实时动态视差遮挡映射与软阴影技术

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“Dynamic Parallax Occlusion Mapping with Soft Shadows”是2006年GDC（游戏开发者大会）上发布的一篇技术文档，由Natalya Tatarchuk在ATI Research, Inc.撰写。该文档主要探讨了动态视差遮蔽贴图（Parallax Occlusion Mapping，简称POM）与近似软阴影相结合的方法，用于实时渲染中提高场景的真实感。在图形学领域，Parallax Occlusion Mapping是一种先进的纹理映射技术，旨在解决传统法线贴图在处理高度变化表面时出现的视觉不准确问题。它通过模拟物体表面的深度，使纹理在视角变化时产生适当的偏移，从而创建出更真实的立体感。与普通法线贴图相比，POM能更好地呈现物体表面的凹凸感，如图1所示，应用POM的鹅卵石人行道（a）比仅使用法线贴图（b）的渲染效果更为逼真。本文档的核心内容是介绍了一种像素级光线追踪算法，该算法能够在实时环境中动态地处理光照对表面的影响，同时实现近似的软阴影效果。在实时渲染中，尤其是在游戏场景中，实现软阴影通常是一个挑战，因为这需要大量的计算资源。而本论文提出的解决方案能够在保证性能的同时，提供更加柔和、自然的阴影效果，增强场景的视觉体验。此外，文档可能还涵盖了以下知识点： 1. **光线追踪算法**：这是一种计算图形学中的方法，用于模拟光如何与物体相互作用。在这里，它被优化以适应实时环境，即在不影响帧率的情况下计算视差遮蔽和阴影。 2. **动态照明**：指的是光线源位置或强度随时间变化的情况。动态照明要求渲染系统能够快速响应这些变化，以确保场景的光照效果始终准确。 3. **近似技术**：为了实现实时渲染，通常需要采用一些近似方法来降低计算复杂性。本文档可能描述了如何用近似算法来模拟真实世界的软阴影，同时保持足够的视觉质量。 4. **性能优化**：在实时渲染中，性能优化至关重要。文档可能讨论了如何通过各种技术，如早期Z剔除、硬件加速等，来减少计算开销。 5. **编程实现**：可能包含关于如何将这种技术整合到游戏引擎或其他实时渲染平台中的技术细节，包括使用的编程语言、API（如OpenGL或DirectX）以及具体的代码实现策略。 6. **应用案例**：除了理论和算法，文档可能会展示实际应用示例，如城市景观的渲染，以证明所提出方法的有效性和实用性。这篇技术文档为图形学和游戏开发领域的专业人士提供了深入理解动态视差遮蔽贴图和软阴影结合的宝贵资料，有助于他们提升实时渲染场景的视觉质量。对于想要学习高级纹理映射和实时阴影技术的读者来说，这是一个不可多得的学习资源。

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I3D 2006, Redwood City, California, 14–17 March 2006.

Dynamic Parallax Occlusion Mapping with Approximate Soft Shadows

Natalya Tatarchuk

ATI Research, Inc.

Figure 1. Realistic city scene rendered using parallax occlusion mapping applied to the cobblestone sidewalk in (a) and using the

normal mapping technique in (b).

Abstract

This paper presents a per-pixel ray tracing algorithm with dy-

namic lighting of surfaces in real-time on the GPU. First, we

propose a method for increased precision of the critical ray-

height field intersection and adaptive height field sampling. We

achieve higher quality results than the existing inverse displace-

ment mapping algorithms. Second, soft shadows are computed by

estimating light visibility for the displaced surfaces. Third, we

describe an adaptive level-of-detail system which uses the infor-

mation supplied by the graphics hardware during rendering to

automatically manage shader complexity. This LOD scheme

maintains smooth transitions between the full displacement

computation and a simplified representation at a lower level of

detail without visual artifacts. The algorithm performs well for

animated objects and supports dynamic rendering of height fields

for a variety of interesting displacement effects. The presented

method is scalable for a range of consumer grade GPU products.

It exhibits a low memory footprint and can be easily integrated

into existing art pipelines for games and effects rendering.

CR Categories: I.3.7 [Computer Graphics]: Three-Dimensional

Graphics and Realism: Color, shading, shadowing, and texture;

Visible line/surface algorithms

Keywords: image-based rendering, motion-parallax, real-time

rendering, displacement mapping, soft shadows, surface details,

adaptive level-of-detail system.

1 Introduction

Texture mapping is essential for creating a compelling impression

of a realistic scene without paying the full cost of rendering

complex geometry. Bump mapping was introduced in the early

days of computer graphics in [Blinn 1978] to avoid rendering

high polygonal count models. Despite its low computational cost

and ease of use, bump mapping fails to account for important

visual cues such as shading due to interpenetrations and self-

occlusion, nor does it display perspective-correct depth at all

angles.

Displacement mapping, introduced by [Cook 1984], addressed the

issues above by actually modifying the underlying surface geome-

try. Ray-tracing based approaches dominated in the offline

domain [Pharr and Hanrahan 1996; Heidrich and Seidel 1998].

These methods adapt poorly to current programmable GPUs and

are not applicable to the interactive application domain due to

high computational costs. Other approaches included software-

based image-warping techniques for rendering perspective-correct

geometry [Oliveira et al. 2000] and precomputed visibility infor-

mation [Wang et al. 2003; Wang et al. 2004; Donnelly 2005].

Despite being interactive, these methods suffer from a large

memory footprint. The majority of these techniques require high

amounts of specialized precomputed data, thus making their

integration into existing art pipelines for game development

unnecessarily complex. Our proposed method requires a low

memory footprint comparable to bump mapping and can be used

for dynamically rendered height fields.

Recent inverse displacement mapping approaches take advantage

of the parallel nature of novel GPUs’ pixel pipelines to render

displacement directly on the GPU ([Doggett and Hirche 2000;

Kautz and Seidel 2001; Hirche et al. 2004; Brawley and

Tatarchuk 2004; Policarpo et al. 2005]. One of the significant

disadvantages of these approaches is the lack of correct object

silhouettes since these techniques do not modify the actual ge-

ometry. Accurate silhouettes can be generated by using view-

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