Single Scattering vs. Multi Scattering

Multi Scattering 现象与 GI 不同。因为空气中的粒子充满整个空间，所以 MS 的效果是连续的。

Ray Marching

  • Ray marching is a popular method to integrate function along a path
  • We use ray marching to calculate final radiance for a given point by single scattering
  • The integrated radiance is usually stored in look-up tables (LUT)

Precomputed Atmospheric Scattering

https://ebruneton.github.io/precomputed_atmospheric_scattering/

空间采样、预计算、查表。

人的视角 (2D)＋阳光角度 (2D) ＝4D
如何参数化地表达 4D 数据

Multi Scattering LUT

一般 N 取 3-4 够用了。预计算部算好，实时部分变得简单高效 非常经典的方法。
大气环境不变前提下，人和太阳可以在任意位置，都能有比较好的效果。

Challenges of Precomputed Atmospheric Scattering

  • Precomputation Cost
    • Multi-scattering iterations are very expensive
    • Hard to generate atmosphere LUT on low-end devices (ie. mobile)
  • Authoring and Dynamic Adjustment of Environments
    • Artist can't change scattering coefficients on the fly
    • Hard to render effects like weather from sunny to rain fog, space travel among planets
  • Runtime Rendering Cost
    • Expensive per-pixel multi high dimensional texture sampling for transmittance LUT and multi scattering LUT (always need to down-sample for efficiency)

A Scalable and Production Ready Sky and Atmosphere Rendering Technique
https://diglib.eg.org/bitstream/handle/10.1111/cgf14050/v39i4pp013-022.pdf

Production Friendly Quick Sky and Atmosphere Rendering

Simplify Multi-scattering Assumption

  • Scattering events with order greater or equal to 2 are executed using an isotropic phase function
  • All points within the neighborhood of the position we currently shade receive the same amount of second order scattered light
  • Visibility is ignored

$$ G_{n+1}=G_n\ast f _{ms} $$

$$ \mathbf{ F_{ms}=1+f_{ms}+f^2_{ms}+f^3_{ms}+\dots = \frac{1}{1-\mathbf{f_{ms}} } } $$

$$ \mathbf{\Psi _ {ms} }=\mathbf{L_ {2^{nd}order} F_ {ms} } $$

假设“散射是各向同性的”。那么，“均匀的入射光到均匀的出射光”的过程，只是一个简单的能衰减过程。所以只需要求出衰减比例，每 bounce 一次就按这个比例衰减就可以了。

Production Friendly Quick Sky and Atmosphere Rendering

Fixed view position and sun position to remove 2 dimensions out of LUT

对上文中的 LUT 的简化：
(1) 假设人所在的高度不变，去掉 height 维
(2) 假设太阳位置不变，去掉入射角的维度仅留下出射光的维度(天顶角、环角)

Production Friendly Quick Sky and Atmosphere Rendering

  • Generated a 3D LUT to evaluate aerial-perspective effects by ray marching

这个方法不保证物理正确，但好处是：
(1) 艺术家友好
(2) 可以创造异星世界效果
(3) 硬件友好

Good Balance of Performance and Effect

  • Scalable from mobile to high-end PCs

Performance for each step of method, as measured on PC (NV 1080) and a mobile device (iPhone 6s)

本文出自CaterpillarStudyGroup，转载请注明出处。
https://caterpillarstudygroup.github.io/GAMES101_mdbook/