Pencahayaan (Lighting) -...

Komputer Grafik 2 (AK045206)

Pencahayaan (Lighting)

Realisme dan Pencahayaan 1/34


Outline

• Pengenalang

• Pengenalan Pencahayaan

• Sumber-sumber Cahaya : Ambient, langsung, titik, dll

• Hukum Cosinus Lambert/ difusi

• Model Phong• Model Phong



IntroductionIntroduction

• Typical three-step yp pdevelopment process1.Understand the real system –

how does light work (Physics)how does light work (Physics)

2.Determine what matters to us –what can we sense (Psyc)

3 E i t th t i3.Engineer a system that remains true to the portion of reality we can appreciate



S l i th Li hti P blSolving the Lighting Problem

– We somewhat understand the perception of light (color)perception of light (color)

– We engineered a solution to representing and generating

l i tcolor using computers

– We need to understand the interplay of light and objects



Optical Illusion



Li htiLighting

• Remember we know how to• Remember, we know how to rasterize– Given a 3-D triangle and a 3-D

viewpoint, we know which pixels represent the triangle

• But what color should thoseBut what color should those pixels be?



Lighting• If we’re attempting to create a p g

realistic image, we need to simulate the lighting of the surfaces in the scenesurfaces in the scene– Fundamentally simulation of

physics and optics

– As you’ll see, we use a lot of approximations (a.k.a perceptually based hacks) to do this simulation fast enough



Definitions

• Illumination: the transport of energy from light sources to surfaces & points– Note: includes direct and indirect

illumination

Images by Henrik Wann Jensen



Definitions

• Lighting: the process of computing the luminous intensity (i.e., outgoing light) at a particular 3-D point, usually p p , yon a surface

• Shading: the process of i i l t i lassigning colors to pixels

(why the distinction?)



Definitions

• Illumination models fall into two categories:– Empirical: simple formulations that

approximate observed phenomenon

– Physically based: models based on the actual physics of light interacting with matter

W tl i i l d l i• We mostly use empirical models in interactive graphics for simplicity

• Increasingly, realistic graphics are using physically based models



C f Ill i iComponents of Illumination

• Two components of illumination: light d f tisources and surface properties

• Light sources (or emitters)– Spectrum of emittance (i.e., color of the

light)g )– Geometric attributes

• Position• Direction• Shape

– Directional attenuation– Polarization



Components of Illumination

• Surface propertiesp p– Reflectance spectrum (i.e., color of

the surface)

– Subsurface reflectance

– Geometric attributes• Position

• Orientation

• Micro-structure



Simplifications for Interactive Graphics

– Only direct illumination from emitters to surfaces

– Simplify geometry of emitters to trivial cases



A bi t Li ht SAmbient Light Sources

• Objects not directly lit are typically still i iblvisible– e.g., the ceiling in this room, undersides of

desks

• This is the result of indirect illumination from emitters, bouncing off intermediate surfaces

• Too expensive to calculate (in real time), so we use a hack called an ambient light source– No spatial or directional characteristics;

illuminates all surfaces equally– Amount reflected depends on surface

properties


p p


Ambient Light Sources

• For each sampled wavelength p g(R, G, B), the ambient light reflected from a surface depends ondepends on– The surface properties, kambient

– The intensity, Iambient, of the ambient light source (constant for all points on all surfaces )

• Ireflected = kambient Iambientreflected ambient ambient



Ambient Light Sources

• A scene lit only with an yambient light source:

Light PositionNot ImportantNot Important

i i iViewer PositionNot Important

Surface AngleNot Important



Di i l Li h SDirectional Light Sources

• For a directional light source we make simplifying assumptions– Direction is constant for all surfaces in

the scene

– All rays of light from the source are parallel

• As if the source were infinitely far away from the surfaces in the scenefrom the surfaces in the scene

• A good approximation to sunlight

• The direction from a surface to the light source is important in lighting the surface


the surface


Directional Light SourcesDirectional Light Sources

• The same scene lit with a directional and an ambient light source

Light PositionNot Important

Surface AngleImportant

Viewer PositionNot Important

Important

p



Point Light Sources

• A point light source emits light p g gequally in all directions from a single point

Th di ti t th li ht f• The direction to the light from a point on a surface thus differs for different points:– So we need to calculate a

normalized vector to the light source for every point we light:y p g

p

l


p


P i t Li ht SPoint Light Sources

• Using an ambient and a point g plight source:

Light PositionLight PositionImportant

Viewer PositionImportant

Surface AngleImportant



Oth Li ht SOther Light Sources

• Spotlights are point sources p g pwhose intensity falls off directionally.

