Illumination Models & Surface-Rendering Methods

Computer Graphics

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Computer Graphics

Chapter 14 Illumination Models & Surface-Rendering Methods 2

Computer Graphics

Chapter 14 Illumination Models & Surface-Rendering Methods 2

Illumination Models & SurfaceRendering Methods •  Illumination model  or  or a lighting model  is the model for calculating light intensity at a single surface point. •  Surface rendering is a procedure for applying a lighting model to obtain piel intensities for all the pro!ected surface  positions in a scene. 3

Surface rendering • Surface rendering can be performed by applying the illumination model to e#ery #isible surface point$ or the rendering can be accomplished by interpolating intensities across the surface from a small set of illumination-model calculations. • Scan-line algorithms use interpolation schemes. • Ray tracing algorithms in#o%e the illumination model at each piel position. • Surface-rendering procedures are termed surfaceshading methods. "

Illumination models 'i#en the parameters( • the optical properties of surfaces )opa*ue+transparent$ shiny+dull$ surface-teture, • the relati#e positions of the surfaces in a scene •  the color and positions of the light sources • the position and orientation of the #ieing plane. Illumination models calculate the intensity pro!ected from a particular surface point in a specified #ieing direction. 

0ecture lane • 0ight Sources • asic Illumination Models mbient 0ight 4iffuse Reflection Specular Reflection & hong Model 5ombine 4iffuse & Specular Reflections ith Multiple 0ight Sources /

0ight Sources • 7hen e #ie an opa*ue nonluminous ob!ect$ e see reflected light from the surfaces of the ob!ect. • 8he total reflected light is the sum of the contributions from light sources and other reflecting surfaces in the scene. • 0ight sources 9 light-emitting sources . • Reflecting surfaces 9 light-reflecting sources.

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0ight Sources 0ight Source

Reflecting Surfaces

Fig. 1 0ight #ieed from an opa*ue surface is in general a combination of reflected light from a light source and reflections of light reflections from other surfaces.

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oint 0ight Source • 8he rays emitted from a point light radially di#erge from the source. • pproimation for sources

Fig. 2 4i#erging ray paths from a point light source.

that are small compared to the sior an illuminated transparent surface$ some of the incident light ill be reflected and some ill be transmitted through the material.

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4iffuse reflection • 'rainy surfaces scatter the reflected light in all directions. 8his scattered light is called diffuse reflection. • 8he surface appears e*ually bright from all #ieing directions. • 7hat e call the color of an ob!ect is the color of the diffuse reflection of the incident light.

Fig. 4 4iffuse reflection from a surface.

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Specular reflection • 0ight sources create highlights$ bright spots$ called specular reflection. More  pronounced on shiny surfaces than on dull.

Fig. 5 Specular reflection superimposed on diffuse reflection #ectors.

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asic Illumination Models 0ighting calculations are based on( • ?ptical properties of surfaces$ such as glossy$ matte$ opa*ue$ and transparent. 8his controls the amount of reflection and absorption of incident light. • 8he bac%ground lighting conditions. • 8he light-source specifications. ll light sources are considered to be point sources$ specified ith a coordinate position and intensity #alue )color,. 1"

mbient 0ight • @#en though an ob!ect in a scene is not directly lit it ill still be #isible. 8his is because light is reflected from nearby ob!ects. Fig. 6 • mbient light has no spatial or mbient light shading. directional characteristics. • 8he amount of ambient light incident on each ob!ect is a constant for all surfaces and o#er all directions. • 8he amount of ambient light that is reflected by an ob!ect is independent of the ob!ects position or orientation and depends only on the optical properties of the surface. 1

mbient 0ight • 8he le#el of ambient light in a scene is a parameter  I a $ and each surface illuminated ith this constant #alue. • Illumination e*uation for ambient light is  I 9 k a I a here  I  is the resulting intensity  I a is the incident ambient light intensity k a is the ob!ectAs basic intensity$ ambientreflection coefficient . 1/

mbient 0ight - @ample

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4iffuse Reflection • 4iffuse reflections are constant o#er each surface in a scene$ independent of the #ieing direction. • 8he amount of the incident light that is diffusely reflected can be set for each surface ith parameter k d$  the diffuse-reflection coefficient $ or diffuse reflectivity . = ≤ k d  ≤ 1 k d near 1 B highly reflecti#e surface   k d near = B surface that absorbs most of the    incident light k d is a function of surface color  

