressible fluids using an interface handling method for boundary conditions in the pressure Poisson equation. We blend particle density computed with one smooth and one spiky kernel to improve the robustness against both fluid–fluid and fluid–solid collisions. To further improve the robustness and efficiency, we present a new interface handling method consisting of two components: free surface handling for Dirichlet boundary conditions and solid boundary handling for Neumann boundary conditions. Our free surface handling appropriately determines particles for Dirichlet boundary conditions using Jacobi‐based pressure prediction while our solid boundary handling introduces a new term to ensure the solvability of the linear system. We demonstrate that our method outperforms the state‐of‐the‐art particle‐based fluid solvers.

A great deal of attention has been devoted to the fabrication of reflectors that can display different color images when viewed from different directions not only in industry but also for the arts. Although such reflectors have previously been successfully fabricated, the number of images displayed has been limited to two or they suffer from ghosting artifacts where mixed images appear. Furthermore, the previous methods need special hardware and/or materials to fabricate the reflectors. Thus, those techniques are not suitable for printing reflectors on everyday personal objects made of different materials, such as name cards, letter sheets, envelopes, and plastic cases. To overcome these limitations, we propose a method for fabricating reflectors using a standard ultraviolet printer (UV printer). UV printer can render a specified 2D color pattern on an arbitrary material and by overprinting the printed pattern can be raised, that is, the printed pattern becomes a microstructure having color and height. We propose using these micro structures to formulate a method for designing spatially varying reflections that can display different target images when viewed from different directions. The microstructure is calculated by minimizing an objective function that measures the differences between the intensities of the light reflected from the reflector and that of the target image. We show several fabricated reflectors to demonstrate the usefulness of the proposed method.

Generating realistic fluid simulations remains computationally expensive, and animators can expend enormous effort trying to achieve a desired motion. To reduce such costs, several methods have been developed in which high-resolution turbulence is synthesized as a post process. Since global motion can then be obtained using a fast, low-resolution simulation, less effort is needed to create a realistic animation with the desired behavior. While much research has focused on accelerating the low-resolution simulation, the problem controlling the behavior of the turbulent, high-resolution motion has received little attention. In this paper, we show that style transfer methods from image editing can be adapted to transfer the turbulent style of an existing fluid simulation onto a new one. We do this by extending example-based image synthesis methods to handle velocity fields using a combination of patch-based and optimization-based texture synthesis. This approach allows us to take into account the incompressibility condition, which we have found to be a important factor during synthesis. Using our method, a user can easily and intuitively create high-resolution fluid animations that have a desired turbulent motion.

The computational cost for creating realistic fluid animations by numerical simulation is generally expensive. In digital production environments, existing precomputed fluid animations are often reused for different scenes in order to reduce the cost of creating scenes containing fluids. However, applying the same animation to different scenes often produces unacceptable results, so the animation needs to be edited. In order to help animators with the editing process, we develop a novel method for synthesizing the desired fluid animations by combining existing flow data. Our system allows the user to place flows at desired positions and combine them. We do this by interpolating velocities at the boundaries between the flows. The interpolation is formulated as a minimization problem of an energy function, which is designed to take into account the inviscid, incompressible Navier-Stokes equations. Our method focuses on smoke simulations defined on a uniform grid. We demonstrate the potential of our method by showing a set of examples, including a large-scale sandstorm created from a few flow data simulated in a small-scale space.

In this paper, we propose a modeling method of clouds from a single photograph. To synthesize realistic images of outdoor scenes, modeling of 3D clouds is necessary. However, synthesizing realistic clouds is still a difficult task. To address this problem, we propose an example-based volume texture synthesis to model nonhomogeneous density volumes. Our method prepares an example cloud volume data using a physically-based fluid simulator. Then the output cloud volume is automatically synthesized from the example cloud volume data so that its rendered image becomes similar to the clouds in the input photograph. In contrast to the previous texture synthesis methods, our method synthesizes density volumes taking into account scattering and transmittance of light. We show several cloud volumes whose rendered images are similar to the input photograph.

Abstract Visual simulation of fire plays an important role in many applications, such as movies and computer games. In these applications, artists are often requested to synthesize realistic fire with a particular behavior. To meet such requirement, we present a feedback control method for fire simulations. The user can design the shape of fire by placing a set of control points. Our method generates a force field and automatically adjusts a temperature at a fire source, based on user specified control points. Experimental results show that our method can control the fire shape.

Clouds play an important role when synthesizing realistic images of outdoor scenes. The realistic display of clouds is therefore one of the important research topics in computer graphics. In order to display realistic clouds, we need methods for modeling, rendering, and animating clouds realistically. It is also important to control the shapes and appearances of clouds to create certain visual effects. In this paper, we explain our efforts and research results to meet such requirements, together with related researches on the visual simulation of clouds.