Requires color point– Requires color, pointdirection, falloffparameters

S t d b O GL– Supported by OpenGL



Other Light Sources

• Area light sources define a 2-D gemissive surface (usually a

disc or polygon)Good example: fluorescent light– Good example: fluorescent light

panels

– Capable of generating soft h d ( h ? )shadows (why? )



L b t’ C i LLambert’s Cosine Law

• Ideal diffuse surfaces reflect according to Lambert’s cosine law:

The energy reflected by a small portion of a surface from a light source in a given direction is proportional to the cosine of the angleis proportional to the cosine of the angle between that direction and the surface normal

• These are often called Lambertian surfacessurfaces

• Note that the reflected intensity is independent of the viewing direction but does depend on thedirection, but does depend on the surface orientation with regard to the light source



L b t’ LLambert’s Law



C ti Diff R fl tiComputing Diffuse Reflection

• The angle between the surface l d th i i li ht inormal and the incoming light is

the angle of incidence:nl

• Idiffuse = kd Ilight cos • In practice we use vector

ith tiarithmetic:

• Idiffuse = kd Ilight (n • l)



Diff Li hti E lDiffuse Lighting Examples

• We need only consider angles y gfrom 0° to 90° (Why?)

• A Lambertian sphere seen at l diff t li htiseveral different lighting

angles:



Ph Li htiPhong Lighting

• The most common lighting model g gin computer graphics was suggested by Phong:

shinynIkI

• The nshiny term is a purely

shinynlightsspecular IkI cos

shiny p yempirical constant that varies the rate of falloff

• Though this model has no

vThough this model has no physical basis, it works (sort of) in practice



Ph Li hti Th TPhong Lighting: The nshiny Term

• This diagram shows how the Phong reflectance term drops off with divergence of the viewing angle from the ideal reflected ray:

Vi i l fl t d l

• What does this term control, visually?

Viewing angle – reflected angle


y


Calculating Phong LightingCalculating Phong Lighting

• The cos term of Phong lighting can be computed using vector arithmetic:

shinynl hl rvIkI

– V is the unit vector towards the viewer

– R is the ideal reflectance direction

shinylightsspecular rvIkI

R is the ideal reflectance direction

• An aside: we can efficiently l l t ?

vcalculate r?

lnlnr 2



Calculating The R Vector

• This is illustrated below:

lnlnr 2

nlnlr 2



Phong Examples

• These spheres illustrate the pPhong model as l and nshiny are varied:



Th Ph Li hti M d lThe Phong Lighting Model

• Let’s combine ambient, diffuse, and specular components:

lights

nisidiambientatotal

shinyrvklnkIIkI#

• Commonly called Phong lighting

i

isidiambientatotal vklnkk1

– Note: once per light

– Note: once per color component

– Do ka, kd, and ks vary with color t?component?



Phong Lighting: Intensity Plots



R f iReferensi• 1: , 4:GraphicsSlides09.pdf , 5: , 8: ,

GlobalIllumination.ppt, localIllumination.ppt , 9:Lecture14

• Buku Teks : 1. F.S.Hill, Jr., COMPUTER GRAPHICS – Using Open GL, Second

Edition, Prentice Hall, 20012. Foley, van Dam, Feiner, Hughes, and Philips, Introduction to

Computer Graphics, Addison Wesley, 2000

• Lecture Notes / Slide-Presentation / Referensi lain yang diperoleh melalui internet :

3. Andries van Dam, Introduction to Computer Graphics, Slide-Presentation, Brown University, 2003, (folder : brownUni)

4. _______________, Interactive Computer Graphic, Slide-Presentation, (folder : Lect_IC_AC_UK)

5 ---------------------Michael McCool CS 488/688 :Introduction to5. Michael McCool, CS 488/688 :Introduction to Computer Graphics, Lecture Notes, University of Waterloo, 2003 (lecturenotes.pdf)

6. _______________, Computer Science 559, Slide-Presentation, Wisconsin University,(folder : Lect_Wisc_EDU)

7. http://graphics.lcs.mit.edu/classses/6.837/F98/Lecture4/Slide23.html , Slide-Presentation, MIT, (folder : MIT_CourseNote)

8 CS 319 : Advance Topic in Computer8. _______________, CS 319 : Advance Topic in Computer Graphics, Slide-Presentation, (folder : uiuc_cs)

9. _______________, CS 445/645 : Introduction to Computer Graphics, Slide-Presentation, (folder :COMP_GRAFIK)

10. Gladimir V.G. Baranoski, CS 488 : Introduction to Computer Graphics , Waterloo University

11. Prof. Peter Panfilov , http://cse yeditepe edu tr/~osertel/courses/CSE484/index html


http://cse.yeditepe.edu.tr/~osertel/courses/CSE484/index.html12. http://www.cl.cam.ac.uk/Teaching/1998/AGraphics/l3a.html

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