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4iffuse Reflection @#en though there is e*ual light scattering in all direction from a surface$ the brightness of the surface does depend on the orientation of the surface relati#e to the light source(

)a,

)b,

Fig. 8  surface perpendicular to the direction of t he incident light )a, is more illuminated than an e*ual-siigure 1= illustrates the illumination ith diffuse reflection$ using #arious #alues of   parameter k d beteen = and1.  

Fig. 10 Series of pictures of sphere illuminated by diffuse reflection model only using different k d  #alues )=."$ =.$ =.6$ =.:$1.=,.

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4iffuse Reflection 7e can combine the ambient and point-source intensity calculations to obtain an epression for the total diffuse reflection. .  I diff  9 k a I a+k d I    l )N L, here both k a and k d depend on surface material    properties and are assigned #alues in the range from = to 1.

Fig. 11 Series of pictures of s phere illuminated by ambient and diffuse reflection model.  I a 9 I l  9 1.=$ k d  9 =." and k a #alues )=.=$ =.1$ =.3=$ =."$ =./=,.

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4iffuse Reflection - @ample

Fig. 12

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Specular Reflection and the hong Model •  Specular reflection is the result of total$ or near total$ reflection of the incident light in a concentrated region around the specular-reflection angle. • Shiny surfaces ha#e a narro specular-reflection range. • 4ull surfaces ha#e a ider reflection range.

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Specular Reflection >igure 13 shos the specular reflection N direction at a point on the 8o 0ight Source R  illuminated surface. In this figure$ L • R  represents the unit #ector in  the direction of specular reflection • L B unit #ector directed toard the Fig. 13 Modeling specular reflection.  point light source •  B unit #ector pointing to the #ieer from the surface position • ngle Φ  is the #ieing angle relati#e to the specular-reflection direction R . θ 

θ 

Φ 

2/

hong Model  Phong model is an empirical model for calculating the

specular-reflection range( • Sets the intensity of specular reflection proportional to cosnsΦ  • ngle Φ  assigned #alues in the range =o to ;=o$ so that cosΦ  #alues from = to 1 •  Specular-reflection parameter  n s is determined by the type of surface$ •  Specular-reflection coefficient  k  s e*ual to some #alue in the range = to 1 for each surface. 26

hong Model • Dery shiny surface is modeled ith a large #alue for n s )say$ 1== or more, • Small #alues are used for duller surfaces. •  >or perfect reflector )perfect mirror,$ n s is infinite N

N R 

L

R  L

Shiny Surface )0arge n s,

4ull Surface )Small n s, Fig. 14 Modeling specular reflection ith parameter n s.

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hong Model cosnsΦ 

Φ 

Fig. 15 lots of cos Φ  for se#eral #alues of specular parameter n s. ns

2;

hong Model hong specular-reflection model(  I  spec 9 k  s I l cosnsΦ  Since  and R are unit #ectors in the #ieing and specular-reflection directions$ e can calculate the #alue of cosnsΦ  ith the dot  product .R .  I  spec 9 k  s I l ).R ,ns

N

8o 0ight Source R 

L θ 

θ  Φ 



Fig. 13 Modeling specular reflection.

3=

hong Model L

N R  L

.

NL

Fig. 16 5alculation of #ector R  by considering pro!ections onto the direction of the normal #ector N.

R E 0 9 )2C.0,C R 9 )2C.0,C-0 31

hong Model N

! R 

L

Φ 



Fig. 17 Falfay #ector ! along the bisector of the angle beteen L and .

 / 2 α  9 Φ  F 9 )0 E D,+G)0 E D,G  I  spec 9 k  s I l )N.!,ns

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Specular Reflection - @ample

Fig. 18

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5ombine 4iffuse & Specular Reflections >or a single point light source$ e can model the combined diffuse and specular reflections from a point on an illuminated surface as  I 9 I diff  E I  spec ns . . 9 k a I a E k d I    l )N L, E k   s I l)   N !, 3"