屏幕空间动态全局光照近似技术

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"GI in image space，也称为屏幕空间方向性遮挡(SSDO)，是环境光遮蔽(AO)的一种进化形式，它不仅包含了AO的遮挡信息，还增加了遮挡方向以及漫反射信息，使得实时渲染中的全局光照效果更加接近物理真实。这种方法在动态全局光照的近似计算中具有重要意义，适用于具有大量几何细节和全动态光照的场景。" 在实时渲染领域，全局光照(Global Illumination, GI)是模拟真实世界光照效果的关键因素，它能考虑光线在场景中多次反射、折射的效果。然而，完全精确的GI计算在实时环境下通常是不现实的，因此需要寻找有效的近似方法。屏幕空间技术因其高效和易于实现的特点，被广泛应用于游戏和实时图形中。屏幕空间环境光遮蔽(Screen Space Ambient Occlusion, SSAO)是一种常用的实时GI近似技术，它通过检查像素周围的几何信息来估算场景中点的局部遮挡程度，从而增加深度感和阴影效果。SSAO虽然有效，但仅能处理遮挡信息，无法提供更复杂的光照交互。 "GI in image space"或"屏幕空间方向性遮挡(SSDO)"则是对SSAO的扩展，它不仅仅是计算遮挡，还引入了遮挡方向信息。这一改进意味着SSDO可以模拟出类似定向阴影的效果，即考虑到光源的方向对遮挡的影响。此外，SSDO还考虑了漫反射间接光的一次反弹，这使得场景中的颜色混合和光照传播更加自然，增强了光照的层次感。与经典SSAO相比，SSDO的开销相对较小，这意味着在保持帧率的同时，可以提升画面的真实感。它可以与其他模拟多通道光传输的方法结合，如延迟着色、屏幕空间反射等，以进一步增强图像质量。这样的综合应用使得在高分辨率下处理具有数百万多边形的复杂动态场景成为可能。 "GI in image space"通过在屏幕空间内计算方向性遮挡和漫反射信息，为实时渲染提供了更加逼真的全局光照效果，同时保持了高效的计算性能。这一技术的不断发展和优化，对于提升实时图形的视觉体验具有重大意义。

Approximating Dynamic Global Illumination in Image Space

Tobias Ritschel Thorsten Grosch Hans-Peter Seidel

MPI Informatik

Figure 1:

We generalize screen-space ambient occlusion (SSAO) to directional occlusion (SSDO) and one additional diffuse indirect bounce of

light. This scene contains 537k polygons and runs at 20.4 fps at 1600×1200 pixels. Both geometry and lighting can be fully dynamic.

Abstract

Physically plausible illumination at real-time framerates is often

achieved using approximations. One popular example is ambient

occlusion (AO), for which very simple and efﬁcient implementations

are used extensively in production. Recent methods approximate

AO between nearby geometry in screen space (SSAO). The key

observation described in this paper is, that screen-space occlusion

methods can be used to compute many more types of effects than just

occlusion, such as directional shadows and indirect color bleeding.

The proposed generalization has only a small overhead compared to

classic SSAO, approximates direct and one-bounce light transport

in screen space, can be combined with other methods that simulate

transport for macro structures and is visually equivalent to SSAO in

the worst case without introducing new artifacts. Since our method

works in screen space, it does not depend on the geometric complex-

ity. Plausible directional occlusion and indirect lighting effects can

be displayed for large and fully dynamic scenes at real-time frame

rates.

CR Categories:

I.3.7 [COMPUTER GRAPHICS]: Three-

Dimensional Graphics and Realism; I.3.3 [COMPUTER GRAPH-

ICS]: Color, Shading, Shadowing and Texture

Keywords: radiosity, global illumination, constant time

1 Introduction

Real-time global illumination is still an unsolved problem for large

and dynamic scenes. Currently, sufﬁcient frame rates are only

achieved through approximations. One such approximation is ambi-

ent occlusion (AO), which is often used in feature ﬁlms and computer

games, because of its high speed and simple implementation. How-

ever, AO decouples visibility and illumination, allowing only for

a coarse approximation of the actual illumination. AO typically

displays darkening of cavities, but all directional information of

the incoming light is ignored. We extend recent developments in

screen-space AO towards a more realistic illumination we call screen-

space directional occlusion (SSDO). The present work explains, how

SSDO

• accounts for the direction of the incoming light,

• includes one bounce of indirect illumination,

• complements standard, object-based global illumination and

• requires only minor additional computation time.

This paper is structured as follows: First, we review existing work in

Section 2. In Section 3 we describe our generalizations of ambient

occlusion for the illumination of meso-structures. Section 4 explains

extensions to improve the visual quality. In Section 5 the integration

of our method into a complete global illumination simulation is de-

scribed. We present our results in Section 6, we discuss in Section 7

before concluding in Section 8.

2 Previous Work

Approximating physically plausible illumination at real-time frame-

rates has recently received much attention. Ambient occlusion (AO)

[Cook and Torrance 1981][Zhukov et al

1998] is used in production

extensively [Landis 2002] because of its speed, simplicity and ease

of implementation. While physically correct illumination computes

the integral over a product of visibility and illumination for every

direction, AO computes a product of two individual integrals: one

for visibility and one for illumination. For static scenes, AO allows

to pre-compute visibility and store it as a scalar ﬁeld over the sur-

face (using vertices or textures). Combining static AO and dynamic

lighting using a simple multiplication gives perceptually plausible

[Langer and B

ulthoff 2000] results at high framerates. To account

for dynamic scenes, Kontkanen et al. [2005] introduced AO Fields,

which allow rigid translation and rotation of objects and specialized

solutions for animated characters exist [Kontkanen and Aila 2006].

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