Microfacet-based interference simulation for multilayer films

It has proved difficult to visualize the iridescent colors of objects coated with multilayer films in ray-based tracers. A physical-based full spectrum scattering model is proposed in this paper; this model applies the multi-beam interference equations to explain the multiple reflection and refraction of light inside the thin films in order to simulate the phase and amplitude variations of light related to iridescent colors. The Fresnel coefficients for the metallic and dielectric films are introduced respectively to photo-realistically render the wave properties of multilayer films. The solution is further extended to take into account the geometry of rough surfaces instead of a smooth averaging surface where the anisotropic and isotropic scattering characteristics are well explained. We demonstrate the validity of the proposed model by visualizing wave effects for diverse metallic or dielectric multilayer film structures and a Morpho butterfly.     

Electromagnetic forward and inverse scattering analysis of a one‐dimensional collision system

A one‐dimensional electromagnetic scattering analysis in presence of a colliding system is presented. A detailed analysis of higher order Doppler effect, due to multiple electromagnetic field reflection upon the two moving objects is performed, invoking the center of momentum frame and taking all possible range of velocity into account. A quasi‐stationary based finite difference time domain numerical method is then performed in order to test the proposed mathematical model for forward scattering analysis. Further, a spectral‐temporal mathematical derivation is performed for the inverse scattering analysis to compute the spatial and temporal information of the target system.     

A fast rendering method for clouds illuminated by lightning taking into account multiple scattering

Methods for rendering natural scenes are used in many applications such as virtual reality, computer games, and flight simulators. In this paper, we focus on the rendering of outdoor scenes that include clouds and lightning. In such scenes, the intensity at a point in the clouds has to be calculated by taking into account the illumination due to lightning. The multiple scattering of light inside clouds is an important factor when creating realistic images. However, the computation of multiple scattering is very time-consuming. To address this problem, this paper proposes a fast method for rendering clouds that are illuminated by lightning. The proposed method consists of two processes. First, basis intensities are prepared in a preprocess step. The basis intensities are the intensities at points in the clouds that are illuminated by a set of point light sources. In this precomputation, both the direct light and also indirect light (i.e., multiple scattering) are taken into account. In the rendering process, the intensities of clouds are calculated in real-time by using the weighted sum of the basis intensities. A further increase in speed is achieved by using a wavelet transformation. Our method achieves the real-time rendering of realistic clouds illuminated by lightning.     

Comparison between a multi-scattering and multi-layer snow scattering model and its parameterized snow backscattering model

Snow Water Equivalent (SWE) is a crucial parameter in the study of climatology and hydrology. Active microwave remote sensing is one of the most promising techniques for estimating the distribution of SWE at high spatial resolutions in large areas. Development of reliable and accurate inversion techniques to recover SWE is one of the most important tasks in current microwave researches. However, a number of snow pack properties, including snow density, particle size, crystal shape, stratification, ground surface roughness and soil moisture, affect the microwave scattering signals and need to be properly modeled and exploited. In this paper, we developed a multi-layer, multi-scattering model for dry snow based on recent theoretical advances in snow and surface modeling. In the proposed multi-layer model, Matrix Doubling method is used to account for scattering from each snow layer; and Advanced Integral Equation Model (AIEM) is incorporated into the model to describe surface scattering. Comparisons were made between the model predictions and field observations from NASA Cold Land Processes Field Experiment (CLPX) during Third Intensive Observation Period (IOP3) and SARALPS-2007 field experiment supported by ESA. The results indicated that model predictions were in good agreement with field observations. With the confirmed confidence, the analyses on multiple scattering, scatterer shape, and snow stratification effects were further made based on the model simulations. Furthermore, a parameterized snow backscattering model with a simple form and high computational efficiency was developed using a database generated by the multiple-scattering model. For a wide range of snow and soil properties, this parameterized model agrees well with the multiple-scattering model, with the root mean square error 0.20 dB, 0.24 dB and 0.43 dB for VV, HH and VH polarizations, respectively. This simplified model can be useful for the development of SWE retrieval algorithm and for fast simulations of radar signals over snow cover in land data assimilation systems.     

多重散射的天空光照效果建模与实时绘制

依据大气散射的物理原理,提出了一种考虑多重散射的天空光照效果建 模与实时绘制方法。该方法首先以太阳和天空光作为光源建立了多重散射的天空光照效果模 型,然后综合多种大气粒子密度, 采用合理的分段采样策略,对天空颜色模型的积分进行简 化,以减少积分近似计算所带来的误差;通过对简化后的模型进行分析提出了采用二维纹理 与三维纹理对光学深度预计算的方法,避免了运行时计算光学深度积分的问题;最后该算法 在GPU 的片段处理器上执行,实现了天空光照效果模型的实时绘制,可以满足各种实时应 用需求。     

一种动态物体次表面散射实时绘制方法

在给出非均匀材质建模描述方法的基础上,通过对光线类型和光的次表面散射物理过程进行分解,分别给出了图像空间非均匀材质单次散射和多次散射的近似计算方法。延迟着色思想的融入,满足了动态物体实时绘制的需要,并可较好地适用于分层非均匀半透明材质。     

Multiple Scattering Approximation in Heterogeneous Media by Narrow Beam Distributions

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.     

