粒子系统技术：模糊对象建模

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"reeves-particles" 粒子系统(Particle Systems)是一种计算机图形学技术，由William T. Reeves在 Lucasfilm Ltd提出，主要用于模拟和渲染那些难以精确建模的模糊对象，如火焰、云朵和水体等。这种技术将物体视为由大量基本粒子构成的云团，这些粒子定义了物体的体积。随着时间的推移，粒子被生成到系统中，然后在系统内移动和变化形状，最终从系统中消失。这种模型能够表现物体的运动、形态变化以及传统基于表面的表示方法无法实现的动力学效果。粒子系统的一个关键特性是粒子可以容易地进行运动模糊处理，从而避免了时间采样不足导致的临时伪影或闪烁现象。在粒子系统的运作过程中，随机过程被用来生成和控制大量粒子的行为，使得模拟的结果具有更自然、更随机的视觉效果。论文提到了将粒子系统应用到电影《星际迷航2：可汗之怒》(Star Trek II: The Wrath of Khan)的Genesis Demo序列中的火墙元素。这展示了粒子系统在电影特效中的潜力，特别是在创造逼真、动态的视觉效果时。根据计算机图形学的分类和主题描述符，粒子系统涉及以下几个领域： 1.3.3[计算机图形学]: 图像/图片生成 - 粒子系统用于生成动态、复杂的图像和动画。 1.3.5[计算机图形学]: 计算几何与对象建模 - 粒子系统提供了一种新的几何表示方法，用于模糊对象的建模。 1.3.7[计算机图形学]: 三维图形 - 粒子系统在三维空间中创建动态效果，增加了图形的维度和真实感。粒子系统是一种创新的技术，它通过模拟大量粒子的行为，为计算机图形学带来了更为逼真和动态的表现力，尤其在模拟自然现象和复杂视觉效果方面有着广泛的应用。

94 • William T. Reeves

where

MeanParts,a r

is the mean per screen area,

VarPartssa r

its variance, and

ScreenArea

the particle system's screen area. This method controls the level of

detail of the particle system and, therefore, the time required to render its image.

For example, there is no need to generate 100,000 particles in an object that

covers 4 pixels on the screen.

To enable a particle system to grow or shrink in intensity, the designer is able

to vary over time the mean number of particles generated per frame (i.e.,

MeanPartsf and MeanPartss~ r

are, as used above, functions of frame number).

Currently, we use a simple linear function

MeanPartsr = InitialMeanParts + DeltaMeanParts × ( f - fo)

MeanPartss~ I = InitialMeanPartss~ + DeltaMeanParts~ × (f- fo ),

where f is the current frame, f0 the first frame during which the particle system

is alive,

InitialMeanParts

the mean number of particles at this first frame, and

DeltaMeanParts

its rate of change. The variance controls,

VarPartsf and

VarParts~a r, are

currently constant over all frames. More sophisticated quadratic,

cubic, or perhaps even stochastic variations in both the mean and variance

parameters would be easy to add.

To control the particle generation of a particle system, therefore, the designer

specifies f0 and either the parameters

InitialMeanParts, DeltaMeanParts,

and

VarParts,

or the parameters

InitialMeanParts,~, DeltaMeanParts, a,

and

VarParts,~.

2.2 Particle Attributes

For each new particle generated, the particle system must determine values for

the following attributes:

(1) initial position,

(2) initial velocity (both speed and direction),

(3) initial size,

(4) initial color,

(5) initial transparency,

(6) shape,

(7). lifetime.

Several parameters of a particle system control the initial position of its particles.

A particle system has a position in three-dimensional space that defines its origin.

Two angles of rotation about a coordinate system through this origin give it an

orientation. A particle system also has a

generation shape

which defines a region

about its origin into which newly born particles are randomly placed. Among the

generation shapes we have implemented are: a sphere of radius r, a circle of

radius r in the

x-y

plane of its coordinate system, and a rectangle of length I and

width w in the

x-y

plane of its coordinate system. Figure 1 shows a typical

particle system with a spherical generation shape. More complicated generation

shapes based on the laws of nature or on chaotic attractors [1] have been

envisioned but not yet implemented.

ACM Transactions on Graphics, Vol. 2, No. 2, April 1983

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