Furniture layout design is a challenging problem, and several methods have recently been proposed. Although the lighting environment in a room has a strong relationship with the furniture functionality, the previous methods completely overlooked it in designing furniture layout. This paper addresses this problem; we propose an efficient method for computing furniture layout taking into account the lighting environment. We propose a new cost function that evaluates the lighting environment taking into account inter-reflections of light. A fast method for evaluating the cost function is also proposed. We demonstrate that our method improves the quality and usability of furniture layout by taking into account the lighting environment.

Abstract The popularity of many‐light rendering, which converts complex global illumination computations into a simple sum of the illumination from virtual point lights (VPLs), for predictive rendering has increased in recent years. A huge number of VPLs are usually required for predictive rendering at the cost of extensive computational time. While previous methods can achieve significant speedup by clustering VPLs, none of these previous methods can estimate the total errors due to clustering. This drawback imposes on users tedious trial and error processes to obtain rendered images with reliable accuracy. In this paper, we propose an error estimation framework for many‐light rendering. Our method transforms VPL clustering into stratified sampling combined with confidence intervals, which enables the user to estimate the error due to clustering without the costly computing required to sum the illumination from all the VPLs. Our estimation framework is capable of handling arbitrary BRDFs and is accelerated by using visibility caching, both of which make our method more practical. The experimental results demonstrate that our method can estimate the error much more accurately than the previous clustering method.

Abstract Fast realistic rendering of objects in scattering media is still a challenging topic in computer graphics. In presence of participating media, a light beam is repeatedly scattered by media particles, changing direction and getting spread out. Explicitly evaluating this beam distribution would enable efficient simulation of multiple scattering events without involving costly stochastic methods. Narrow beam theory provides explicit equations that approximate light propagation in a narrow incident beam. Based on this theory, we propose a closed‐form distribution function for scattered beams. We successfully apply it to the image synthesis of scenes in which scattering occurs, and show that our proposed estimation method is more accurate than those based on the Wentzel‐Kramers‐Brillouin (WKB) theory.

Abstract We propose a stable and efficient particle‐based method for simulating highly viscous fluids that can generate coiling and buckling phenomena and handle variable viscosity. In contrast to previous methods that use explicit integration, our method uses an implicit formulation to improve the robustness of viscosity integration, therefore enabling use of larger time steps and higher viscosities. We use Smoothed Particle Hydrodynamics to solve the full form of viscosity, constructing a sparse linear system with a symmetric positive definite matrix, while exploiting the variational principle that automatically enforces the boundary condition on free surfaces. We also propose a new method for extracting coefficients of the matrix contributed by second‐ring neighbor particles to efficiently solve the linear system using a conjugate gradient solver. Several examples demonstrate the robustness and efficiency of our implicit formulation over previous methods and illustrate the versatility of our method.

Abstract In this paper, we propose a method for the simulation of clouds using particles exclusively. The method is based on position based fluids, which simulates fluids using position constraints. To reduce the simulation time, we have used adaptive splitting and merging to concentrate computation on regions where it is most needed. When clouds are formed, particles are split so as to add more details to the generated cloud surface and when they disappear, particles are merged back. We implement our adaptive method on the Graphics Processing Unit (GPU) to accelerate the computation. While the splitting portion is easily parallelizable, the merge portion is not. We develop a simple algorithm to address this problem and achieve reasonable simulation times. Copyright © 2015 John Wiley & Sons, Ltd.

We propose a method of three-dimensional (3D) modeling of volumetric fluid phenomena from sparse multi-view images (e.g., only a single-view input or a pair of front- and side-view inputs). The volume determined from such sparse inputs using previous methods appears blurry and unnatural with novel views; however, our method preserves the appearance of novel viewing angles by transferring the appearance information from input images to novel viewing angles. For appearance information, we use histograms of image intensities and steerable coefficients. We formulate the volume modeling as an energy minimization problem with statistical hard constraints, which is solved using an expectation maximization (EM)-like iterative algorithm. Our algorithm begins with a rough estimate of the initial volume modeled from the input images, followed by an iterative process whereby we first render the images of the current volume with novel viewing angles. Then, we modify the rendered images by transferring the appearance information from the input images, and we thereafter model the improved volume based on the modified images. We iterate these operations until the volume converges. We demonstrate our method successfully provides natural-looking volume sequences of fluids (i.e., fire, smoke, explosions, and a water splash) from sparse multi-view videos. To create production-ready fluid animations, we further propose a method of rendering and editing fluids using a commercially available fluid simulator.

Abstract This paper proposes an interactive rendering method of cloth fabrics under environment lighting. The outgoing radiance from cloth fabrics in the microcylinder model is calculated by integrating the product of the distant environment lighting, the visibility function, the weighting function that includes shadowing/masking effects of threads, and the light scattering function of threads. The radiance calculation at each shading point of the cloth fabrics is simplified to a linear combination of triple product integrals of two circular Gaussians and the visibility function, multiplied by precomputed spherical Gaussian convolutions of the weighting function. We propose an efficient calculation method of the triple product of two circular Gaussians and the visibility function by using the gradient of signed distance function to the visibility boundary where the binary visibility changes in the angular domain of the hemisphere. Our GPU implementation enables interactive rendering of static cloth fabrics with dynamic viewpoints and lighting. In addition, interactive editing of parameters for the scattering function (e.g. thread's albedo) that controls the visual appearances of cloth fabrics can be achieved.

