An Analytic Model forFull Spectral Sky-Dome Radiance

SIGRAPH 2012

2012/12/05ked

Authors

Lukas Hosek, Alexander Wilkie

Outline

Goal Why is the sky blue? Modeling Simulation Results

Outline

Goal Why is the sky blue? Modeling Simulation Results

A new sky model is required

Accurate simulation is slow Environment maps are limited Fix the flaws of old model

A new sky model is required

Accurate simulation is slow Environment maps are limited

Fix the flaws of old model

A new sky model is required

Accurate simulation is slow Environment maps are limited

Fix the flaws of old model

?

A new sky model is required

Accurate simulation is slow Environment maps are limited

Fix the flaws of old model Preetham model

Limited turbidity range Fails at some spectrum

A new sky model is required

Accurate simulation is slow Environment maps are limited

Fix the flaws of old model Preetham model

Limited turbidity range Fails at some spectrum

A new sky model is required

Demo

A new sky model is required

Accurate simulation is slow Environment maps are limited

Fix the flaws of old model Preetham model

Limited turbidity range Fails at some spectrum

how?

Outline

Goal Why is the sky blue? Modeling Simulation Results

Scattering

http://www.sciencemadesimple.com/sky_blue.html

Scattering

Effected by Haze: dust particles, water droplets, …. (1859) Air molecules (1899) And finally settle by Einstein (1910)

http://www.sciencemadesimple.com/sky_blue.html

“ 方勵之—天空為什麼是藍色的”

Scattering – the sky near the horizen

http://www.sciencemadesimple.com/sky_blue.html

http://www.research-in-germany.de/25326/german-countryside-bildergalerie,templateId=popup,currentContentId=25294.html

The red sunset

http://www.sciencemadesimple.com/sky_blue.html

http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

Outline

Goal Why is the sky blue? Modeling Simulation Results

Luminance model

Perez (1993)

The parameters A to E and Yz are used to tune the luminance distribution.

Spectral radiance model

Preetham (1999)

CIE xyY color space

Spectral radiance model

Preetham (1999) The parameters of Perez model (A to E and

Yz) are linear functions of condition, turbidity. Turbidity,

Ratio of the optical thickness to pure air T=2: very clear, Arctic-like sky T=3: clear sky in temperate climate T=6: warm, moist day

The new model

Hosek (2012) Extend the Perez model

The new model

Hosek (2012) Extend the Perez model

The new model

Hosek (2012) Extend the Perez model

The new model

Hosek (2012) Extend the Perez model

The parameters

A, B, C, D, E, F, G, H, I, and 10 parameters

The parameters

A, B, C, D, E, F, G, H, I, and

: control point

: albedo : parameter

The parameters

John von Neumann “With four parameters I can fit an elephant, and

with five I can make him wiggle his trunk”

Outline

Goal Why is the sky blue? Modeling Simulation Results

Scattering models

Molecules: Rayleigh scattering Haze: Mie scattering

Scattering models

Molecules: Rayleigh scattering Cross section: Density: Phase function:

Haze: Mie scattering Cross section: Density: Phase function:

Scattering models

Molecules: Rayleigh scattering Cross section: Density: Phase function:

Haze: Mie scattering Cross section: Density: Phase function:

Scattering models

Molecules: Rayleigh scattering Cross section: Density: decays exponentially with altitude( ) Phase function:

Haze: Mie scattering Cross section Density Phase function:

Scattering models

Molecules: Rayleigh scattering Cross section: Density: decays exponentially with altitude( ) Phase function:

Angle between outgoing and incoming light directions

Haze: Mie scattering Cross section Density: Phase function:

Scattering models

Molecules: Rayleigh scattering Cross section: Density: decays exponentially with altitude( ) Phase function:

: angle between outgoing and incoming light directions

Haze: Mie scattering Cross section: pre-calculated tabulated values Density: decays exponentially with altitude( ) Phase function:

g: anisotropy factor

Scattering models

Phase functions

http://home.comcast.net/~vinelandrobotics/

Ray tracer100km cover 0.0005%

Lambertian diffuse surface

Ray tracer100km cover 0.0005%

path tracing

Lambertian diffuse surface

Ray tracer100km cover 0.0005%

path tracing

ignored

Lambertian diffuse surface

Ray tracercover 0.0005%

path tracing

ignored

Lambertian diffuse surface

100km

10

720

Outline

Goal Why is the sky blue? Modeling Reference data generation Results

Photos vs. simulation

Turbidity values

Solar elevation = 4o

Ground albedo

Solar elevation = 40o, T = 4 In high turbidity settings, changing ground albedo

alters the brightness of the whole sky-dome

New model vs. Perez model

SNR Reference: path tracing results

Photograph vs. new model

Feature work

Reference

為什麼天空是藍的，方勵之 www.sciencemadesimple.com/sky_blue.htm A Practical Analytic Model for Daylight, A.J.

Preetham, P. Shirley, and B. Smits. Unbiased Global Illumination with Participating

Media, M. Raab, D. Seibert, and A. Keller.

Thx.