一种真实感雪场景模拟方法

探讨已有降雪和积雪模拟方法,针对粒子系统难以同时兼得真实性与实时性的问题,提出一种真实感较强的雪场景实时绘制算法。算法用矩形基本粒子进行纹理叠加映射来建模雪粒子,用温度调节控制雪粒子大小及雪密度大小,归一化生命周期并用于颜色混合。飘雪阶段,引入层次细节(LOD)技术,根据雪粒子真实受力特点模仿其运动过程,并将其受力全面综合考虑并简化,提高降雪模拟效果真实度,并降低计算复杂度。积雪阶段,获取场景模型暴露面及其高度场,以暴露面中点的高度及其点坐标为基础,得到雪粒子落地位置,进而模拟积雪高度变化效果;将整个地面作为地面雪粒子发射器,模拟积雪融化效果,提高真实性。     

大规模雪场景的实时绘制

大规模雪场景的真实感实时绘制在虚拟现实、雪灾的预防和救援、军事仿真及游戏设计等领域有着广泛的应用价值,但现有方法难以同时生成大规模动态雪场景的积雪及飘雪效果.为此提出并实现了一种交互式大规模雪场景建模与实时绘制的新方法.为了精细地模拟场景的积雪效果,提出一种基于视点的自适应降雪遮挡图模型,能在实时更新地物的遮挡关系的同时大大减少大规模雪场景中积雪的计算量,并提高了计算精度;对于场景的飘雪,采用一种基于视点的雪粒子分层建模技术来减少雪粒子数量,将视点变换及降雪粒子系统移至GPU中进行加速计算;采用动态多旋转纹理来模拟飘落雪花的形状以增加其真实感;采用几何与纹理混合绘制的方式来减少大场景的复杂度.最终成功地实现了野外和城市两个大规模雪场景的实时漫游,在场景中可看到压雪累累的树枝雪挂景象及轿车上厚厚积雪等冬天美景.     

虚拟场景中的实时降雪模拟

降雪是一种重要的自然现象,对降雪的模拟可以大大提高虚拟场景的逼真度。文中在分析粒子系统建模方法的基础上,给出了一种雪粒子模型,该模型对雪的物理模型进行了适当简化:通过控制雪粒子的大小、形状、密度(数量)来实现雪强度的模拟;通过控制雪粒子的运动来模拟风对雪的影响,并对雪粒子的产生域进行了讨论,使其满足视点运动的要求。实践表明,该粒子系统模型可在复杂的三维场景中实时地模拟降雪现象,仿真效果逼真。     

虚拟场景中的实时降雪模拟

降雪是一种重要的自然现象，对降雪的模拟可以大大提高虚拟场景的逼真度。文中在分析粒子系统建模方法的基础上，给出了一种雪粒子模型，该模型对雪的物理模型进行了适当简化：通过控制雪粒子的大小、形状、密度（数量）来实现雪强度的模拟；通过控制雪粒子的运动来模拟风对雪的影响，并对雪粒子的产生域进行了讨论，使其满足视点运动的要求。实践表明，该粒子系统模型可在复杂的三维场景中实时地模拟降雪现象，仿真效果逼真。     

基于粒子系统的雪景模拟算法研究与实现

文章在分析粒子系统建模方法的基础上,给出了一种雪粒子模型,该模型对雪的物理模型进行了适当简化：通过控制雪粒子的大小、形状、密度（数量）来实现雪强度的模拟;通过控制雪粒子的运动来模拟风对雪的影响,并对雪粒子的产生域进行了讨论,使其满足视点运动的要求。实践表明,该粒子系统模型可在复杂的三维场景中实时地模拟降雪现象,仿真效果逼真。     

大尺度三维复杂场景中雪动态模拟的方法

在大尺度三维复杂场景中,提出了降雪和积雪快速模拟的方法,采用视景体相关和线性分组变换的方法更新雪粒子系统,改进了降雪绘制的视觉效果。针对场景尺度大、复杂度高的特点,创建视点相关遮挡图（Occlusion Map）来精细绘制近处实体的表面积雪细节,生成积雪灰度纹理图来近似模拟远处地物的整体积雪效果,实现了大场景多细节层次积雪的动态模拟。在积雪绘制过程中,近处积雪细节模拟所需的视景体参数和遮挡图通过CPU获取,将复杂的积雪位置的判定和积雪量的计算从CPU移到GPU中,利用自定义的GPU顶点和片段操作加快了积雪模拟的速度。实验结果表明,在大尺度三维复杂场景交互式操作下,提出的方法能较好地实现雪的绘制和模拟。     