We present a technique for computing the shape of a transparent object that can generate user-defined caustic patterns. The surface of the object generated using our method is smooth. Thanks to this property, the resulting caustic pattern is smooth, natural, and highly detailed compared to the results btained using previous methods. Our method consists of two processes. First, we use a differential geometry approach to compute a smooth mapping between the distributions of the incident light and the light reaching the screen. Second, we utilize this mapping to compute the surface of the object. We solve Poisson's equation to compute both the mapping and the surface of the object.

Abstract We propose an efficient rendering method for dynamic scenes under all‐frequency environmental lighting. To render the surfaces of objects illuminated by distant environmental lighting, the triple product of the lighting, the visibility function and the BRDF is integrated at each shading point on the surfaces. Our method represents the environmental lighting and the BRDF with a linear combination of spherical Gaussians, replacing the integral of the triple product with the sum of the integrals of spherical Gaussians over the visible region of the hemisphere. We propose a new form of spherical Gaussian, the integral spherical Gaussian, that enables the fast and accurate integration of spherical Gaussians with various sharpness over the visible region on the hemisphere. The integral spherical Gaussian simplifies the integration to a sum of four pre‐integrated values, which are easily evaluated on‐the‐fly. With a combination of a set of spheres to approximate object geometries and the integral spherical Gaussian, our method can render object surfaces very efficiently. Our GPU implementation demonstrates realtime rendering of dynamic scenes with dynamic viewpoints, lighting, and BRDFs.

Abstract Pixel art is a kind of digital art that through per‐pixel manipulation enables production of a diverse array of artistic images. In this paper, we present a new way for people to experience and express pixel art. Our digital art consists of a set of sticks made of acrylate resin, each of which refracts light from a parallel light source, in certain directions. Artistic users are able to easily rearrange these sticks and view their digital art through the refracted light projection on any planar surface. As we demonstrate in this paper, a user can generate various artistic images using only a single set of sticks. We additionally envision that our pixel art with rearrangeable sticks would have great entertainment appeal, e.g., as an art puzzle.

Clouds play an important role in creating realistic images of outdoor scenes. Many methods have therefore been proposed for displaying realistic clouds. However, the realism of the resulting images depends on many parameters used to render them and it is often difficult to adjust those parameters manually. This paper proposes a method for addressing this problem by solving an inverse rendering problem: given a non-uniform synthetic cloud density distribution, the parameters for rendering the synthetic clouds are estimated using photographs of real clouds. The objective function is defined as the difference between the color histograms of the photograph and the synthetic image. Our method searches for the optimal parameters using genetic algorithms. During the search process, we take into account the multiple scattering of light inside the clouds. The search process is accelerated by precomputing a set of intermediate images. After ten to twenty minutes of precomputation, our method estimates the optimal parameters within a minute.

We present a new technique for bi-scale material editing using Spherical Gaussians (SGs). To represent large-scale appearances, an effective BRDF that is the average reflectance of small-scale details is used. The effective BRDF is calculated from the integral of the product of the Bidirectional Visible Normal Distribution (BVNDF) and BRDFs of small-scale geometry. Our method represents the BVNDF with a sum of SGs, which can be calculated on-the-fly, enabling interactive editing of small-scale geometry. By representing small-scale BRDFs with a sum of SGs, effective BRDFs can be calculated analytically by convolving the SGs for BVNDF and BRDF. We propose a new SG representation based on convolution of two SGs, which allows real-time rendering of effective BRDFs under all-frequency environment lighting and real-time editing of small-scale BRDFs. In contrast to the previous method, our method does not require extensive precomputation time and large volume of precomputed data per single BRDF, which makes it possible to implement our method on a GPU, resulting in real-time rendering.

This paper presents a method for controlling simulated explosions by prediction and optimization. Many methods have been proposed for simulating realistic explosion based on numerical fluid analysis. These methods are widely used in applications such as movies and computer games. In these applications, there is often a requirement for the explosion to conform to a specified final shape. The goal of our research is to meet this requirement by controlling the simulation so that the explosion evolves into a target shape specified by the user. The key ideas of our method are optimization of the initial velocities at the explosion source and predictive control of the simulation. Our method optimizes the initial velocities during the preprocessing step. Next, during the simulation, it predicts the future shape from the history of the explosion shape and controls the simulation to minimize the difference between the target shape and the predicted shape.

