Computer Graphics Inf4/MSc Computer Graphics Lecture Notes #16 Image-Based Lighting

Computer Graphics Inf4/MSc

Computer Graphics

Lecture Notes #16

Image-Based Lighting


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Global Illumination and Image-Based Lighting

• Traditional Computer Graphics involves:

– Modelling with matter: geometry with reflectance properties.

• Image-Based Lighting allows:

– Combination of real and synthetic graphics with consistent illumination, using images as light sources.

– Extension of Environment mapping


Environment Mapping

• Computing the color according to the reflection vector

• No self-shadowing, no inter-reflection


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Image Based Lighting

• Add models or objects to scenes and allow them to be manipulated in the scenes.

• Modelling with light allows added objects to be illuminated consistent with image existent lighting - photorealistic inclusions.

• We start with the answer by finding the scene illumination.


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• Real Scene

• Goal: place synthetic objects on table

(all pictures P. Debevec 98-99)


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Extracting scene lighting

• Capture illumination using illumination sphere


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Real scene


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Image Based Lightingcaptured illumination field


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Real scene

light based model

local scene

synthetic objects


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Image Based Lighting• Use global illumination - compute effects of

synthetic objects on local scene

local scene (brdf estimated)synthetic objects (brdf known)

light based model


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• Render into the scene

background


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• Render synthetic objects


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Image Based Lighting• Effect of local scene on real scene


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Image Based Lighting• Add differences to image


Some more results


Light Probe Images

• Spherical Environment Maps• In most digital images, pixel values

aren’t proportional to the light levels in the scene. – light levels are encoded nonlinearly so they

appear either more correctly or more pleasingly on CRTs

Computer Graphics Inf4/MSc High-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

Computer Graphics Inf4/MSc High-Dynamic Range

Photography


HDR images


Computer Graphics Inf4/MSc Comparison: Radiance map

versus single image


Eyes For Relighting

• Extracting environment maps from eyes for use in integrating and relighting scenes

• Apply IBL to the 3D objects added into the image

• http://www.youtube.com/watch?v=MllTSofXt8E


Shape from Shading

• Obtaining the 3D shape from an image• Assuming Lambertian reflection• Can recover the normal vector if we cast the

light from 3 different directions

zzyyxx nLnLnL nL

LnL, )cos(

Object

N

L

Example: sphere(lit from left)


• Integrate the normals over the surface to compute the 3D surface

• We can also estimate the colour information and use this for texture


Surface inserted into new scenes• lit according to the global illumination of that scene.• This illumination can of course come from eyes!


Acquiring the Reflectance Field of a Human Face

• Simulating reflectance on human faces requires a lot of effort and computation– Need to acquire the reflectance model – Subsurface scattering

• Instead of simulating the global illumination, we can simply illuminate the face by real-lights from different directions

• Creating images of arbitrary light conditions by combining the captured data


The Light Stage:60-second exposure

Computer Graphics Inf4/MSc Light Stage Data

Original Resolution:

6432

Original Resolution:

6432

Lighting through image recombination: Haeberli ‘92, Nimeroff ‘94, Wong ‘97Lighting through image recombination: Haeberli ‘92, Nimeroff ‘94, Wong ‘97

Computer Graphics Inf4/MSc Reflectance Functions


Lighting Reflectance Functions

normalized light map

normalized light map

reflectance function

reflectance function

lighting productlighting product

rendered pixel

rendered pixel

1


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References

• Rendering Synthetic Objects into Real Scenes, SIGGRAPH 98

• Acquiring the Reflectance Field of a Human Face, SIGGRAPH 2000 http://www.debevec.org

• Eyes For Relighting, Nishino SIGGRAPH 2004

Documents

Computer Graphics Inf4/MSc Computer Graphics Lecture Notes #16 Image-Based Lighting