基于OSG粒子系统的雪景模拟

为了模拟真实降雪场景,建立逼真的雪景仿真系统,提出了一种基于OSG (open scene graph)粒子系统的仿真模拟技术,建立以雪粒子受力情况分析为核心的自定义雪粒子模型.该模型分析了雪粒子的具体受力情况,随机确定雪粒子的各初始属性,并在每帧中动态的赋予雪粒子不同的风力值以模拟真实降雪场景中的风力情况,解决了传统模型中雪粒子受力不够真实的情况.实验结果表明,自定义雪粒子模型能够更加真实的模拟降雪场景,具有逼真的仿真效果.     

基于粒子系统和Vega的实时雨雪模拟

在分析粒子系统实现原理的基础上,基于Vega提出了一种在大型场景漫游系统中实时模拟雨、雪的方法.基本思想是基于Vega特效模块粒子属性的设置,采用一种始终面向视点的多边形来表示雨、雪粒子模型.试验结果表明,该方法交互性强,能够有效地再现三维场景中的降雨、降雪景象,在实时交互漫游的前提下表现出较强的真实感.     

一种基于雪粒径演化过程的积雪亮温模拟新方法

本研究采用HUT模型、DMRT模型和MEMLS模型模拟积雪雪粒子与不同波段（18.7 GHz和36.5 GHz）微波相互作用（吸收和消光）,并用于辐射传输模型。而雪粒径的获取一直是一个难点,本研究由Jordan91雪粒径演化模型演化得到雪粒径,并将其作为辐射传输模型的输入参数,基于像元内实测数据进行混合像元18.7和36.5 GHz水平极化亮温模拟。结果表明：采用HUT模型、DMRT模型和MEMLS模型的消光系数在18.7 GHz时模拟亮温的偏差分别为-3.6、-1.8和-0.7 K,在36.5 GHz时分别为4.0、10.4和14.4 K。对于18.7 GHz水平极化和36.5 GHz水平极化,基于有效雪粒径的亮温模拟与基于雪粒径演化过程的亮温模拟精度呈现出很好的线性关系。因此,基于雪粒径演化过程的方法是一种合适的获取辐射传输模型中雪粒径参数的方法。     

一种大场景环境下降雪和积雪仿真的新方法

在分析已有的降雪和积雪模拟方法的基础上,提出了一种大场景环境下降雪和积雪模拟的新方法。将降雪和积雪过程看成雪景模拟的两个阶段,分别用不同的方法进行模拟。在降雪方面,引入粒子的旋转,粒子动态纹理和动态颜色等,提高了降雪模拟的真实性和灵活性;在积雪方面,引入噪声技术,解决了已有积雪模拟方法不能用于大场景模拟的问题。实验表明,降雪积雪模拟方法模拟效果真实,实时性良好,适合大场景尤其是飞行模拟系统中。     

Hybrid‐based snow simulation and snow rendering with shell textures

In this study, we develop a system for interactive snow simulation and rendering. Snow is modeled as a hybrid structure that handles movable snow and static snow separately. We develop a simple approach to convert between these two types of snow. Movable snow is represented by a set of particles, whereas static snow is modeled as grid cells. For a piece of movable snow (e.g., snowflake), the particles are connected using springs. Thus, we can model snow as a type of brittle material, such as a snow pile split into chunks of smaller snow pieces after a collision. To render the snow, we adopt a shell structure that has a series of concentric, semitransparent, textured shells. We applied our system to several examples, with the rendered snow appearing similar to real snow. Copyright © 2015 John Wiley & Sons, Ltd.     

复杂气象条件下的机器人视觉

文中系统地总结了近年来团队提出的雨雪及散射的建模与去除算法,包括基于近场光照散射成像模型的深度估计和散射去除算法、基于远场平行光照散射成像模型和区域优化的远场图像去雾算法及基于低秩分解的雪花去除算法和基于矩阵分解的雨雪去除算法.     