Real-time rendering of dynamic scenes illuminated by complex, all-frequency lighting with highly glossy BRDFs remains a challenging problem. Although several methods have been proposed to render dynamic scenes, these methods can not handle deformable objects [Zhou et al. 2005], all-frequency lighting [Ren et al. 2006; Nowrouzezahrai et al. 2009], or highly glossy BRDFs [Annen et al. 2008].

Synthetic images of outdoor scenes generated by computer graphics usually contain the sky as background. The appearance of the sky plays an important role in enhancing the reality of scenes. In this paper, we propose an intuitive and interactive system for synthesizing sky images that can reflect the user's intention. Our system consists of two steps. The first step is a sky generation step. The luminance distribution and colors of the sky are generated using our intuitive user interface. The second step is a cloud composition step. We have prepared a database of photographs of real clouds. Our system searches the database for cloud photographs that are suitable for the synthesized sky. The user selects the clouds in the search results and they are composited into the sky image. By using our system, the user can easily generate desired sky images.

Abstract Photo‐realistic rendering of inhomogeneous participating media with light scattering in consideration is important in computer graphics, and is typically computed using Monte Carlo based methods. The key technique in such methods is the free path sampling, which is used for determining the distance (free path) between successive scattering events. Recently, it has been shown that efficient and unbiased free path sampling methods can be constructed based on Woodcock tracking. The key concept for improving the efficiency is to utilize space partitioning (e.g., kd‐tree or uniform grid), and a better space partitioning scheme is important for better sampling efficiency. Thus, an estimation framework for investigating the gain in sampling efficiency is important for determining how to partition the space. However, currently, there is no estimation framework that works in 3D space. In this paper, we propose a new estimation framework to overcome this problem. Using our framework, we can analytically estimate the sampling efficiency for any typical partitioned space. Conversely, we can also use this estimation framework for determining the optimal space partitioning. As an application, we show that new space partitioning schemes can be constructed using our estimation framework. Moreover, we show that the differences in the performances using different schemes can be predicted fairly well using our estimation framework.

Interactive rendering under complex real world illumination is essential for many applications such as material design, lighting design, and virtual realities. For such applications, interactive manipulations of viewpoints, lighting, BRDFs, and positions of objects are beneficial to designers and users. This paper proposes a system that acquires complex, all-frequency lighting environments and renders dynamic scenes under captured illumination, for lighting and material design applications in cyber worlds. To capture real world lighting environments easily, our method uses a camera equipped with a cellular phone. To handle dynamic scenes of rigid objects and dynamic BRDFs, our method decomposes the visibility function at each vertex of each object into the occlusion due to the object itself and occlusions due to other objects, which are represented by a nonlinear piecewise constant approximation, called cuts. Our method proposes a compact cut representation and efficient algorithm for cut operations. By using our system, interactive manipulation of positions of objects and real time rendering with dynamic viewpoints, lighting, and BRDFs can be achieved.

Abstract In this paper, we propose a simple method for modeling clouds from a single photograph. Our method can synthesize three types of clouds: cirrus, altocumulus, and cumulus. We use three different representations for each type of cloud: two‐dimensional texture for cirrus, implicit functions (metaballs) for altocumulus, and volume data for cumulus. Our method initially computes the intensity and the opacity of clouds for each pixel from an input photograph, stored as a cloud image. For cirrus, the cloud image is the output two‐dimensional texture. For each of the other two types of cloud, three‐dimensional density distributions are generated by referring to the cloud image. Since the method is very simple, the computational cost is low. Our method can generate, within several seconds, realistic clouds that are similar to those in the photograph.

Abstract The visual simulation of natural phenomena has been widely studied. Although several methods have been proposed to simulate melting, the flows of meltwater drops on the surfaces of objects are not taken into account. In this paper, we propose a particle‐based method for the simulation of the melting and freezing of ice objects and the interactions between ice and fluids. To simulate the flow of meltwater on ice and the formation of water droplets, a simple interfacial tension is proposed, which can be easily incorporated into common particle‐based simulation methods such as Smoothed Particle Hydrodynamics. The computations of heat transfer, the phase transition between ice and water, the interactions between ice and fluids, and the separation of ice due to melting are further accelerated by implementing our method using CUDA. We demonstrate our simulation and rendering method for depicting melting ice at interactive frame‐rates.

Realistic rendering of participating media is one of the major subjects in computer graphics. Monte Carlo techniques are widely used for realistic rendering because they provide unbiased solutions, which converge to exact solutions. Methods based on Monte Carlo techniques generate a number of light paths, each of which consists of a set of randomly selected scattering events. Finding a new scattering event requires free path sampling to determine the distance from the previous scattering event, and is usually a time-consuming process for inhomogeneous participating media. To address this problem, we propose an adaptive and unbiased sampling technique using kd-tree based space partitioning. A key contribution of our method is an automatic scheme that partitions the spatial domain into sub-spaces (partitions) based on a cost model that evaluates the expected sampling cost. The magnitude of performance gain obtained by our method becomes larger for more inhomogeneous media, and rises to two orders compared to traditional free path sampling techniques.

