Real-Time Volume Graphics [07] Global Volume Illumination

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Real-Time Volume Graphics07 Global Volume
Illumination
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Why Global Illumination

• Local illumination
• might sufficient for many application areas in
  scientific visualization (e.g medicine)
• Not sufficient for visual arts/photorealism!
• Appearance of many common objects is dominated by
  scattering effects
• Smoke, clouds
• Wax, skin, translucent materials

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Surface vs Volume Illumination

• Surface Lighting
• Light transport in vacuum
• Lighting calculation is performed at surface
  points
• Reflectivity from BRDF

• Volume Lighting
• Light transport in participating medium
• Lighing Calculation atevery point
• Scattering from phase function

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Surface Illumination
The incoming radiance , at a point from
direction will partially be reflectedinto
direction
To obtain the radiance at , we must account for
all possibleincoming directions
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Phase Functions

• For surfaces, the BRDF describes the probability
  of light being reflected from one direction
  on the hemisphere into another direction .
• For volumes, the phase function describes the
  probability of light being scattered from
  direction into direction

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Physical Model of Radiative Transfer
Increase
true emission
in-scattering
Decrease
true absorption
out-scattering
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Scattering
Phase function
Incomingradiance
Out-scattering coefficient

• In-scattering
• Volume Rendering Equation

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Volume Illumination

• Up until now External light is not attenuated
• Now Attenuation of light as it travels through
  the volume

• Single scattering, no attenuation.
• Light reaches every point unimpededly
• Light is scattered once before it reaches
  the eye
• Not physically plausible

• Single scattering with attenuation.
• Light is attenuated along ist way through
  the volume (Volumetric shadows)
• Light is scattered once before it reaches
  the eye

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Shadows Maps

• For Surfaces

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Shadow Maps

• For Volume Rendering
• Update image and shadow buffer slice-by-slice
• Need proxy geometry, that can be rendered from
  twodifferent views.

Half Angle Slicing
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Light Source Attenuation

1. Draw slice into light buffer
2. Draw slice into image buffer
3. Proceed with next slice

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Light Source Attenuation
Example Visible Human CT Head Direct Light
Attenuation
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Volume Illumination

• Up until now External light is attenuated by
  volume
• Now Light is scattered inside the volume

• Single scattering with attenuation.
• Light is attenuated along ist way through
  the volume (Volumetric shadows)
• Light is scattered once before it reaches
  the eye

• Multiple scattering
• Light is scattered multiple times before it
  reaches the eye (Global illumination)

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Scattering

• Approximation

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Volumetric Scattering
2. Draw slice into image buffer Sample light
buffer multiple timesjittered around the
original position
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Volumetric Scattering

• Scattering part
• Chromatic out-scattering term can be
  used to change color of light as it travels
  throught the volume

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Volumetric Scattering
Volumetric Scattering Chromatic Attenuation
Photographof realwax block
Bright bluereflective color
Directattenuationonly
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Examples Phase Function
Henyey-Greenstein Lit from front
Henyey-Greenstein Lit from behind
Henyey-Greenstein Lit 45 degrees from above
Henyey-Greenstein Mie Phase function
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Examples
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Examples
Direct light only
Achromatic indirect light
Chromatic indirect light
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Examples

• Approximation of opalescent materials (lit from
  above)

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Examples
Direct Light Shadows
Surface Shading
Direct LightIndirect Light
Direct LightIndirect Light Surface shading on
the leaves only