Envelopment Surface Rendering of Falling Waterfor Particle and Volume Based Virtual Liquid
Manipulation Model
Shunsuke Miyashita 1) , Kenji Funahashi 2)
Nagoya Institute of Technology
1) miyasita@center.nitech.ac.jp, 2) kenji@nitech.ac.jp

1Introduction

This paper describes a method to render envelopment surface of falling water in an interactive manipulation model of virtual liquidthat we have proposed for virtual reality system. In the CG ﬁeld, although researchers
have developed the method to simulate the behavior of ﬂuid with particle method e.g. SPH
method [1], it needs enormous computation
time and usually needs parallel processing torealize real-time processing. Therefore, wehave already proposed an interactive manipulation model of virtual liquid like water which
focus on user impression and real-time processing rather than exact behavior simulation[2]. The model can make user feel realistic sensation from the water behavior occurred byuser’s manipulation. However, free fall condition water is just simulated with particles
and rendered as a particle simply. Althoughmetaball and marching cubes method [3] arefamous to render surface calculated with particle system, these method are not appropriate
for interactive manipulation system becauseof much calculation time and many times ofdata scan. In this paper, we propose an efﬁcient and eﬀective rendering method of free
fall condition water. To reduce data scanningfrequency and searching pattern, it is classiﬁed
into simple patterns to create process of envelopment surface from particles in the method.
Then envelopment surface is rendered in realtime without parallel processing.

improved the stay condition water, that is, wedeveloped the method to express wave, vortex and diﬀusion of color as surface eﬀects ofwater [4]. The improved model can make user
feel more realistic sensation. However, free fallcondition water, or each particle is rendered asa simple square polygon, and every polygon is
directed to the viewpoint.
3

Envelopment Surface Model

The new model renders envelopment surface
for free fall condition water that was rendered
as a particle simply (Fig. 1). Although eachprocess to make the surface is described andillustrated as two dimension in the following
explanation, it is actually rendered in three
dimensional space. At ﬁrst, the 3D space isdivided into grid (Fig. 2(a), (b)). Then thefollowing steps are processed every drawingframe.
Step 1: Set the grid value where any particle exists to “1” (Fig. 2(c)). The calculation
amount of this process is proportional to thenumber of particles that exist in virtual space.
Step 2: Scan and draw each grid as one of thepredeﬁned patterns (Fig. 3 ）according to the
8-neighbor of it (Fig. 2(d)). The calculation
amount is proportional to the grid size.
The grid is often big size, but the number oftimes the grid is scanned is just one per aframe in our model.

y2yParticle and Volume Based Model

Our proposed model represents the water asfollowing two conditions to simplify calculation; the stay condition which exists in a container and the free fall condition which ﬂowsfrom/to container. The stay condition wateris treated with the volume value, and the freefall condition one is treated as a simple particle system that particles do not interfere eachother. Using this model, total calculation timeis reduced drastically, and an interactive water
manipulation is realized in real-time. We havexxzz

Fig. 1: Rendering outline

(a)

(b)

0000000000010000100000010000100011000110000 0(c)0000 0(d)0Fig. 4: Appearance of experimental system

Fig. 2: Rendering procedure

Parentpattern

00 1 0010 1 10・・・

ChildepatternFig.

41 10 1 1 ・・・
0 010 1 0010 0 10

Fig. 5: Envelopment surface and particles

・・・
1 10 1 0 ・・・
01 10 0 1 ・・・
03: Examples of 12 predefined parentpatterns, each pattern has 0–7 childpatterns in 2DExperiment

Using the model mentioned above, we implemented a virtual liquid manipulation system (Fig. 4). User can move a container withPOLHEMUS 3SPACE ISOTRAK II. We compared this new model that renders envelopment surface (Fig. 5(left)) with previous modelthat renders particles directly (Fig. 5(right)).
The envelopment surface is rendered and covers particles which decide the behavior of freefall liquid, so user feel more realistic sensation
at depth and spatial eﬀect. The processing
speed is 29 fps with about 1500 number of particles where the grid size is 120 × 120 × 60.
It means interactive manipulation is realized
in real-time.
5Conclusion

In this paper, we proposed an eﬃcient andeﬀective rendering model of free fall water withenvelopment surface. It can make more real-istic sensation. As a future work, we should
improve visual eﬀects of envelopment surface,e.g. at shading process, and rendering speed
not only for experimental system but for application system. We are now promoting toapply this virtual water manipulation modelto new e-learning system based on VR technique e.g. virtual experience system of chemical laboratory. Realistic sensation felt fromthe liquid manipulation and real-time processing on low-priced PC are important than theexact behavior of water for the system like described above.
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