111/17/2015 17:21 graphics ii 91.547 global rendering and radiosity session 9
TRANSCRIPT
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Graphics II 91.547
Global Rendering and Radiosity
Session 9
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A More Sophisticated View ofThe Nature of Light
Ray Oriented View
• Used in Phong shading and in ray tracing • Single direction• Zero width• Notion of intensity
Flux intensity view
• Vector field represents energy flow per unit time per unit area
• Finite beam widths
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Energy Flow across a Surface
dA
n
( )
E dA
E dA
( ) ( )
( ) ( )cos
n
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Flux and Energy Conservation
dd,),A(dd'd,)',,k(dd,
'ddd,),',k(dd,
VVS
VV
pppppppp
ppppp
+ ++=
Emission + in scattering = streaming + outscattering + absorption
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Radiance:Energy emitted from a surface per unit projected area, per unit solid angle of direction
dAdpLd
pLpL
cos),,(
),(),,(
Energy emitted per surface area, per steradian (solid angle)
Therefore:
dAp
d
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Radiosity: Energy emitted from a surface per unit area
dApBd
dpLpB
)(
),()(
Therefore:
dAp
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Irradiance: Energy arriving at a surface per unit area
dpLdA
dpE
dApEd
cos),(),(
)(
dAp
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Reflectance:Bi-Directional Reflectance Function (BDRF)
n
i r
),(),,(),( irir pEpfpL
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The Radiance Equation
At any surface: radiance = emitted radiance + total reflected radiance
For any incoming direction the reflected radiance in direction is the Irradiance multiplied by the BRDF:
i
iii dpLpf cos),(),,(
Integrating over the hemisphere of all incoming directions at p gives:
iii dpLpf cos),(),,(reflection total
The radiance equation for outgoing radiance is therefore:
iiiie dpLpfpLpL cos),(),,(),(),(
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iiiie dpLpfpLpL cos),(),,(),(),(
The Radiance Equation:All we really need to know for rendering?
Material Surface Properties
Light Sources
PlenopticFunction
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Types of Solution to Radiance Equation
Local Global
View Dependent OpenGL Phong Lighting Recursive Ray Tracing
Monte Carlo Ray Tracing
View Independent Flat or Smooth Defined Color (No Lighting)
Radiosity
Monte Carlo Photon Tracing
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iiiie dpLpfpLpL cos),(),,(),(),(
The Radiance Equation:Defined Color: No Lighting
Solved atVertices
All objects are “emitters” according toglColor*()
No reflections considered
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iiiie dpLpfpLpL cos),(),,(),(),(
The Radiance Equation:OpenGL Lighting Model
Objects can beemitters
Restricted to Point Light Sources
PhongBDRF
Solved atVertices
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iiiie dpLpfpLpL cos),(),,(),(),(
The Radiance Equation:Recursive Ray Tracing
Objects can beemitters
Restricted to Point Light Sources +Single Reflected &Refracted Ray
PhongBDRF
Solved forRays ThroughPixels
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iiiie dpLpfpLpL cos),(),,(),(),(
The Radiance Equation:Monte Carlo Ray Tracing
Objects can beemitters
Rays cast recursively,chosenaccording to BDRF
ActualBDRF
Solved forRays ThroughPixels
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iiiie dpLpfpLpL cos),(),,(),(),(
The Radiance Equation:Radiosity
Objects can beemitters – emissionassumed constant andindependent of angle
Constantreflectivity
Assumed constantover surface “patches”independent of angle
Assumed constantover surface “patches”independent of angle
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Perfectly Diffuse Reflectivity
i
)(L
Energy is reflected uniformly in alldirections
)(rL
1,0
),,(
ipf
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Radiosity
0 Based on the theory of heat transfer (energy) between surfaces (Siegel 1984)
0 Adapted to computer graphics by Goral et al. (Goral 1984)0 Based upon conservation of energy0 Surfaces are assumed to be perfectly diffuse (lambertian)
