Physically based animatio n of sandstorm

http://kucg.korea.ac.kr

PHYSICALLY BASED ANIMA-TION OF SANDSTORM

Shiguang Liu, Zhangye Wang, Zheng Gong, Lei Huang, and Qunsheng Peng

(presented by Kam, Hyeong Ryeol)


CONTENTS Abstract Introduction Related Work Modeling of the Sandstorm Rendering of Sandstorm Scene Results and Discussion Conclusion and Future Works


ABSTRACT Physically based method for modeling and

animating sandstorm Stable incompressible multiple fluid model Based on Reynold-average Navier-Stokes equa-

tions. The sand and dust particle flow is computed tak-

ing interaction among the wind, sand, and dust particles into account.

Multi-Fluid Solver is designed and implemented on GPU.

Various illumination effects of sandstorm scenes can be simulated by spectral sampling scattering calculation.


INTRODUCTION There were little attention to this area We propose a fast, physically based, and eas-

ily implemented method for modeling and animating realistic sandstorm scenes.

Sandstorm carries huge amount of sand and dust

The wind is caused by convection currents created by intense heating of the ground.

Air is unstable when heated This causes the mixture of higher winds in the

troposphere with winds in the lower atmosphere, incurring strong surface winds.


INTRODUCTION We propose a physically based method for

modeling and animating sandstorm 1. We establish the unstable wind field of sand-

storm based on RANS. The motion of sand and dust particle is regarded as the

continuous flows (expressed by the non-viscosity fluid model)

2. We propose a GPU-based Multi-Fluid Solver for dynamic sandstorm scene.

3. Spectral sampling of the scattering light. 4. According to the statistical distribution of the

size of sand and dust particles, fantastic illumi-nation effects of sandstorm in different areas and at different stages are rendered.


RELATED WORK Studying the motion of sandstorm by experi-

mental data or numerical analysis method Joseph et al. studied the relationship between the

weather conditions and velocity of sandstorm. Aiming at calculating the movement of sandstorm

Too complicated to be visualized For simulation of fluid-like natural phenomena,

most works adopt fluid models. Volume of fluid

Conserving mass, tracking and locating the free surface Moving particle semi-implicit

Incompressible fluid / gridless particle method


RELATED WORK Chemical reaction may exist.

ex) combustion Two-Fluid model

Two-Fluid Lattice Boltzmann model Miscible binary mixtures

Volcanic clouds Gas-solid flow / Conveyed by the velocity field Interaction during explosion / Drag force

Sand particle are conveyed by the velocity field AND the interaction between sand parti-cles and the air flow.


RELATED WORK Realistic simulation of sandstorm

A method of modeling granular materials(sand, grains) Helps to simulate phenomena like splashing or avalanches

Modeling and rendering realistic desert scene include sand dunes and wind ripples Bump-mapping using Level of Detail.


MODELING OF THE SANDSTORM We consider sandstorm as a multi-fluid com-

posed of wind, sand, and small dust particles flows. Wind field Sand and dust particle flow model Interaction among wind, sand, and dust particle

flow Multi-fluid solver on GPU


MODELING OF THE SANDSTORM- WIND FIELD Wind Field

Stable near-surface air flow : Navier-Stokes equations. Unstable air flow : Reynold-average Navier-Stokes

Sandstorm - (considering the effects of the atmospheric tur-bulence)


MODELING OF THE SANDSTORM- WIND FIELD The velocity distribution

around a sand particle Reynold shear stress is

(a) not considered (b) considered


MODELING OF THE SANDSTORM- SAND AND DUST PARTICLE FLOW MODEL Since a sandstorm consists of a huge number

of sands and dust particles, so tracing each particle is not feasible. Particles’ movements obey statistical distribution

like fluid so we can approximate the motion of sand and dust particle as non-viscosity, incom-pressible fluid like below:


MODELING OF THE SANDSTORM- SAND AND DUST PARTICLE FLOW MODEL The force of a single particle in air flow

Suppose the sand and dust particles : spherical The particles move in XOY plane

This force consists of The valid gravity of sand particle The entrainment force by air flow : most important

produced by the velocity difference between the air flow and the sand particle flow


MODELING OF THE SANDSTORM- SAND AND DUST PARTICLE FLOW MODEL The force of a single particle in air flow

the valid gravity of particle

the entrainment force

the coefficient of resistance

Buoyancy of the sand particle in the

air flow


MODELING OF THE SANDSTORM- INTERACTION AMONG WIND, SAND, AND DUST PARTICLE FLOW The sand and dust particles are entrained by

the wind. The velocity of the wind will be affected by

the counterforce of the sand and dust parti-cle flow

Sandstorm’s external force is the interaction force between sand particle flow and air flow (caused by the velocity difference between them)


MODELING OF THE SANDSTORM- INTERACTION AMONG WIND, SAND, AND DUST PARTICLE FLOW The wind field, sand, and dust particle flows can

be regarded as continuous fluid. Interaction among these can be modeled as that be-

tween wind field and a group of particles We account the sand and dust particles in a unit vol-

ume as a whole The counterforce to the wind field by the sand and

dust particle flow is Equivalent to adding a body force to the wind field model


MODELING OF THE SANDSTORM- INTERACTION AMONG WIND, SAND, AND DUST PARTICLE FLOW The diameter distribution of sand and dust

particles in sandstorms

L : low / M : moderate / S : High visibilities


MODELING OF THE SANDSTORM- INTERACTION AMONG WIND, SAND, AND DUST PARTICLE FLOW Due to the diameter of sand and dust particle is

very small, the interaction force between them can be ignored.

