Objectives Objectives

Learn to shade objects so their images appear three-Learn to shade objects so their images appear three-dimensionaldimensional

Introduce the types of light-material interactionsIntroduce the types of light-material interactions Build a simple reflection model---the Phong Build a simple reflection model---the Phong

model--- that can be used with real time graphics model--- that can be used with real time graphics hardwarehardware

CSC461: Lecture 22CSC461: Lecture 22 Lighting and Shading Lighting and Shading

Why we need shadingWhy we need shading Suppose we build a model of a sphere using Suppose we build a model of a sphere using

many polygons and color it with many polygons and color it with glColorglColor. . We get something likeWe get something like

But we wantBut we want

ShadingShading Why does the image of a real sphere look likeWhy does the image of a real sphere look like

Light-material interactions cause each point to have Light-material interactions cause each point to have a different color or shadea different color or shade

Need to consider Need to consider Light sourcesLight sources Material propertiesMaterial properties Location of viewerLocation of viewer Surface orientationSurface orientation

ScatteringScatteringLight strikes A Light strikes A

Some scatteredSome scattered Some absorbedSome absorbed

Some of scattered light strikes BSome of scattered light strikes B Some scatteredSome scattered Some absorbedSome absorbed

Some of this scatterdSome of this scatterdlight strikes Alight strikes Aand so onand so on

Rendering EquationRendering Equation

The infinite scattering and absorption of The infinite scattering and absorption of light can be described by the light can be described by the rendering rendering equation equation CannotCannot bebe solved in generalsolved in general Ray tracing is a special case for perfectly Ray tracing is a special case for perfectly

reflecting surfacesreflecting surfaces Rendering equation is global and includesRendering equation is global and includes

ShadowsShadows Multiple scattering from object to objectMultiple scattering from object to object

Global EffectsGlobal Effects

translucent surface

shadow

multiple reflection

Color SourcesColor Sources RGB model is usedRGB model is used Light is simulated with the color modelLight is simulated with the color model Three component intensity or Three component intensity or luminanceluminance

functionfunction

Each component of a light source can be Each component of a light source can be independently calculatedindependently calculated

b

g

r

I

I

I

I

Local vs Global RenderingLocal vs Global Rendering

Correct shading requires a global calculation Correct shading requires a global calculation involving all objects and light sourcesinvolving all objects and light sources Incompatible with pipeline model which shades Incompatible with pipeline model which shades

each polygon independently (local rendering)each polygon independently (local rendering) However, in computer graphics, especially real However, in computer graphics, especially real

time graphics, we are happy if things “look time graphics, we are happy if things “look right”right” Exist many techniques for approximating global Exist many techniques for approximating global

effectseffects

Light-Material InteractionLight-Material Interaction Light that strikes an object is partially Light that strikes an object is partially

absorbed and partially scattered (reflected)absorbed and partially scattered (reflected) The amount reflected determines the color The amount reflected determines the color

and brightness of the objectand brightness of the object A surface appears red under white light because A surface appears red under white light because

the red component of the light is reflected and the the red component of the light is reflected and the rest is absorbedrest is absorbed

The reflected light is scattered in a manner The reflected light is scattered in a manner that depends on the smoothness and that depends on the smoothness and orientation of the surfaceorientation of the surface

Light SourcesLight Sources

General light sources are difficult to work General light sources are difficult to work with because we must integrate light with because we must integrate light coming from all points on the source coming from all points on the source

Simple Light SourcesSimple Light Sources Point sourcePoint source

Model with position and colorModel with position and color Distant source = infinite distance away (parallel)Distant source = infinite distance away (parallel)

SpotlightSpotlight Restrict light from ideal point sourceRestrict light from ideal point source

Ambient light Ambient light Same amount of light everywhere in sceneSame amount of light everywhere in scene Can model contribution of many sources and Can model contribution of many sources and

reflecting surfacesreflecting surfaces

Surface TypesSurface Types The smoother the surface, the more the reflected The smoother the surface, the more the reflected

light resembles a mirrorlight resembles a mirror A very rough surface scatters light in all directionsA very rough surface scatters light in all directions

smooth surface rough surface

Phong ModelPhong Model A simple model that can be computed rapidlyA simple model that can be computed rapidly The light source model has three termsThe light source model has three terms

DiffuseDiffuse SpecularSpecular AmbientAmbient

Uses four vectors at the Uses four vectors at the point ppoint p

l: to light sourcel: to light source v: to viewerv: to viewer n: Normaln: Normal r: Perfect reflectorr: Perfect reflector

Light SourcesLight Sources In the Phong Model, we add the results from each In the Phong Model, we add the results from each

light sourcelight source Each light source has separate Each light source has separate diffusediffuse, , specularspecular, and , and ambientambient terms to allow for maximum flexibility even terms to allow for maximum flexibility even though this form does not have a physical though this form does not have a physical justificationjustification

