10illumination

Graphics

Graphics Lab @ Korea University

Illumination Model

고려대학교 컴퓨터 그래픽스 연구실

CGVR


Illumination

How do We Compute Radiance for a Sample Ray? Must derive computer models for ...

Emission at light sources Scattering at surfaces Reception at the camera

Wireframe WithoutIllumination

DirectIllumination

CGVR


Overview

Direct Illumination Emission at light sources Scattering at surfaces

Global Illumination Shadows Refractions Inter-object reflections

Direct Illumination

CGVR


Overview



Direct Illumination

CGVR


Modeling Light Source

IL(x,y,z,) Describes the intensity of energy, Leaving a light source Arriving at location(x,y,z) From direction () With wavelength

CGVR


Empirical Model

Ideally Measure Irradiant Energy for “All” Situations Too much storage Difficult in practice

CGVR


Light Source Model

Simple Mathematical Models: Point light Directional light Spot light

CGVR


Point Light Source

Models Omni-Directional Point Source (E.g., Bulb) Intensity (I0) Position (px, py, pz) Factors (kc, kl, kq) for attenuation with distance (d)

2q1c

0

kkkI

ddIL

CGVR


Directional Light Source

Models Point Light Source at Infinity (E.g., Sun) Intensity (I0) Direction (dx,dy,dz)

No attenuationwith distance 0ILI

CGVR


Spot Light Source

Models Point Light Source with Direction (E.g., Luxo) Intensity (I0), Position (px, py, pz) Direction (dx, dy, dz) Attenuation

2

q1c

0

kkkLDI

ddIL

CGVR


Overview



Direct Illumination

CGVR


Modeling Surface Reflection

Rs(,) Describes the amount of incident energy Arriving from direction () Leaving in direction (,) With wavelength

CGVR


Empirical Model

Ideally Measure Radiant Energy for “All” Combinations of Incident Angles Too much storage Difficult in practice

CGVR


Reflectance Model

Simple Analytic Model: Diffuse reflection + Specular reflection + Emission + “Ambient”

Based on modelproposed by Phong

CGVR


Reflectance Model



CGVR


Diffuse Reflection

Assume Surface Reflects Equally in All Directions Examples: chalk, clay

CGVR


Diffuse Reflection

How Much Light is Reflected? Depends on angle of incident light dLdAcos

CGVR


Diffuse Reflection

Lambertian Model Cosine law (dot product)

LDD IKI LN

CGVR


Reflectance Model



CGVR


Specular Reflection

Reflection is Strongest Near Mirror Angle Examples: mirrors, metals

CGVR


Specular Reflection

How Much Light is Seen? Depends on angle of incident light and angle to

viewer

CGVR


Specular Reflection

Phong Model {cos()}n

Ln

SS IKI RV

CGVR


Reflectance Model



CGVR


Emission

Represents Light Emitting Directly From Polygon Emission ≠ 0

CGVR


Reflectance Model



CGVR


Ambient Term

Represents Reflection of All Indirect Illumination

This is a total hack (avoids complexity of global illumination)!

CGVR


Reflectance Model


CGVR


Reflectance Model


CGVR


Reflectance Model

Sum Diffuse, Specular, Emission, and Ambient

CGVR


Surface Illumination Calculation

Single Light Source:

Ln

SLDALAE IKILNKIKII RV

CGVR


Surface Illumination Calculation

Multiple Light Sources:

i i

niSiiDALAE IKILNKIKII )RV(

CGVR


Overview



Global Illumination

CGVR


Global Illumination

CGVR


Shadows

Shadow Terms Tell Which Light Sources are Blocked Cast ray towards each light source Li

Si = 0 if ray is blocked, Si = 1 otherwise

L LL

nSDAAE ISKLNKIKII )RV(

ShadowTerm

CGVR


Ray Casting

Trace Primary Rays from Camera Direct illumination from unblocked lights only

L LL

nSDAAE ISKLNKIKII )RV(

CGVR


Recursive Ray Tracing

Also Trace Secondary Rays from Hit Surfaces Global illumination from mirror reflection and

transparency

TTRSL LLn

SDAAE IKIKISKLNKIKII )RV(

CGVR


Mirror Reflection

Trace Secondary Ray in Direction of Mirror Reflection Evaluate radiance along secondary ray and include it into

illumination model

TTRSL LLn


Radiancefor mirror

reflection ray

CGVR


Transparency

Trace Secondary Ray in Direction of Refraction Evaluate radiance along secondary ray and include it into

illumination model

TTRSL LLn


Radiance forrefraction ray

CGVR


Transparency

Transparency coefficient is fraction transmitted KT = 1 if object is translucent, KT = 0 if object is opaque 0 < KT < 1 if object is semi-translucent

TTRSL LLn


TransparencyCoefficient

CGVR


Refractive Transparency

For Thin Surfaces, Can Ignore Change in Direction Assume light travels straight through surface

LT

CGVR


Refractive Transparency

For Solid Objects, Apply Snell’s Law: r sin risin i

LηηN)coscos

ηη(T

r

iri

r

i

CGVR


Summary

Direct Illumination Ray casting Usually use simple analytic approximations for light

source emission and surface reflectance

Global illumination Recursive ray tracing Incorporate shadows, mirror reflections,

and pure refractions

CGVR


Illumination Terminology

Radiant power [flux] (Φ) Rate at which light energy is transmitted (in Watts).

Radiant Intensity (I) Power radiated onto a unit solid angle in direction( in Watt/sr)

e.g.: energy distribution of a light source (inverse square law) Radiance (L)

Radiant intensity per unit projected surface area( in Watts/m2sr) e.g.: light carried by a single ray (no inverse square law)

Irradianc (E) Incident flux density on a locally planar area (in Watts/m2 )

Radiosity (B) Exitant flux density from a locally planar area ( in Watts/m2 )

Date post:	25-Jan-2017
Category:	Documents
Upload:	rahul-ramesh
View:	834 times
Download:	1 times

10illumination

Documents