computer graphics & visualization global illumination effects
TRANSCRIPT
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computer graphics & visualization
How to make a PIXAR movie
Global Illumination Effects
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Motivation• Realistic illumination of the scene
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Motivation• Soft shadows
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Motivation• Subsurface scattering
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Motivation• Many algorithms exist– Photon mapping– Ambient Occlusion– …
• Common goal: Solving parts of the Rendering Equation
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Problems• Scene changes -> New computation• Still not possible in real-time• Uses Raytracing or Radiosity
Already explainedGoing to be explained now
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Photons• Have energy
• h: Planck constant• v: Frequency of light
hvE
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Radiometric QuantitiesRadiant energy J
Radiant power W
Irradiance W/m²
Radiosity W/m²
Radiant intensity W/sr
dt
dQ
dA
dE
dA
dB
d
dI
Q
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Radiance
cos²
dAd
dL
• θ: angle between surface‘s normal and ω• cosθ: Lambertian law• Constant along a ray
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Irradiance
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
BRDF
iiii
rrrir dxL
xdLxf
cos),(
),(),,(
Bidirectional reflectance distribution function
How much light is reflected
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Reflection Equation
iiiirirrr dxLxfxL
cos),(),,(),(
Integrate over the hemisphere
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Rendering Equation
Sy
yirirrer dAyxGyxVyLxfxLxL
),(),(),(),,(),(),(
Radiance
Emitted light
Surfaces
BRDF
Visibility
Geometry factor
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Radiosity• Ideal diffuse reflection can be simulated with
Radiosity• Uses finite elements• Introduced by Goral et al.
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Radiosity• Origin: Thermal heat transfer• Developed in 1984, still in use• Modelling of diffuse lighting– Doesn‘t account for specular lighting– Independent of viewer– Therefore: Stays constant in constant scene
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Radiosity Equation
j
ijjieii FBBB
RadiosityEmissivity
Reflectivity Form factor
Sy
yirirrer dAyxGyxVyLxfxLxL
),(),(),(),,(),(),(
constant
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Form Factors
i jA A
ijji
iij dAdA
rAF
²
coscos1
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Form Factors
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Nusselt Analog• Simple geometric analog for calculating form
factors
B B
AFij
A
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Hemicube Algorithm• Hemicube instead of hemisphere
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Hemicube Algorithm• Idea:– Precompute delta form factors analytically– Count covered pixels– Sum up covered delta form factors to the true form
factor
q
qij FF
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Hemicube Example
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Hemicube Algorithm on GPU• Use projection center as viewport• Use current face as viewing plane• Do the rendering• Grab the colour buffer (IDs of patches)• Count coloured pixels• Visibility test performed by depth test
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Radiosity Algorithm• Compute form factors• Solve linear equation system
for i = 1, … , nj
ijjieii FBBB
nnnnnnnnn
n
n
E
E
E
B
B
B
FFF
FFF
FFF
2
1
2
1
21
22222212
11121111
1
1
1
)1( T
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Von Neumann Series
...
...
...
²
)3()2()1()0(
3210
BBBB
ETETETET
BTTEETBEB
0 Bounces 1 Bounce 2 Bounces 3 Bounces
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Jacobi Iteration
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Shooting / Gathering
N
j
kjijii
ki BFEB
X
X
X
XXXXX
1
)()1( )(
N
j
kj ij i i
kiB F E B
X
X
X
X
X
X
X
X
X
X
1
) ( ) 1 () (
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Radiosity Result
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Radiosity vs. Ray Tracing
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Radiosity Conclusion• Old, but still in use• Used for simulating diffuse lighting• Result can be used in combination with other GI
algorithms
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Ambient Occlusion Motivation• Ambient term constant in Phong model• Not very realistic• Idea: Compute occlusion of each face
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Ambient Occlusion• Result: Occluded areas appear darker than
brigther ones• Multiply with usual Phong model• 2 possibilities:– Screen space– Object space
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Screen Space Ambient Occlusion• Can be completely done on GPU• No preprocessing• Independent of scene complexity• Idea: Instead of performing full raytracing use
occlusion information from z-buffer
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Screen Space Ambient Occlusion• Take 3D samples around each point• Determine occlusion of each point by testing
against the depth buffer• Far samples with less influence• Use blurring for smooth results
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Screen Space Ambient Occlusion
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Object Space Ambient Occlusion• Define surface element as an oriented disk• Use Heron‘s formula ,• Store position, normal and area in texture for
pixel shader
))()(( csbsass 2
cbas
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Object Space Ambient Occlusion• Compute accessibility value ateach element (% of hemisphere)• Approximation based on thesolid angle of an oriented disk
• Strongly dependent on scenecomplexity
²
)cos4,1max(cos1
rA
r RE
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Object Space Ambient Occlusion
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Ambient Occlusion Results
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Ambient Occlusion Results
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Ambient Occlusion Conclusion• Can be preprocessed for each object• Used in the current version of PIXAR‘s
RenderMan
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Outlook• Faster computation– Cheaper– Artists can see results faster
• More realistic lighting
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Conclusion• Very important for any animated movie• Computation time not too important
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computer graphics & visualization
Global Illumination EffectsChristian A. Wiesner
Thanks for your attention!