实时渲染任意表面的毛发技术：粒子系统与lapped纹理

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Real-TimeFur.pdf是一篇由Jerome Lengyel、Emil Praun、Adam Finkelstein和Hugues Hoppe合作的研究论文，发表在Microsoft Research和Princeton University的研究者们之间。本文介绍了一种创新的方法，旨在实现实时渲染任意拓扑表面的毛发（fur）。该方法的关键步骤包括： 1. **虚拟头发模拟与粒子系统**：首先，通过使用粒子系统来模拟真实的虚拟头发。这一步为后续的渲染过程提供了基础数据。 2. **体积纹理生成**：将模拟出的头发样本存储到一个体积纹理（volume texture）中，作为动态渲染的资源。体积纹理允许对头发的密度和方向进行连续的表示。 3. **lapped textures参数化**：为了适应不同形状表面的贴图，作者们开发了一种名为“lapped textures”的技术。这种方法通过反复粘贴纹理块覆盖整个表面，实现了纹理的全局参数化，使得毛发的生长方向能在整个表面上保持一致。 4. **半透明介质壳渲染**：在运行时，体积纹理被划分为一系列同心层，并以半透明的方式呈现，形成毛发的立体效果。 5. **增强边缘视觉质量**：为了提高毛发在轮廓线附近的表现，论文提出在表面法线方向添加“鳍状”元素（fins），这些使用常规的二维纹理映射，从体积纹理中按头发生长方向采样。 6. **实时修改与控制**：该方法的优势在于支持实时调整观察视角和照明条件，以及对局部毛发颜色、长度和方向的精细控制，这对于交互式应用来说是至关重要的，尤其是在处理中等复杂度的模型时。 Real-TimeFur.pdf提供了一种高效且逼真的实时毛发渲染技术，能够实现对任意拓扑表面的毛发表现，适用于各种实时图形应用，如游戏、影视制作等，为艺术家和设计师提供了实时调整和控制毛发细节的能力。

Real-Time Fur over Arbitrary Surfaces

Jerome Lengyel

Microsoft Research

http://research.microsoft.com/~jedl

Emil Praun

Princeton University

http://www.cs.princeton.edu/~emilp

Adam Finkelstein

Princeton University

http://www.cs.princeton.edu/~af

Hugues Hoppe

Microsoft Research

http://research.microsoft.com/~hoppe

Abstract

We introduce a method for real-time rendering of fur on surfaces

of arbitrary topology. As a pre-process, we simulate virtual hair

with a particle system, and sample it into a volume texture. Next,

we parameterize the texture over a surface of arbitrary topology

using “lapped textures” — an approach for applying a sample

texture to a surface by repeatedly pasting patches of the texture

until the surface is covered. The use of lapped textures permits

specifying a global direction field for the fur over the surface. At

runtime, the patches of volume textures are rendered as a series of

concentric shells of semi-transparent medium. To improve the

visual quality of the fur near silhouettes, we place “fins” normal

to the surface and render these using conventional 2D texture

maps sampled from the volume texture in the direction of hair

growth. The method generates convincing imagery of fur at

interactive rates for models of moderate complexity. Further-

more, the scheme allows real-time modification of viewing and

lighting conditions, as well as local control over hair color, length,

and direction.

Additional Keywords: hair rendering, lapped textures, volume textures.

1. Introduction

A distinguishing characteristic of mammals is that they have hair.

For many computer graphics applications, a sense of immersion

requires the presence of realistic virtual creatures. Unfortunately,

generating convincing imagery of people and animals remains a

challenging problem, in part because the hair often looks artificial.

This paper presents a method for rendering realistic fur over

surfaces of arbitrary topology at interactive frame rates. In this

paper, we use the terms fur and hair interchangeably.

Perhaps the most effective imagery of fur is due to Kajiya and

Kay [4], who ray-traced a model with explicit geometric detail

represented as volume textures. In the computer animation indus-

try, fine geometric modeling of fur led to convincing artificial

mammals, such as the lemurs in Disney’s Dinosaur [1]. Another

example is the dog fur n 101 Dalmatians [2], which was repre-

sented using a stochastic (rather than geometric) model.

However, rendering hair is computationally expensive, and the

majority of the methods to date are too slow for interactive use

(see Thalmann et al. [10] for a survey). In an interactive setting,

Van Gelder and Wilhelms [11] showed that various parameters of

fur can be manipulated in real time. However, their renderer is

limited to drawing polylines, which limits the number of hairs that

can be drawn per frame as well as the realism of the result.

Recent work by Meyer and Neyret [6][7] showed that rendering

volume textures at interactive rates is possible by exploiting

graphics card hardware. Lengyel [5] subsequently developed a

similar method optimized for the specific case of fur. The method

renders a furry surface as a series of concentric, semi-transparent,

textured shells containing samples of the hair volume. By exploit-

ing conventional texture mapping, the method allows interactive

rendering of furry models represented by thousands of polygons.

Our approach is based on the shell method and thus achieves

interactive frame rates. However, in this project, we address three

limitations of previous work. First, the shell method requires a

global parameterization of the surface. While this is easy to achieve

for special cases (such as a torus or disc), it prevents application of

this method to a surface of arbitrary topology, without first cutting

the surface into pieces and rendering them separately. Second, the

method requires a significant amount of texture memory, because

each shell needs a separate texture covering the whole surface and

these textures must be large enough to resolve individual hairs.

Finally, the shell method provides an effective approximation to

volume textures only when the viewing direction is approximately

normal to the surface. Near silhouettes where shells are seen at

grazing angles, the hair appears to be overly transparent, and gaps

become evident between the shells.

In our work, we address the first two limitations of the shell

method – parameterization and texture memory size – by using

the lapped textures of Praun et al. [8]. Lapped textures cover a

surface of arbitrary topology by repeatedly pasting small patches

of example texture over the surface ( ). Because the

surface is covered by a collection of patches, we only need local

parameterizations within the individual patches, as opposed to a

global parameterization. Furthermore, the many patches instanti-

ated over the surface can all share the same texture, thus greatly

reducing texture memory overhead.

Figure 1

The third limitation – silhouettes – is not specific to the shell

method. In interactive settings such as games, designers use low-

polygon-count models to maintain high frame rates. Detailed

textures help to embellish these coarse models, except at the

silhouette where tangent discontinuities detract from the visual

quality. Furthermore, in the specific case of fur, the visual cues at

the silhouette play a critical role in perceiving the characteristics

of the fur. Polygonal artifacts may be alleviated by using higher

resolution models, or by clipping to a high-resolution 2D contour

as described by Sander et al. [9]. To address the silhouette prob-

lem, we introduce a scheme for rendering textured fins normal to

the surface near silhouettes. The fin textures are created using the

same volumetric model for hair as the shell textures, but sampled

in a different direction more appropriate for oblique viewing.

Alternatively, an artist may create a fin texture directly.

We offer interactive control over local and global properties of the

fur, such as its direction, length, or color. For example, the user

may globally adjust the angle of the hair with respect to the

surface or may locally adjust the hair direction using a “combing

tool”.

The principal contributions of this work are: (1) integrating the

shell method with lapped textures to allow for arbitrary topology

and to reduce texture requirements; (2) improving the visual

quality of silhouettes by rendering fins; and (3) demonstrating

interactive local and global control over hair properties.

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