Visual simulation of natural phenomena has become one of the most important research topics in computer graphics. Such phenomena include water, fire, smoke, clouds, and so on. Recent methods for the simulation of these phenomena utilize techniques developed in computational fluid dynamics. In this paper, the basic equations (Navier-Stokes equations) for simulating these phenomena are briefly described. These basic equations are used to simulate various natural phenomena. This paper then explains our applications of the equations for simulations of smoke, clouds, and aerodynamic sound.

Abstract A rendering system for interior scenes is proposed in this paper. The light reaches the interior scene, usually through small regions, such as windows or abat‐jours, which we call portals. To provide a solution, suitable for rendering interior scenes with portals, we extend the traditional precomputed radiance transfer approaches. In our approach, a bounding sphere, which we call a shell, of the interior, centered at each portal, is created and the light transferred from the shell towards the interior through the portal is precomputed. Each shell acts as an environment light source and its intensity distribution is determined by rendering images of the scene, viewed from the center of the shell. By updating the intensity distribution of the shell at each frame, we are able to handle dynamic objects outside the shells. The material of the portals can also be modified at run time (e.g. changing from transparent glass to frosted glass). Several applications are shown, including the illumination of a cathedral, lit by skylight at different times of a day, and a car, running in a town, at interactive frame rates, with a dynamic viewpoint.

Abstract Recently, many techniques using computational fluid dynamics have been proposed for the simulation of natural phenomena such as smoke and fire. Traditionally, a single grid is used for computing the motion of fluids. When an object interacts with a fluid, the resolution of the grid must be sufficiently high because the shape of the object is represented by a shape sampled at the grid points. This increases the number of grid points that are required, and hence the computational cost is increased. To address this problem, we propose a method using multiple grids that overlap with each other. In addition to a large single grid (a global grid) that covers the whole of the simulation space, separate grids (local grids) are generated that surround each object. The resolution of a local grid is higher than that of the global grid. The local grids move according to the motion of the objects. Therefore, the process of resampling the shape of the object is unnecessary when the object moves. To accelerate the computation, appropriate resolutions are adaptively‐determined for the local grids according to their distance from the viewpoint. Furthermore, since we use regular (orthogonal) lattices for the grids, the method is suitable for GPU implementation. This realizes the real‐time simulation of interactions between objects and smoke.

Clouds play an important role for creating realistic images of outdoor scenes. In order to generate realistic clouds, many methods have been developed for modeling and animating clouds. One of the most effective approaches for synthesizing realistic clouds is to simulate cloud formation processes based on the atmospheric fluid dynamics. Although this approach can create realistic clouds, the resulting shapes and motion depend on many simulation parameters and the initial status. Therefore, it is very difficult to adjust those parameters so that the clouds form the desired shapes. This paper addresses this problem and presents a method for controlling the simulation of cloud formation. In this paper, we focus on controlling cumuliform cloud formation. The user specifies the overall shape of the clouds. Then, our method automatically adjusts parameters during the simulation in order to generate clouds forming the specified shape. Our method can generate realistic clouds while their shapes closely match to the desired shape.

In this paper, we propose a fast global illumination solution for interactive lighting design. Using our method, light sources and the viewpoint are movable, and the characteristics of materials can be modified (assuming low-frequency BRDF) during rendering. Our solution is based on particle tracing (a variation of photon mapping) and final gathering. We assume that objects in the input scene are static, and pre-compute potential light paths for particle tracing and final gathering. To perform final gathering fast, we propose an efficient technique called Hierarchical Histogram Estimation for rapid estimation of radiances from the distribution of the particles. The rendering process of our method can be fully implemented on the GPU and our method achieves interactive frame rates for rendering scenes with even more than 100,000 triangles.

Haptic interfaces enable us to interact with a virtual world using our sense of touch. This paper presents a method for realizing haptic interaction with water. Our method displays forces acting on rigid objects due to water with a high frame rate (500 Hz). To achieve this, we present a fast method for simulating the dynamics of water. We decompose the dynamics into two parts. One is a linear flow expressed by a wave equation used to compute water waves. The other is a more complex and non-linear flow around the object. The fluid forces due to the non-linear flow is precomputed by solving Navier-Stokes equations, and stored in a database, named the Fluid Resistance Map. The precomputed non-linear flow and the linear flow are combined to compute the forces due to water.

In a cel animation workplace, nonphotorealistic rendering techniques are often applied to a 3D scene model, which is constructed by referring to the 2D hand-drawn images of the scene. There exist a lot of scene features that must be modeled from the 2D information. The accentuation effects are amongst the most important features in a hand-drawn scene. In this context, accentuation refers to adding hand-drawn shading of the scene. The specific region that an artist wishes the viewer to observe is drawn brighter than the other regions. A method is proposed for the extraction of the accentuation effects from a single 2D painted image of an animated scene. This method enables the automatic addition of the accentuation effects to each frame of the animation, observed from other viewpoints.

