Real-time Generation of CG and Sound of Liquid with Bubble

Masataka Imura∗ Yoshinobu Nakano Yoshihiro Yasumuro Yoshitsugu Manabe Kunihiro Chihara

Graduate School Information Science, Nara Institute of Science and Technology

1 Introduction

Recently, we can render real-time CG of liquid which shows variousphenomena by global lighting and physical simulation accordingto the improvement of the computing power. However, the effectsound for real-time CG of liquid should be made corresponding toall the behavior of the liquid beforehand. Therefore, it is difficultfor high reproducibility to be made for all sounds by hand. In liquid,the sound is generated according to the change of the radius of themoving bubble. Therefore, making the effect sound of liquid shouldconsider the movement of bubbles.

In this paper, we propose the technique for enhancing SmoothedParticle Hydrodynamics(SPH), expressing the liquid with bubblesby introducing the particles that compose the liquid and the gas.Moreover, the technique for outputting the effect sound by syn-chronizing with rendering of CG is proposed. This technique dis-tinguishes the number of bubbles spatially generated by using theproximity-area search lattice of SPH. In addition, this technique canautomatically generate the effect sound for real-time CG of the liq-uid by obtaining the characteristic frequency from the bubble size.The proposal technique enables to generate animation and the effectsound of the liquid with bubbles in real-time.

2 Enhancing Smoothed Particle Hydrody-namics

Figure 1 shows the block diagram of the proposed system. Thesystem can be divided into computation fluid dynamics simulationmodule, graphics rendering module and sound rendering module.

To make appropriate sound from physical simulation of liquids, themovement of bubbles, which are main sources of the sound, mustbe integrated into the simulation. We enhance Smoothed ParticleHydrodynamics, one of the particle-based fluid simulation meth-ods, to treat two phases: gas and liquid. In SPH, the Navier-Stokesequation is discretized into particle-particle interaction. In the en-hanced SPH, there are interaction between gas-liquid particles andliquid-liquid particles, but no interaction between gas-gas particles.

Physical Simulator

Position of Particles

Sound Rendering Unit

Bubble (List, Radius, Position)

Graphic Rendering Unit

波形テーブル波形テーブル Sound Judgment

Model of Generation

Model of Explosion

Wave Selector

Frequency Modulation

OutputModule

(OpenAL)

♪♪♪♪

Wave

Select

Flag

Radius

Wave

Frequency

Marching CubesShadow curved surface is generated.

Shader that considers bubblesSurface Rendering.

Enhanced SPHTo treat two kinds of particles.

The frequency is calculated from the radius of the bubble.

Effect Soundis generated !

SystemSystemSystemSystem

Table of Waves

The position of liquid and air particles is calculated.

The parameter of the bubble is calculated by clustering of air particle.

Generation and explosion of bubbles are judged.The shape of waves is

selected.

radiusfrequency

α＝Rendering

Shadow Curved Surface

Figure 1: Proposed System

∗e-mail: imura@is.naist.jp

3 Automatic Generation of Sound of Liquid

To find the position and size of bubbles in liquid, we use clusteringof gas particles. For clustering, first we map the gas particles intoregular lattice, then merge neighboring grids which contain gas par-ticles. The position of a certain bubble is the center of gravity of theparticles which consist of one bubble and the radius of the bubbleis the maximum distance between particles in the bubble. The po-sitions of bubbles are tracked frame by frame, then the generationand extinction of bubbles, which are the event accompanied withsound, are detected. Figure 2 shows the typical result of cluster-ing of gas particles. The particles which belong to the same clusterhave the same color.

Figure 2: Result of Clustering

The typical sound of bubble generation and extinction are recordedbeforehand and are used as sound source. However, the principalfrequency of the liquid sound depends on the size of the bubble. Inthe proposed system, the appropriate sound source is selected basedon the event type (generation or extinction), then the frequency ofthe selected sound source is modulated based on the bubble radius.After modulated, the sound is outputted using three-dimensionalsound generation system.

4 Result

The prototype system was constructed with OpenGL for graphicsrendering and OpenAL for sound rendering on Intel Core2 DuoPC. We used 6750 liquid particles and 1000 gas particles. Figure 3shows the typical result of the generation of graphics and sounds.We can render the proper graphics with bubbles inside the liquidand the sound is added to the animation. The speed of calculationand rendering is about 15 frames per second.

Average Delay time : 15 ms Average FPS : 15 fps

Figure 3: Result