Separate Separate redred, , greengreen and and blueblue components for each components for each termterm

Hence, 9 coefficients for each point source – Hence, 9 coefficients for each point source – illumination matrixillumination matrix Diffuse -- IDiffuse -- Idrdr, I, Idgdg, I, Idbdb Specular -- ISpecular -- Isrsr, I, Isgsg, I, Isbsb Ambient -- IAmbient -- Iarar, I, Iagag, I, Iabab

Material PropertiesMaterial Properties Material properties match light source Material properties match light source

propertiesproperties Nine absorption coefficients – reflection matrixNine absorption coefficients – reflection matrix

Diffuse -- kDiffuse -- kdrdr, k, kdgdg, k, kdbdb

Specular -- kSpecular -- ksrsr, k, ksgsg, k, ksbsb

Ambient --Ambient -- k karar, k, kagag, k, kabab

Shininess coefficient Shininess coefficient

Adding up the ComponentsAdding up the Components For each light source and each color component, For each light source and each color component,

the Phong model can be written (without the the Phong model can be written (without the distance terms) asdistance terms) as

I =kI =kdd I Idd ll · · n n ++ kkss I Iss ( (vv · · r r ))+ k+ kaa I Iaa

For each color component For each color component we add contributions from we add contributions from all sourcesall sources

ExamplesExamples

Only differences in these teapots are the parametersin the Phong model

Ambient ReflectionAmbient Reflection Ambient light is the result of multiple Ambient light is the result of multiple

interactions between (large) light sources and interactions between (large) light sources and the objects in the environmentthe objects in the environment

Amount and color depend on both the color of Amount and color depend on both the color of the light(s) and the material properties of the the light(s) and the material properties of the objectobject

All points have the same light intensityAll points have the same light intensity Ambient effect kAmbient effect kaa I Iaa for all points for all points

reflection coefficient intensity of ambient light

Diffuse Reflection -- Lambertian SurfaceDiffuse Reflection -- Lambertian Surface Perfectly diffuse reflector -- Light scattered equally Perfectly diffuse reflector -- Light scattered equally

in all directionsin all directions Characterized by rough surfaces Characterized by rough surfaces Lambertian surface – no preferred reflection angle Lambertian surface – no preferred reflection angle Modeled with Lambert’s law: Amount of light Modeled with Lambert’s law: Amount of light

reflected is proportional to the vertical component of reflected is proportional to the vertical component of incoming lightincoming light reflected light ~cos reflected light ~cos ii

cos cos ii = = ll · · n n if vectors normalizedif vectors normalized

There are also three coefficients, kThere are also three coefficients, krr, k, kbb, k, kgg that show how that show how

much of each color component is reflectedmuch of each color component is reflected

Diffuse effect: kDiffuse effect: kdd I Idd ll · · n n

Specular SurfacesSpecular Surfaces Most surfaces are neither ideal diffusers nor perfectly Most surfaces are neither ideal diffusers nor perfectly

specular specular Smooth surfaces show specular highlights due to Smooth surfaces show specular highlights due to

incoming light being reflected in directions incoming light being reflected in directions concentrated close to the direction of a perfect concentrated close to the direction of a perfect reflection or a mirrorreflection or a mirror

specularhighlight

Modeling Specular RelectionsModeling Specular Relections Phong proposed an approximate model: add a term to Phong proposed an approximate model: add a term to

the calculation of diffuse reflectionthe calculation of diffuse reflection The term drops off as the angle between the viewer The term drops off as the angle between the viewer

(v) and the ideal reflection (r) increases(v) and the ideal reflection (r) increases

Ir ~ ks Is cosks Is (r·v)α

shininess coef

absorption coef

incoming intensityreflectedintensity

The Shininess CoefficientThe Shininess Coefficient Values of Values of between 100 and 500 correspond to metals between 100 and 500 correspond to metals Values between 5 and 10 give surface that look like plasticValues between 5 and 10 give surface that look like plastic As As increases, the reflected light concentrates in a increases, the reflected light concentrates in a

narrower regionnarrower region As As goes to infinity, get a mirrorgoes to infinity, get a mirror

cos

90-90

Distance TermsDistance TermsThe light from a point source that reaches a The light from a point source that reaches a

surface is inversely proportional to the square surface is inversely proportional to the square of the distance between themof the distance between them

We can add a factor of theWe can add a factor of the form 1/(a + bd +cdform 1/(a + bd +cd22) to) to the diffuse and specular the diffuse and specular termstermsThe constant and linear terms soften the effect The constant and linear terms soften the effect

of the point sourceof the point source