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Image-Based Lighting
cs129: Computational PhotographyJames Hays, Brown, Spring 2011
© Eirik Holmøyvik
Slides from Alexei Efros and Paul Debevec
adapted by Klaus Mueller, added some slides from Jian Huang, UTK
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Inserting Synthetic Objects
Why does this look so bad?• Wrong orientation• Wrong lighting• No shadows
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Environment Maps
Simple solution for shiny objects• Models complex lighting as a panoramic image• i.e. amount of radiance coming in from each direction• A plenoptic function!!!
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reflective surface
viewer
environment texture image
v
n
r
projector function converts reflection vector (x, y, z) to texture image (u, v)
Environment Mapping
Reflected ray: r=2(n·v)n-v
Texture is transferred in the direction of the reflected ray from the environment map onto the objectWhat is in the map?
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What approximations are made?The map should contain a view of the world with the
point of interest on the object as the Center of Projection• We can’t store a separate map for each point, so one map is
used with the COP at the center of the object• Introduces distortions in the reflection, but we usually don’t
notice• Distortions are minimized for a small object in a large room
The object will not reflect itself!
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Environment MapsThe environment map may take various forms:
• Cubic mapping• Spherical mapping• other
Describes the shape of the surface on which the map “resides”
Determines how the map is generated and how it is indexed
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Cubic Mapping
The map resides on the surfaces of a cube around the object• Typically, align the faces of the cube with the coordinate axes
To generate the map:• For each face of the cube, render the world from the center of
the object with the cube face as the image plane– Rendering can be arbitrarily complex (it’s off-line)
• Or, take 6 photos of a real environment with a camera in the object’s position
To use the map:• Index the R ray into the correct cube face• Compute texture coordinates
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Cubic Map Example
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Calculating the Reflection Vector
Normal vector of the surface : NIncident Ray : IReflection Ray: R N,I,R all normalized
R = I -2 N ( N . I )
The texture coordinate is based on the reflection vector
Assuming the origin of the vector is always in the center of the cube environment map
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Indexing Cubic Maps
Assume you have R and the cube’s faces are aligned with the coordinate axes, and have texture coordinates in [0,1]x[0,1]• How do you decide which face to use?• Use the reflection vector coordinate with
the largest magnitude
(0.3, 0.2, 0.8) face in +z direction
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Indexing Cubic Maps
How do you decide which texture coordinates to use?• Divide by the coordinate with the largest magnitude• Now ranging [-1,1]• Remapped to a value between 0 and 1.
(0.3,0.2,0.8) ((0.3/0.8 +1)*0.5, ((0.2/0.8 +1)*0.5) = (0.6875, 0.625)
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A Sphere Map
• A mapping between the reflection vector and a circular texture
• Prepare an image/texture in which the environment is mapped onto a sphere
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Sphere Mapping
To generate the map: Take a photograph of a shiny sphere • Mapping a cubic environment map onto a sphere• For synthetic scenes, you can use ray tracing
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Sphere Map
Compute the reflection vector at the surface of the objectFind the corresponding texture on the sphere mapUse the texture to color the surface of the object
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Indexing Sphere Maps
Given the reflection vector R (Rx,Ry,Rz)
• (u,v) on the spherical map
1,0 vu
222 1
2
1
2 v,
2
1
2
zyx
yx
RRRm
m
R
m
Ru
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Indexing Sphere Maps
The normal vector is the sum of the reflection vector and the eye vector
Normalization of the vector gives
If the normal is on a sphere of radius 1, its x, y coordinates are also location on the sphere map
Finally converting them to make their range [0,1]
m
R
m
R
m
R
RRRm
zyx
zyx
1,,
1 222
n
)1,,( zyx RRRN
222 1
2
1
2 v,
2
1
2
zyx
yx
RRRm
m
R
m
Ru
m
R
m
R
m
R
RRRm
zyx
zyx
1,,
1 222
n
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Non-linear Mapping
Problems:• Highly non-uniform sampling• Highly non-linear mapping
Linear interpolation of texture coordinates picks up the wrong texture pixels• Do per-pixel sampling or use high resolution polygons• Can only view from one direction
Correct Linear
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Example
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What about real scenes?
From Flight of the Navigator
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What about real scenes?
from Terminator 2
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Real environment mapsWe can use photographs to capture environment maps
How do we deal with light sources? Sun, lights, etc?• They are much much brighter than the rest of the
enviarnment
User High Dynamic Range photography, of course!