雨雾天气下光线散射效果的实时绘制

提出一种适用于雨雾天气的天空光照的多粒子散射模型,实现了雨雾场景中大气散射效果及雨后彩虹的真实感绘制.同时,通过对传统点光源单散射模型公式的解析简化,实现了各向异性光源散射效果的实时绘制.最终实时绘制出多种不同条件下真实感较强的雨雾场景.     

基于生物组织表面特性的体绘制光照模型

传统的光照模型侧重于模拟光能在微面元上的分布,它将表面反射分为镜面反射和漫反射,并采用表面法向与入射光夹角的余弦来模拟漫反射。由于生物组织表面富含水份,因此具有确定性介质和随机介质的双重特性。从物组织表面的漫反射光,就是由于光线射入介质表面产在介质内部被散射,而后射出表面的光能。因此,淡了逼真地表面生物组织的表面特性,必须同时考虑生物介质的表面反射和介质内部体散射双重特性。文中根据生物组织的特点,     

Anti-aliased and real-time rendering of scenes with light scattering effects

Recently, for real-time applications such as games, the rendering of scenes with light scattering effects in the presence of volumetric objects such as smoke, mist, etc., has gained much attention. Slice-based methods are well-known techniques for achieving fast rendering of these effects. However, for real-time applications, it is necessary to reduce the number of slice planes that are used. As a result, aliasing (striped patterns) can appear in the rendered images. In this paper, we propose a real-time rendering method for scenes containing volumetric objects that does not generate aliasing in the rendered images. When a scene consists of volumetric and polygonal objects, the proposed method also does not generate aliasing at the boundaries between the polygonal and the volumetric objects. Moreover, we are able to reduce aliasing at shadows inside a volumetric object that are cast by polygonal objects by interpolating the occlusion rates of light at several locations. The proposed method can be efficiently implemented on a GPU.     

An efficient layered simulation workflow for snow imprints

This paper introduces a novel workflow to generate snow imprints, and model the interaction of snow with dynamic objects. We decoupled snow simulation into three components: a base layer, snow particles, and snow mist. The base layer consists of snow that has not been in contact with a dynamic object yet, and is stored as a level set. Snow particles model the interaction between the snow and the dynamic objects. They are added when the dynamic objects collide with the base layer, and are animated using an adapted granular material simulation. The very thin and powdery snow released by airborne snow particles is modeled by the snow mist. This component is greatly influenced by the surrounding air medium; thus, it is animated using a fluid simulation. This decomposition allows to focus memory expensive and time-consuming computations only where dynamic objects interact with the snow, which is much more efficient than relying on a full-scale simulation.     

具有散射效果的室内外光束实时绘制

依据空气散射的物理原理,提出一种高效的能够实时模拟动态自然光束的真实感绘制方法.首先推导出散射计算的解析表达式以取代传统的数值积分方法;然后基于 GPU 特点设计了一种有效的光束体表示,作为光束体和阴影体统一形式共同参与 GPU 着色器的运算,从而生成具有阴影效果的光束.该方法不仅可以模拟单个光源形成的光束,还可以实时模拟动态太阳光源产生的多个自然光束.由于采用了 GPU 加速计算,可以满足实时应用需求.     

Review on tabletop true 3D display

Tabletop three‐dimensional (3D) display is an attractive display technology that allows multiple individuals around the table to view the reconstructed 3D objects simultaneously, which can be applied to a variety of application scenarios such as desktop conference and board games. In this review paper, the tabletop true 3D display has been characterized and classified into four categories based on the technologies of light field display, integral imaging, and volumetric 3D and holographic 3D displays. Moreover, the comparisons of these technologies are listed, and the prospect of the tabletop 3D display is discussed.     

均匀参与介质的渲染方法研究

参与介质在现实世界中广泛存在,光线在参与介质中的传播过程比在表面上的传播过程更加复杂,比如在高度散射参与介质中会发生成千上万次反射、在低散射参与介质中由于表面聚集出现体焦散效果,从而使得光线的模拟过程非常耗时。目前常用的方法包括点、光束和路径统一模型法（unifying points,beams and paths,UPBP）以及流型探索梅特罗波利斯光线传递方法（manifold exploration Metropolis light transport,MEMLT）等,这些方法在一定程度上改进了原有方法,但是在一些特殊情况下仍然需要很长时间才能收敛。本文介绍几种针对均匀参与介质的高效渲染方法。1）基于点的参与介质渲染方法,主要通过在参与介质内分布一些点来分别加速单次、二次和多次散射的计算,在GPU （graphics processing unit）实现的基础上,最终达到可交互的效率,并且支持对任意的均匀参与介质的编辑。2）基于多次反射的预计算模型,预计算出无限参与介质中的多次散射分布,通过分析光照分布的对称性,将该分布的维度从4维减低为3维,并且将该分布应用到多种蒙特卡洛渲染方法中,比如MEMLT、UPBP等,从而提高效率。3）参与介质中的路径指导方法,通过学习光线在参与介质中的分布,该分布用SD-tree （spatial-directional tree）来表示,与相位函数进行重采样来产生出射方向。以上3种方法分别从不同角度加快了参与介质的渲染效率。     

基于物理模型的快速单幅图像去雾方法

在雾、霾等天气条件下,大气粒子的散射作用导致捕获的图像严重降质.本文提出一种新的基于物理模型的快速单幅图像去雾算法.该算法从大气散射模型出发,通过对大气光照进行白平衡,从而简化大气散射模型;利用快速双边滤波方法估计大气耗散函数,进而恢复场景反照率.本文算法的时间复杂度达到图像像素数的线性函数,具有很快的执行速度.实验结...     