The interaction force between sand particles in a unit volume and the air flow :


MODELING OF THE SANDSTORM- MULTI-FLUID SOLVER ON GPU Our model describes a multiple fluid system

Air flow Sand and dust particle flows.

We solve the multiple Navier-Stokes equa-tions in parallel in one rendering pass by combining multiple field data texture into one texture. It reduces the calculating time

Flat 3D texture technique It’s easy to read and store velocity data


MODELING OF THE SANDSTORM- MULTI-FLUID SOLVER ON GPU The calculation flow

1. Initialize the air flow and sand particle flow 2. Set the initial condition and boundary condition 3. Solve the NS equations on GPU by the Semi-La-

grange methods.

The size of our flat 3D texture is several times as large as that of the previous method, but it doesn’t affect the calculation efficiency for the lin-ear calculation function of GPU


MODELING OF THE SANDSTORM- MULTI-FLUID SOLVER ON GPU Flow chart of Multi-Fluid

Solver With this, we can solve

multiple NS in parallel in one rendering pass.


RENDERING OF SANDSTORM SCENE To show realistically, we must consider the

interaction of various types of components of sandstorm with light. Scattering / Absorption effect of particles

Our rendering model of sandstorm scene is based on multiple Mie scattering theory

We adopt pre-computation technique to ac-celerate the rendering rate.


RENDERING OF SANDSTORM SCENE- MIE SCATTERING MODEL FOR NATURAL LIGHT Theory for scattering of spherical particles

For particle sizes larger than a wavelength Produces a pattern like an antenna lobe, with a sharper

and more intense forward lobe for larger particles. Not strongly wavelength dependent and produces the

almost white glare



RENDERING OF SANDSTORM SCENE- CALCULATION OF SCATTERING IN SANDSTORM The shape of the majority of sand and dust parti-

cles is spherical The effect of scattering can be determined by

measuring the intensity Isca of a light ray after traveling l distance.

If I0 is the intensity of the light source, the ratio is

According to Bougure Law


RENDERING OF SANDSTORM SCENE- CALCULATION OF SCATTERING IN SANDSTORM Considering the distribution of sand particles

in sandstorm, we define the scattering coeffi-cient of sand particles in a unit volume as

Since the computation of the Mie scattering is very complicated including calculation of scatter-ing section and scattering coefficient, we use a new method to pre-compute these terms of sand particles


RENDERING OF SANDSTORM SCENE- RENDERING OF SANDSTORM SCENE Multiple scattering effect of sands

discrete the space filled with sandstorm into voxels

For each voxel Pi,j, its incident radiance from direction w includes the direct light from the light source in direction w and multiple scat-tered light from other voxels.

Multiple scattering model is :


RENDERING OF SANDSTORM SCENE- CALCULATION OF SCATTERING IN SANDSTORM In-scattering from the six neighboring voxels

are sampled, so

For scattering of sand particles is almost isotropic, we consider the phase function as constant


RENDERING OF SANDSTORM SCENE- CALCULATION OF SCATTERING IN SANDSTORM Rendering method : a two-pass algorithm

We pre-compute the shading of sandstorm scene according to the position of each voxel and the incident direction of light source in the first pass. ▼

We use the shading result To render the scene under fixed viewpointin the second pass


RESULTS AND DISCUSSION With the increase of the density of sand and dust

particles, the scattering color is changing gradu-ally from light yellow to yellow, then to red, and the visibility decreases correspondingly The color change is mainly caused by the change of

density distribution of sand particles.

Our method is based on physical theory The appearance of sandstorm and scattering effects

of the road lamp looks realistic.


RESULTS AND DISCUSSION Figure 8

(a) rendering result (b) the real photo



(a) rendering result (b) the real photo



(a) high visibility (b) moderate visibility (c) low visibility



Near the viewpoint



Sandstorm scenes with moderate visibility


CONCLUSION AND FUTURE WORKS Our method adopts multi-phase fluid models to

simulate the motion of air, sand, and dust parti-cles in the sandstorm.

The wind field is established by RANS equations The sand and dust particle flow is built with the

non-viscosity fluid model taking the statistical dis-tribution of particles of varied size into account.

We design a Multi-Fluid Solver and implement it on GPU

By spectral sampling of the light scattering, the peculiar illumination effect of dynamic sandstorm scenes is revealed.


CONCLUSION AND FUTURE WORKS Contributions

1. It is the first time to simulate dynamic sandstorm scene based on physical principles.

2. We adopt multiple fluid model on GPU to deal with the motion and the complex interaction fast.

3. System is easy to implement. Users can generate various realistic sandstorm scenes with different visibil-ity at different stages.


CONCLUSION AND FUTURE WORKS Future Work

This model can be extended to simulate other phe-nomena of multiple gas-solid mixtures But, oil-water-like phenomena is our next goal

Dynamic sandstorm model is still far from perfect It’s like fog when it is close to the view point Euler-based method combining with particle system suggests

a potential way for overcoming this limitation. We will simulate other natural disastrous phenomena

Such as debris flow, avalanche, etc


THE END Any Questions??

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Physically based animatio n of sandstorm