Several ways to acquire environment maps:• Stitching mosaics• Fisheye lens• Mirrored Balls
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Stitching HDR mosaicsStitching HDR mosaics
http://www.gregdowning.com/HDRI/stitched/http://www.gregdowning.com/HDRI/stitched/
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Scanning Panoramic CamerasScanning Panoramic CamerasPros:
very high res (10K x 7K+)Full sphere in one scan – no stitchingGood dynamic range, some are HDR
Issues:More expensiveScans take a while
Companies: Panoscan, Sphereon
Pros:very high res (10K x 7K+)Full sphere in one scan – no stitchingGood dynamic range, some are HDR
Issues:More expensiveScans take a while
Companies: Panoscan, Sphereon
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Fisheye ImagesFisheye Images
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Mirrored SphereMirrored Sphere
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Sources of Mirrored BallsSources of Mirrored Balls
2-inch chrome balls ~ $20 ea. McMaster-Carr Supply Company
www.mcmaster.com
6-12 inch large gazing balls Baker’s Lawn Ornaments
www.bakerslawnorn.com
Hollow Spheres, 2in – 4in Dube Juggling Equipment
www.dube.com
FAQ on www.debevec.org/HDRShop/
2-inch chrome balls ~ $20 ea. McMaster-Carr Supply Company
www.mcmaster.com
6-12 inch large gazing balls Baker’s Lawn Ornaments
www.bakerslawnorn.com
Hollow Spheres, 2in – 4in Dube Juggling Equipment
www.dube.com
FAQ on www.debevec.org/HDRShop/
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0.34 0.34
0.580.58
=> 59% Reflective=> 59% Reflective
Calibrating Mirrored Sphere Reflectivity
Calibrating Mirrored Sphere Reflectivity
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Real-World HDR Lighting Environments
Lighting Environments from the Light Probe Image Gallery:http://www.debevec.org/Probes/Lighting Environments from the Light Probe Image Gallery:http://www.debevec.org/Probes/
FunstonBeach
UffiziGallery
EucalyptusGrove
GraceCathedral
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Acquiring the Light ProbeAcquiring the Light Probe
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Assembling the Light ProbeAssembling the Light Probe
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Not just shiny…Not just shiny…
We have captured a true radiance map
We can treat it as an extended (e.g spherical) light source
Can use Global Illumination to simulate light transport in the scene So, all objects (not just shiny) can be
lighted What’s the limitation?
We have captured a true radiance map
We can treat it as an extended (e.g spherical) light source
Can use Global Illumination to simulate light transport in the scene So, all objects (not just shiny) can be
lighted What’s the limitation?
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Illumination ResultsIllumination Results
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Comparison: Radiance map versus single imageComparison: Radiance map versus single image
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Putting it all togetherPutting it all together
Synthetic Objects +Real light!
Synthetic Objects +Real light!
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CG Objects Illuminated by a Traditional CG Light Source
CG Objects Illuminated by a Traditional CG Light Source
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Paul Debevec. A Tutorial on Image-Based Lighting. IEEE Computer Graphics and Applications, Jan/Feb 2002.
Paul Debevec. A Tutorial on Image-Based Lighting. IEEE Computer Graphics and Applications, Jan/Feb 2002.
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Rendering with Natural LightRendering with Natural Light
SIGGRAPH 98 Electronic Theater
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RNL Environment mapped onto
interior of large cube
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MOVIE!MOVIE!
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Illuminating a Small SceneIlluminating a Small Scene
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We can now illuminatesynthetic objects with real light.
How do we add synthetic objects to a real scene?
We can now illuminatesynthetic objects with real light.
How do we add synthetic objects to a real scene?
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Real Scene ExampleReal Scene Example
Goal: place synthetic objects on tableGoal: place synthetic objects on table
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Light Probe / Calibration GridLight Probe / Calibration Grid
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real scenereal scene
Modeling the SceneModeling the Scene
light-based modellight-based model
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The Light-Based Room ModelThe Light-Based Room Model
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real scenereal scene
Modeling the SceneModeling the Scene
synthetic objectssynthetic objects
light-based modellight-based model
local scenelocal scene
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The Lighting ComputationThe Lighting Computation
synthetic objects(known BRDF)
synthetic objects(known BRDF)
distant scene (light-based, unknown BRDF)distant scene (light-based, unknown BRDF)
local scene(estimated BRDF)
local scene(estimated BRDF)
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Rendering into the SceneRendering into the Scene
Background PlateBackground Plate
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Rendering into the SceneRendering into the Scene
Objects and Local Scene matched to SceneObjects and Local Scene matched to Scene
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Differential RenderingDifferential Rendering
Local scene w/o objects, illuminated by modelLocal scene w/o objects, illuminated by model
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Differential Rendering (2)Difference in local sceneDifferential Rendering (2)Difference in local scene
-- ==
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Differential RenderingDifferential Rendering
Add differences to images -- Final ResultAdd differences to images -- Final Result
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High-Dynamic Range PhotographyHigh-Dynamic Range Photography
• Standard digital images typically represent only a small fraction of the dynamic range—the ratio between the dimmest and brightest regions accurately represented—present in most realworld lighting environments.
The bright light saturates the pixel colour
Need a high dynamic range image which can be produced by combining images of different shutter speed
• Standard digital images typically represent only a small fraction of the dynamic range—the ratio between the dimmest and brightest regions accurately represented—present in most realworld lighting environments.
The bright light saturates the pixel colour
Need a high dynamic range image which can be produced by combining images of different shutter speed
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High-Dynamic Range PhotographyHigh-Dynamic Range Photography
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HDR imagesHDR images
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IMAGE-BASED LIGHTING IN FIAT LUXPaul Debevec, Tim Hawkins, Westley Sarokin, H. P. Duiker, Christine Cheng, Tal Garfinkel, Jenny Huang
SIGGRAPH 99 Electronic Theater
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HDR Image SeriesHDR Image Series
2 sec2 sec 1/4 sec1/4 sec 1/30 sec1/30 sec
1/250 sec1/250 sec 1/2000 sec1/2000 sec 1/8000 sec1/8000 sec
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Stp1 PanoramaStp1 Panorama
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Assembled PanoramaAssembled Panorama
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Light Probe ImagesLight Probe Images
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Capturing a Spatially-Varying Lighting EnvironmentCapturing a Spatially-Varying Lighting Environment
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The MovieThe Movie