2008년 MUE학회 논문 발표자료 요약 In computer graphics, fluid-like motion is an important and challenging problem with many applications. Currently, the graphical performance and resolution of the mobile phone is highly improved; therefore, mobile 3D games are more and more popular items in the field of mobile services. Yet interactive physically-based fluid simulation is still challenging for mobile platforms. Stable and fast fluid simulation methods are well developed in PC and console games, but fluid simulation and interactive fluid models still have many problems in the mobile environment. We studied and implemented physically-based models for fluids like fire and smoke effects on a Mobile 3D environment.

1. An Interactive Fire Animation on a
Mobile Environment
DongGyu Park, SangHyuk Woo, MiRiNa Jo
Dept. of Information and Communications Engineering,
Changwon National University
DoHoon Lee
Research Institute of Computer Information and Communication,
Pusan National University

2. Motivation and Contribution
• Fluid simulation
– Physics based on realistic animations of water, smoke, explosions,
and related phenomena
• Advances in the mobile handset
– The development of mobile hardware has accelerated.
– The mobile handset which can process 4 million polygons.
– Expected that the fluid simulation will be possible.
• Physics based models in Mobile Game
– As PC games are converted to mobile, we expect that physically
based modeling will be implemented in mobile environment.

3. Introduction-Mobile hardware
• Display
– Small screen
– 48x84 1bit pixel -> 320x240,
256 pixel(WIPI minimum
requirement)
• Mobile Chipset
– ARM 9 family : 200 Mhz
– ARM 10 family : 400 ~ 600 Mhz
– ARM 11 family : 400Mhz ~
1Ghz processor for Smart
phone
• Small Memory size

4. Introduction-Mobile 3D API
• OpenGL-ES
– Low-level Graphic API
– Widely adopted standard
for Mobile device
– Most OS’s, thousands of
applications
– A foundation for high level
APIs
• OpenInventor, Java3D,
game engines.
• JSR-184
– Java Standardization
Request(JSR)
– Mobile 3D Graphics API for
J2ME

5. WIPI & NF3D
• WIPI(Wireless Internet Platform for Interoperability)
– Standard wireless platform in Korea
• About NF3D
– Mobile 3D API based on OpenGL-ES
– Support 3D camera, object handling functions

6. Fire simulation on Mobile Games
• Mobile Game – “Rupee Story”
– 3D adventure games
– Interactive fire simulation on realtime game
– WIPI + NF3D API

7. Navier-Stokes Equation
• NS Equation describes the flow of fluid.
• The sum of inflow and outflow is 0.
(Conservation of mass)
• Conservation of momentum
– u : velocity vector field
– t : time step
– : fluid density
– p : fluid pressure
– v : coefficient of viscosity
– f : external force
fu
1
u)u(
u
0u
2






p
t

fu
1
u)u(
u
0u
2






p
t
Advection
Pressure
DiffuseExternal force

8. Stable Fluid Solver
• Stable Navier-Stokes Solver
– Analysis of evaluating solution is difficult.
• Evaluate approximation Solution using numerical method
– Finite difference : Having instability that don’t convergence quickly.
• When the time interval is large, it is unstable.
• the fluid velocity, we must use a small time interval
– Stable Fluid
• Semi-Lagrangian : the method of getting velocity by back tracking to
the particle of current cell
• Can tolerate the use of larger-than-desired time intervals

9. Fire simulation
• Fire consists of the components: Fuel, Oxygen, Heat,
Inert gas
– Fuel + Oxygen  Heat + Waste
– If there is insufficient oxygen and fuel , the fire makes smoke.
• Physical phenomena around the fire
– As temperature rises, the surrounding air(smoke) has rising force
– As the air moves away from the source of fire, the temperature go
es fall down

10. Fire simulation
• Fire and smoke can be simulated by using buoyancy
force and cooling factor
– Buoyancy force
• This can raise high temperature of gaseous.
– Cooling factor
• As fire and smoke go away from the energy source, the
temperature goes down.
• The temperate goes fall down by certain ratio and is
conducted to neighbor cells
• We can draw the color of fire and smoke based on its
temperature.

11. Interaction
• Use the external force term in NS equation.
• When the game character approaches to the fire, external
force affect the velocity field.
• Interaction for various effects.
– Perturbing the flame and scattering smoke.
Apply external forces for interaction

12. Coloring the Fire and Smoke
• The temperature(wavelength) of the fire determines the
color of the flame.
• As the temperature increases; the color changes from red,
green (yellow), blue and white
– EX) As the temperature changes, the color of the flame is not
same on burning tree.
850 ℃
1100 ℃
1500 ℃
Low temperature
High temperature

13. Coloring the Fire and Smoke
• The temperature of the fire determines the color of the
flame.
• Set the standard temperature
– Separate fire and smoke by specific temperature
• Define the temperature value of grid cell for simulation
– Float value between 0 and 1
– To use RGB color system for coloring flame and smoke
– Set five basic colors: linear interpolation of assigned colors
Linear interpolation
Low
temperature
High
temperature
< The color of the flame >

14. Billboard
• Orienting the polygon based on the view direction
– Two dimensional animated texture maps
– As the view changes, the orientation of the polygon change
– Using to show complex geometry model as one image
• EX) tree, cloud
– Alpha texturing + Fire animation  explosion and smoke effect
Simulated billboard on
Mobile 3DSimulated billboard on PC

15. Billboard
• Using fixed point operation of NF3D
• Using off-screen buffer
– Save simulation result to off-screen buffer
– Copy it as image
– Texturing on billboard
– White background process alpha blending transparently
128
128
Off-screen buffer
Image
Billboard
128
128
Off-screen buffer
Image
Billboard

16. Billboard
• Using alpha blending
– Control transparency of Billboard
– Specified color is complete clearness
(a) does not apply
alpha blending
(b) apply alpha
blending (50%)
(b) erased original
white background
and apply alpha
blending

17. Implementation
• The simulation are applied on our mobile 3D game
“Rupee Story”
• Real-time Interaction with Game Characters, demo

18. Performances
• NF3D supports Frame per second check function
• Tested on emulator and FPS(Frame Per Second) check
function
Simulation and rendering results
0
5
10
15
20
25
30
32*32 48*48 64*64 96*96 128*128
Simulation grid
FPS

19. Performances
• The change of billboard refreshment period
– According to importance of game and fluid simulation
• The change of simulation grid size
– Simulation speed is depend on the size of the grid
– For realtime game application we should have to find appropriate
grid size
– By an experiment, 64*64 grid size is good for our realtime game
application

20. Conclusions and Future Works
• Physically based model for animating and rendering fire
and smoke with using billboard technique on a mobile
environment.
• We demonstrated that this model could be used to
produce realistic looking fire and smoke animation and
plausible realtime interaction for mobile games.
• More realistic fluid effect on new mobile 3D platforms
• Implement water and ocean effects on mobile platforms
• Implement with level-set methods that are very effective
methods for tracking fluid surfaces.

21. Thank you