15 Atomospheric and Glare E ff ects Steve Marschner CS5625 Spring - PowerPoint PPT Presentation

15 Atomospheric and Glare E ff ects Steve Marschner CS5625 Spring 2020

Plan Physics of the Air Bibliography • scattering due to gases • Nishita et al., “Display of The Earth Taking into Account Atmospheric • scattering due to aerosols/particles Scattering,” SIGGRAPH 1993. • distribution of atmosphere • Preetham, Shirley, Smits, “A Practical Analytic Model for Daylight,” Atmospheric Phenomena SIGGRAPH 1999. • sunlight • Hosek & Wilkie, “An Analytic Model • skylight for Full Spectral Sky-dome • aerial perspective Radiance,” SIGGRAPH 2012. • clouds Computational Models • ray and path tracing • analytic approximations

pure air 256 exceptionally clear 128 64 very clear 32 16 clear R m (km) 8 light haze 4 haze 2 thin fog 1 0 . 5 1 2 4 8 16 32 64 Turbidity Preetham, Shirley, & Smits SIGGRAPH 1999

θ v θ s W N γ φ s S E Preetham, Shirley, & Smits SIGGRAPH 1999

42°, T=1 .5°, T=1 .5°, T=6 .25°, T=7 Hosek & Wilkie CG&A 2013

(a) T = 2 (b) T = 4 Hosek & Wilkie SIGGRAPH 2012 (c) T = 6 (d) T = 8

Obtaining Reference Data

Result: Raw Data

Empirical analytic sky models CIE Standardized sky model • parameters A…E are tabulated for various conditions and solar elevations Equations: Hosek & Wilkie SIGGRAPH 2012 2 )+ E cos 2 γ ) F CIE 2003 ( θ , γ ) = (1+ Ae B/ cos θ )(1+ C ( e D γ − e D π (2) • Preetham provides empirical analytic functions for these coe ffi cients in x, y, Y Hosek extended sky model B cos θ +0 . 01 ) · ( C + De E γ + F ( θ , γ ) =(1 + Ae + F cos 2 γ + G · χ ( H, γ ) + I · cos 1 2 θ ) 1 + cos 2 α χ ( g, α ) = 3 (1 + g 2 − 2 g · cos α ) 2 • They provide tabulated values for A…I, fitted to simulation; models turbidity quite a bit better.

Sky Colour Patterns (sunset)

(a) Sunrise (b) α = 0 . 1 T=3 (e) Daytime sky (f) α = 0 . 9 T=7 Hosek & Wilkie SIGGRAPH 2012

Aerial perspective Attenuation removes light from the viewing ray • more blue removed, resulting 
 in warmer colors Inscattering adds light 
 to the viewing ray Preetham, Shirley, & Smits SIGGRAPH 1999 • more blue added (usually), 
 resulting in blue contribution 
 (away from sunrise/sunset) Computing both requires 
 integration along ray • density, sun radiance change with h • analytic approximations used 
 for fast performance

T=2 T=6 morning evening Preetham, Shirley, & Smits SIGGRAPH 1999 T=2 T=10 evening overcast

Scattering in the eye Scattering also happens inside the eye Spencer, Shirley, Zimmerman, Greenberg SIGGRAPH 1995 Causes “flare” from bright sources to add with other parts of the image Amount of flare • depends on angle 
 between the source 
 and the pixel 
 receiving flare • angle ~= image-space 
 distance, so model 
 as a convolution

Bloom and lenticular flare Spencer, Shirley, Zimmerman, Greenberg SIGGRAPH 1995