Caustics are patterns of light formed by reflection or refraction of light from objects, and several methods have been developed to render caustics because of their visually beautiful patterns. This paper proposes a method for the rapid rendering of caustics formed by refracted light through transparent objects. First, a preprocess is used to generate sampling rays incident on each vertex of the object, and, taking refraction into account, to trace the rays until they leave the object. The position and direction of each ray that finally leaves the transparent object are obtained and stored in a lookup table. Next, in the rendering process, when the object is illuminated, the position and direction of the rays leaving the object are calculated using the lookup table. This makes it possible to render refractive caustics due to transparent objects at interactive frame rates even if the direction of the light changes and the object is translated or rotated.

In this paper, we propose a radiosity method for the point-sampled geometry to compute diffuse inter reflection of light. Most traditional radiosity methods subdivide the surfaces of objects into small elements such as quadrilaterals. However, the point-sampled geometry includes no explicit information about surfaces, presenting a difficulty in applying the traditional approach to the point-sampled geometry. The proposed method addresses this problem by computing the inter reflection without reconstructing any surfaces. The method realizes lighting simulations without losing the advantages of the point-sampled geometry.

Abstract Sound is an indispensable element for the simulation of a realistic virtual environment. Therefore, there has been much recent research focused on the simulation of realistic sound effects. This paper proposes a method for creating sound for turbulent phenomena such as fire. In a turbulent field, the complex motion of vortices leads to the generation of sound. This type of sound is called a vortex sound. The proposed method simulates a vortex sound by computing vorticity distributions using computational fluid dynamics. Sound textures for the vortex sound are first created in a pre‐process step. The sound is then created at interactive rates by using these sound textures. The usefulness of the proposed method is demonstrated by applying it to the simulation of the sound of fire and other turbulent phenomena. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism ‐ Animation; I.6.8 [Simulation and Modeling]: Types of Simulation ‐ Animation; I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism; I.6.3 [Simulation and Modeling]: Applications; H.5.5 [Information Interfaces and Presentation]: Sound and Music Computing ‐ Methodologies and techniques, Modeling.

In this paper, we present a physics motivated modeling method for volcanic clouds as a two fluids model. Some previous methods model smoke or clouds as one fluid, but the volcanic clouds can not be treated as one fluid. The volcanic clouds consist of the pyroclasts, the volcanic gas and the entrained air. Since the pyroclasts and the volcanic gas can be treated as one fluid, called magma, the volcanic clouds are regarded as two fluids, the magma and the entrained air. The modeling in the 3D analysis space can be simplified to enhance the performance. Since our approach is physics motivated, it can be used to generate physically reasonable and realistic images of volcanic clouds from the volcanic initial eruption to the equilibrium situation.

Abstract In order to synthesize realistic images of scenes that include water surfaces, the rendering of optical effectscaused by waves on the water surface, such as caustics and reflection, is necessary. However, rendering causticsis quite complex and time‐consuming. In recent years, the performance of graphics hardware has made significantprogress. This fact encourages researchers to study the acceleration of realistic image synthesis. We present herea method for the fast rendering of refractive and reflective caustics due to water surfaces. In the proposed method,an object is expressed by a set of texture mapped slices. We calculate the intensities of the caustics on the objectby using the slices and store the intensities as textures. This makes it possible to render caustics at interactive rateby using graphics hardware. Moreover, we render objects that are reflected and refracted due to the water surfaceby using reflection/refraction mapping of these slices. Categories and Subject Descriptors (according to ACM CCS): I.3.1 [Computer Graphics]: Hardware Architecture I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism

In computer graphics, most research focuses on creating images. However, there has been much recent work on the automatic generation of sound linked to objects in motion and the relative positions of receivers and sound sources. This paper proposes a new method for creating one type of sound called aerodynamic sound. Examples of aerodynamic sound include sound generated by swinging swords or by wind blowing. A major source of aerodynamic sound is vortices generated in fluids such as air. First, we propose a method for creating sound textures for aerodynamic sound by making use of computational fluid dynamics. Next, we propose a method using the sound textures for real-time rendering of aerodynamic sound according to the motion of objects or wind velocity.