视频中的实时降雪与积雪模拟

雪景模拟,特别是基于视频和图像的实时模拟,在计算机动画、游戏和影视特效制作领域有着广泛的应用.但是现有雪景模拟方法中,计算机图形学方法的三维背景建模复杂,而图像处理的相关方法多没有结合背景信息.结合图形和图像的方法,提出基于深度图信息在视频或图像上模拟具有层次感的粒子系统降雪,并在背景图像上添加动态积雪的效果;为增强真实感,对雪花运动建模并实现实时风场;同时加入运动模糊、色调调整、雾气渲染等方式以改进视觉效果.实验结果证明,该方法渲染出的雪景十分真实,并且速度达到了实时要求.     

Analysis and synthesis of textured motion: particles and waves

Natural scenes contain a wide range of textured motion phenomena which are characterized by the movement of a large amount of particle and wave elements, such as falling snow, wavy water, and dancing grass. In this paper, we present a generative model for representing these motion patterns and study a Markov chain Monte Carlo algorithm for inferring the generative representation from observed video sequences. Our generative model consists of three components. The first is a photometric model which represents an image as a linear superposition of image bases selected from a generic and overcomplete dictionary. The dictionary contains Gabor and LoG bases for point/particle elements and Fourier bases for wave elements. These bases compete to explain the input images and transfer them to a token (base) representation with an O(10(2))-fold dimension reduction. The second component is a geometric model which groups spatially adjacent tokens (bases) and their motion trajectories into a number of moving elements--called "motons." A moton is a deformable template in time-space representing a moving element, such as a falling snowflake or a flying bird. The third component is a dynamic model which characterizes the motion of particles, waves, and their interactions. For example, the motion of particle objects floating in a river, such as leaves and balls, should be coupled with the motion of waves. The trajectories of these moving elements are represented by coupled Markov chains. The dynamic model also includes probabilistic representations for the birth/death (source/sink) of the motons. We adopt a stochastic gradient algorithm for learning and inference. Given an input video sequence, the algorithm iterates two steps: 1) computing the motons and their trajectories by a number of reversible Markov chain jumps, and 2) learning the parameters that govern the geometric deformations and motion dynamics. Novel video sequences are synthesized from the learned models and, by editing the model parameters, we demonstrate the controllability of the generative model.     

基于层次分布域和粒子旋转的降雪模拟 *

传统降雪场景的模拟往往由于粒子数量的巨大而降低实时性。针对这一问题 ,在粒子系统的基础上采用梯形层次分布域来发射粒子 ,减少了不可见粒子的浪费 ;同时用伪粒子系统来体现粒子的旋转并引入了层叠纹理的方法来增加逼真度 ,在粒子的消亡条件判断上 ,采用部分判断的算法来提高实时性。实验表明 ,该方法能实时高效地绘制出较为真实的降雪现象。     

颗粒物料输送过程运动特性的离散元模拟

为揭示颗粒物料输送过程中的运动规律以及卸料轨迹的影响因素,采用离散单元 法建立胶带输送机运输模型,对颗粒物料的输送和抛射行为进行模拟。通过颗粒物料输送过程 的运动速度和抛射轨迹分析发现：颗粒在胶带输送机上的运动分为加速、抛射和碰撞3 个阶段, 颗粒在碰撞阶段与挡板和其他颗粒发生多次碰撞,使其速度和方向不断改变,是影响颗粒卸料 落点的主要因素;颗粒物料按照入料顺序分为入料初期颗粒、入料中间颗粒和入料末期颗粒, 入料中间颗粒在抛射和碰撞反弹阶段都会受到其他颗粒的干扰,其卸料轨迹和落点存在较大差 异,碰撞过程中能量损失较大,是影响输送效率的重要原因。     

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.     

Real-time snowing simulation

A snowing scene has a unique fascination for people due to its incomparable beauty. However, little work has been presented on the real-time generation of a dynamic snowing scene, partially due to the difficulty that the simulation of a dynamic snowing process involves the complex modeling of the wind field and the interaction between wind and snow. In this paper, by fully considering the physical characteristics of wind and snow, we construct a three-dimensional wind field based on the discrete form of the Boltzmann equation. According to the interaction laws between wind and snow, we simulate the falling of snow, deposition and erosion in 3D space. Experimental results show that realistic wind-driven snow scenes under different speeds of wind with different amounts of snowfall can be rendered in real-time.