Extinction Transmittance Maps

彬彬

The interaction between light and participating media involves

complex physical phenomena such as light absorption and scattering.

The radiance transmitted through a medium then depends on

the variations of scattering and extinction along potentially complex

light paths, yielding soft light shafts and shadowing (Figure 1).

Computing light scattering in these media usually requires complex

offline computations [Cerezo et al. 2005]. Some real-time applications

are based on heavy precomputations [Zhou et al. 2008].

Some others introduce restrictions such as approximate diffusion

schemes, or specific volume representations.

In particular, [Jansen and Bavoil 2010] extend the concept of deep

shadow maps [Lokovic and Veach 2000] by representing the variations

of opacity within the medium in Fourier space: each pixel

of the Fourier Opacity Map stores a set of projection coefficients.

Opacity variations are then unprojected from the coefficients to

evaluate the overall opacity of the medium for each visible particle.

This technique is highly effective, and the map is built by accumulating

the contributions of each particle independently using

alpha blending. However, this method only represents an opacity

information and cannot account for actual light scattering.

This limitation is raised in [Delalandre et al. 2011]: instead of

representing opacities, each pixel of the Transmittance Function

Map (TFM) stores a Fourier transform of the medium transmittance

along light rays. Single scattering is then estimated in a ray marching

process, in which the light reduced intensity is deduced from

the transmittance values. While providing accurate results and realtime

performance, the evaluation of the medium transmittance at

a point requires the knowledge of the overall extinction along the

entire light path. The generation of the map then requires a ray

marching through the participating medium. This technique is then

usable only for voxelized media, overlooking other representations

such as the dynamic particle clouds massively used in video games.

Extinction Transmittance Maps:

We introduce Extinction Transmittance Maps (ETM), a technique

combining the advantages of both Fourier Opacity Maps and Transmittance

Function Maps while avoiding their respective drawbacks.

As for [Jansen and Bavoil 2010] and [Delalandre et al. 2011], our

method borrows from the principle of shadow mapping where a virtual

camera is oriented towards the medium from the location of the

light source. This camera is used to create the Extinction Transmittance

Map, into which the contents of the medium are rendered to

build a set of Fourier coefficients representing the local variations

of the extinction parameters (Figure 2). To generate the final image

we reformulate the transmittance function to evaluate the contribution

of each visible volume sample directly from the ETM.

彬彬

菜刀吻电线

頂。。。

晃晃

不错哦,顶一下......

tc

很经典，很实用，学习了!

晃晃

沙发不解释

菜刀吻电线

不会吧，太恐怖了

奇

不错不错，收藏了

tc

俺是新人,这厢有礼了!