Abstract The display of realistic natural scenes is one of the most important research areas in computer graphics. Therendering of water is one of the essential components. This paper proposes an efficient method for renderingimages of scenes within water. For underwater scenery, the shafts of light and caustics are attractive and importantelements. However, computing these effects is difficult and time‐consuming since light refracts when passingthrough waves. To address the problem, our method makes use of graphics hardware to accelerate the computation.Our method displays the shafts of light by accumulating the intensities of streaks of light by using hardware colorblending functions. Making use of a Z‐buffer and a stencil buffer accelerates the rendering of caustics. Moreover,by using a shadow mapping technique, our method can display shafts of light and caustics taking account ofshadows due to objects. ACM CSS: I. 3.1 Computer Graphics—Hardware Architecture, I. 3.7 Computer Graphics—Three‐DimensionalGraphics and Realism

The simulation of various natural phenomena is one of the important research fields in computer graphics. In particular, aspects such as sky, clouds, water, fire, trees, smoke, terrains, desert scenes, snow and fog are indispensable for creating realistic images of natural scenes, flight simulators and so on. Therefore, a lot of researchers have been trying to develop methods for simulating and rendering these. In this paper, we focus on sky, clouds, smoke, desert scenes and atmospheric effects, such as shafts of light. These phenomena have the common feature that they consist of the effects of small particles. To create realistic images, physical based simulation and rendering are required. In particular, the color greatly depends on the properties of light scattering due to particles. In general, however, the simulation and rendering of these images is assumed to be very time-consuming. This paper describes efficient methods for creating realistic images of such natural phenomena.

A number of methods have been developed for creating realistic images of natural scenes. Their applications include flight simulators, the visual assessment of outdoor scenery, etc. Previously, many of these methods have focused on creating images under clear or slightly cloudy days. Simulations under bad weather conditions, however, are one of the important issues for realism. Lightning is one of the essential elements for these types of simulations. The paper proposes an efficient method for creating realistic images of scenes including lightning. Our method can create photo-realistic images by taking into account the scattering effects due to clouds and atmospheric particles illuminated by lightning. Moreover, graphics hardware is utilized to accelerate the image generation. The usefulness of our method is demonstrated by creating images of outdoor scenes that include lightning.

The display of realistic natural scenes is one of the most important research areas in computer graphics. The rendering of water is one of the essential components. The paper proposes an efficient method for rendering images of scenes within water. For underwater scenery, the shafts of light and caustics are attractive and important elements. However, computing these effects is difficult and time-consuming, since light refracts when passing through waves. To address the problem, our method makes use of graphics hardware to accelerate the computation. Our method displays the shafts of light by accumulating the intensities of streaks of light by using hardware color blending functions. The rendering of caustics is accelerated by making use of a Z-buffer and a stencil buffer. Moreover, by using a shadow mapping technique, our method can display shafts of light and caustics, taking account of shadows due to objects.

The simulation of natural phenomena such as clouds, smoke, fire and water is one of the most important research areas in computer graphics. In particular, clouds play an important role in creating images of outdoor scenes. The proposed method is based on the physical simulation of atmospheric fluid dynamics which characterizes the shape of clouds. To take account of the dynamics, we used a method called the coupled map lattice (CML). CML is an extended method of cellular automaton and is computationally inexpensive. The proposed method can create various types of clouds and can also realize the animation of these clouds. Moreover, we have developed an interactive system for modeling various types of clouds.

Recently graphics hardware has increased in capability, and is, moreover, now available even on standard PCs. These advances have encouraged researchers to develop hardware-accelerated methods for rendering realistic images. One of the important elements in enhancing reality is the effect of atmospheric scattering. The scattering of light due to atmospheric particles has to be taken into account in order to display shafts of light produced by studio spotlights and headlights of automobiles, for example. The purpose of the paper is to develop a method for displaying shafts of light at interactive rates by making use of the graphics hardware. The method makes use of hardware-accelerated volume rendering techniques to display the shafts of light.

This paper proposes a simple and computationally inexpensive method for animation of clouds. The cloud evolution is simulated using cellular automaton that simplifies the dynamics of cloud formation. The dynamics are expressed by several simple transition rules and their complex motion can be simulated with a small amount of computation. Realistic images are then created using one of the standard graphics APIs, OpenGL. This makes it possible to utilize graphics hardware, resulting in fast image generation. The proposed method can realize the realistic motion of clouds, shadows cast on the ground, and shafts of light through clouds.

Recently, simulation of natural phenomena, such as water, smokes, fire, clouds, have been attempted in computer graphics. Clouds play an important role when generating images of outdoor scenes, the earth viewed from outer space and the visualization of weather information. This paper describes the modeling methods of clouds, and an efficient calculation method for light scattering due to clouds taking into account both multiple scattering and sky light. The methods discussed here are also useful for displaying snow.

The paper proposes an image based modeling of clouds where realistic clouds are created from satellite images using metaballs. The intention of the paper is for applications to space flight simulators, the visualization of the weather information, and the simulation of surveys of the Earth. In the proposed method, the density distribution inside the clouds is defined by a set of metaballs. Parameters of metaballs, such as center positions, radii, and density values, are automatically determined so that a synthesized image of clouds modeled by using metaballs is similar to the original satellite image. We also propose an animation method for clouds generated by a sequence of satellite images taken at some interval. The usefulness of the proposed method is demonstrated by several examples of clouds generated from satellite images of typhoons passing through Japan.