reflectors0 Environment is divided into “patches”0 Radiosity of a patch is the total rate of energy leaving the
patch- Assumed constant over the patch- Equal to sum of emitted and reflected energy
0 Interaction among patches modeled by unitless form factors- Fij defined as the fraction of energy leaving dAi that
arrives at dAj
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The Basic Radiosity Relationship
jij
j
jiiiii FdABdAEdAB
Radiosity x area = emitted energy + reflected energy
For an environment divided into n patches:
jjiiij
j
jjijiiiii
AFAF
AFBAEAB
(reciprocity)
B E B Fi i i jj
n
ij
1
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Resulting System of Equations
1
1
1
1 11 1 12 1 1
2 21 2 22 2 2
3 31 3 32 3 3
1 2
1
2
3
1
2
3
F F F
F F F
F F F
F F F
B
B
B
B
E
E
E
E
n
n
n
n n n n n nn n n
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Stages in Radiosity Solution
Discretized environment
Form factorcalculations
Full matrix solution
Standard renderer
Change scenegeometry
Change colorsor lighting
Change view
Specific View
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Calculating the Form Factors:Energy reaching Aj from Ai
AidAi
i
d ij
j
Aj
dAj
dE dA I d dA
ddA
r
dE dAI dA dA
r
i i i i ij i
ij jj
i ii i j i j
cos
cos
cos cos
2
2
Differential energy leaving Ai that reaches Ajis given by:
Solid angle subtended by Aj at Ai can be expressed:
Substituting gives:
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Calculating the Form Factors:Calculating the Energy Fraction
FdA dA
dA
dA I d dA I
FI dA dA
dA I r
dA
r
FdA
r
FA
dA dA
r
dAidAjj i
i
i i i i i
dAidAji i j i j
i i
i j j
dAiAji j j
Aj
A A
i
i j j i
i j
Radiative energy reaching from
Total energy leaving in all directions
Total energy = cos
cos cos cos cos
cos cos
cos cos
2 2
2
2
1
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The Nusselt Analogue (Siegel 1984)
Patch
Projection onto surfaceof hemisphere
Projection onto base ofcircle
F FdA
rij dAiAji j j
Aj
cos cos
2
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Equivalent Projection Areas
A
B
C
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Hemicube
Patch i
Patch j
Projection of patch jonto hemicube “pixels”
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Summing Delta Form Factors onPixels onto which Aj Projects
Patch i
FaFb
FcFd
Patch j
F F F F Fij a b c d
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Hemicube Algorithm HandlingOcclusion Issue
Patch i
Patch j
Patch k
FikFij
Hemicube
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Stages in Radiosity Solution
Discretized environment
Form factorcalculations
Full matrix solution
Standard renderer
Change scenegeometry
Change colorsor lighting
Change view
Specific View
N x N
N x N
ComputationCost Storage
N x N
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“Gathering”
B
B
B
B
E
E
E
E
F F F
B
B
B
B
i
n
i
n
i i i i i in i
n
1
2
1
2
1 2
1
2
Patch i
Fi1Fi2
Fi3
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“Shooting”
ii
ii
ii
i
nn F
F
F
B
E
E
E
B
B
B
1
1
1
2
1
2
1
Patch i
Fi1Fi2
Fi3
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Progressive Refinement Radiosity Algorithm
repeat for (each patch i) [Position a hemicube on patch I and calculate form factors Fij for the first iteration] for (each patch j (j!=I)) do
rad = j Bi Fij Ai/Aj Bj = Bj + rad Bj=Bj+ rad Bi=0until convergence
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Number of Patches = 124
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Number of Patches = 829
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Number of Patches = 124Number of Elements = 829
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Number of Patches = 58Number of Elements = 1135
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Comparison of Images
124 Patches 5.36 min. 829 Patches 96.46 min.
124 Patches, 829 Elements 32.6 min. 58 Patches, 1135 elements 23.59 min
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Rendering CausticsArvo, 1986
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Combining Radiosity and Ray Tracing
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Combined Radiosity and Ray Tracing