基于暗通道先验和Retinex理论的快速单幅图像去雾方法

针对雾霾天气下捕获的图像存在低对比度、低饱和度和色调偏移等现象, 提出了一种基于暗通道先验和Retinex理论的快速单幅图像去雾方法.该方法从大气散射模型出发, 利用暗通道先验法则,通过灰度开运算对大气光值进行区间估计,同时获得介质传输率的初始估计, 并通过白平衡简化大气散射模型; 其次,基于Retinex理论,利用高斯滤波获得介质传输率的粗略估计, 并通过线性映射实现灰度值搬移; 然后,将介质传输率的初始估计和粗略估计进行像素级融合, 利用快速联合双边滤波进行边缘优化,同时通过参数自适应调整的方法对雾图中大片天空区域的介质传输 率进行修正; 最后,通过简化大气散射模型和色调调整得到复原图像.与几种典型的图像去雾算法相比, 本文算法具有很快的运算速度,能有效提高复原图像的清晰度和对比度,同时获得较好的图像颜色.     

A comprehensive geometrical optics application for wave rendering

This paper presents a novel method to model wave effects in a ray tracer which attempts to account for the attenuation, scattering and absorption of light affected by participating media along rays. Inspired by diffraction shaders (DS), we propose an extension of this model to construct a new Bidirectional Reflectance Distribution Function (BRDF) to simultaneously take into consideration the phase and amplitude variation of light. The new method can simulate diffraction effects of a variety of materials, where we introduce the Fresnel factor and a microfacet scattering metric which affect the absorption and geometrical attenuation of photons. Incorporating Wigner Distribution Function (WDF), our method is further extended to model interference effects after multiple bounces by deferring the phase calculation. An acceleration algorithm is also implemented to real-time model diffraction effects of different apertures. We demonstrate the validity of our method by generating wave patterns for diverse scenes.     

Modeling and Rendering of Heterogeneous Granular Materials: Granite Application

Light‐matter interactions is one of the most important factors of realistic rendering. While a lot of work has already been performed in the light transport and simulation area, we believe that “virtual materials” have not yet been studied enough to achieve a high degree of realism. Some good models exist in order to take into account homogeneous materials. However, there are only a few studies of heterogeneous granular materials. In this paper, we propose a method based on mimicking natural phenomena to take into account these materials. Our study focuses on granite, which can be considered as a heterogeneous agglomerate of individually homogeneous grains. First we present a nucleation/growth process inspired technique giving a full 3D model of granite. Then, we use a rendering process taking into account each material component and subsurface scattering in a simple way.     

Precomputed Atmospheric Scattering

We present a new and accurate method to render the atmosphere in real time from any viewpoint from ground level to outer space, while taking Rayleigh and Mie multiple scattering into account. Our method reproduces many effects of the scattering of light, such as the daylight and twilight sky color and aerial perspective for all view and light directions, or the Earth and mountain shadows (light shafts) inside the atmosphere. Our method is based on a formulation of the light transport equation that is precomputable for all view points, view directions and sun directions. We show how to store this data compactly and propose a GPU compliant algorithm to precompute it in a few seconds. This precomputed data allows us to evaluate at runtime the light transport equation in constant time, without any sampling, while taking into account the ground for shadows and light shafts.     

Multistreaming of 3-D Scenes With Optimized Transmission and Rendering Scalability

Three-dimensional (3D) graphic scenes require considerable network bandwidth to be transmitted and computing power to be rendered on a user's terminal. Toward high-quality display in real time, we propose a sender-driven mechanism for streaming 3D scenes in a resource-constrained environment. In doing so, objects are encoded into multiresolutions to provide transmission and rendering scalability, and a weighted distortion metric is developed to measure the quality of a scene rendered with multiresolution objects, modeling objects' unequal importance regarding display. To preserve the manipulation independency of multiple objects in data delivery while provide preferential treatment for different objects as well as different layers of each object, transmission of the objects is performed over multiple streams in a partially sequenced and partially reliable fashion. A rate-distortion optimization framework is developed, which determines an optimal level of reliability for every chunk of data in each stream, taking into account the rendering importance of the object, the distortion-rate performance of the data chunks, and the statistics of the network link. Compared with heuristical methods, simulation results show that the proposed framework maximizes the display quality of the scene while minimizing the amount of data that needs to be processed by the client's rendering engine