The display of natural scenes such as mountains, trees, the earth as viewed from space, the sea, and waves have been attempted. Here a method to realistically display snow is proposed. In order to achieve this, two important elements have to be considered, namely the shape and shading model of snow, based on the physical phenomenon. In this paper, a method for displaying snow fallen onto objects, including curved surfaces and snow scattered by objects, such as skis, is proposed. Snow should be treated as particles with a density distribution since it consists of water particles, ice particles, and air molecules. In order to express the material property of snow, the phase functions of the particles must be taken into account, and it is well‐known that the color of snow is white because of the multiple scattering of light. This paper describes a calculation method for light scattering due to snow particles taking into account both multiple scattering and sky light, and the modeling of snow.

Computer graphics are being used for visual environmental assessment or architectural designs. Displaying the sky as a background is indispensable in generating photorealistic images for such applications. In this paper, we propose a fast display method of the sky colour by expressing the intensity distribution of the sky using basis functions, even if the sun position and/or the camera position are altered. In the proposed method, cosine functions are used as basis functions. The sun altitude is altered at certain intervals and the distributions of the sky colour for each sun altitude are precalculated and stored efficiently using basis functions. The colour of the sky in the view direction of an arbitrary sun position can be obtained from the stored distributions and displayed quickly. © 1997 by John Wiley & Sons, Ltd.

Article Free Access Share on Display of clouds taking into account multiple anisotropic scattering and sky light Authors: Tomoyuki Nishita Fukuyama University, Sanzo, Higashimura-cho, Fukuyama, 729-02 Japan Fukuyama University, Sanzo, Higashimura-cho, Fukuyama, 729-02 JapanView Profile , Yoshinori Dobashi Hiroshima University, 1-4-1, kagamiyama, Higashi-hiroshima, 739 Japan Hiroshima University, 1-4-1, kagamiyama, Higashi-hiroshima, 739 JapanView Profile , Eihachiro Nakamae Hiroshima Prefectural University, Nanatsuka-cho, Shoubara City, 727 Japan Hiroshima Prefectural University, Nanatsuka-cho, Shoubara City, 727 JapanView Profile Authors Info & Claims SIGGRAPH '96: Proceedings of the 23rd annual conference on Computer graphics and interactive techniquesAugust 1996 Pages 379–386https://doi.org/10.1145/237170.237277Published:01 August 1996Publication History 92citation2,177DownloadsMetricsTotal Citations92Total Downloads2,177Last 12 Months145Last 6 weeks14 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteeReaderPDF

Abstract Recently, computer graphics are frequently used for both architectural design and visual environmental assessment. Using computer graphics, designers can easily compare the effect of the natural light on their architectural designs under various conditions, such as different times of day, seasons, atmospheric conditions (clear or overcast sky) or building wall materials. In traditional methods of calculating the luminance due to sky light, however, all calculation must be performed from scratch if such conditions undergo change. Therefore, to compare the architectural designs under different conditions, a great deal of time has to be spent on generating the images. This paper proposes a new method of quickly generating images of an outdoor scene, taking into account glossy specular reflection, even if such conditions change. In this method, luminance due to sky light is expressed by a series of basis functions, and basis luminances corresponding to each basis function are precalculated and stored in a compressed form in the preprocess. Once the basis luminances are calculated, the luminance due to sky light can be quickly calculated by the weighted sum of the basis luminances. Several examples of an architectural design demonstrate the usefulness of the proposed method.

Abstract When designing interior lighting effects, it is desirable to compare a variety of lighting designs involving different lighting devices and directions of light. It is, however, time‐consuming to generate images with many different lighting parameters, taking interreflection into account, because all luminances must be calculated and recalculated. This makes it difficult to design lighting effects interactively. To address this problem, this paper proposes a method of quickly generating images of a given scene illustrating an interreflective environment illuminated by sources with arbitrary luminous intensity distributions. In the proposed method, the luminous intensity ditribution is expressed with basis functions. The proposed method uses a series of spherical harmonic functions as basis functions, and calculates in advance each intensity on surfaces lit by the light sources whose luminous intensity distribution are the same as the spherical harmonic functions. The proposed method makes it possible to generate images so quickly that we can change the luminous intensity distribution interactively. Combining the proposed method with an interactive walk‐through that employs intensity mapping, an interactive system for lighting design is implemented. The usefulness of the proposed method is demonstrated by its application to interactive lighting design, where many images are generated by altering lighting devices and/or direction of light.

Abstract It is inevitable for indoor lighting design to render a room lit by natural light, especially for an atelier or an indoor pool where there are many windows. This paper proposes a method for calculating the illuminance due to natural light, i.e. direct sunlight and skylight, passing through transparent planes such as window glass. The proposed method makes it possible to efficiently calculate such illuminance accurately, because it takes into account both non‐uniform luminous intensity distribution of skylight and the distribution of transparency of glass according to incident angles of light. Several examples including the lighting design in an indoor pool, are shown to demonstrate the usefulness of the proposed method.