Real-Time Volume Graphics
[07] Global Volume Illumination
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
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
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
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 at
every point
Scattering from phase
function
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Surface Illumination
The incoming radiance
at a point
from direction
,
will partially be reflected
into direction
To obtain the radiance at , we must account for all possible
incoming directions:
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
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
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Increase
Physical Model of Radiative Transfer
Decrease
true emission
true absorption
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
in-scattering
out-scattering
Eurographics 2006
Scattering
In-scattering
Outscattering
coefficient
Phase
function
Incoming
radiance
Volume Rendering Equation:
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Volume Illumination
Up until now: External light is not attenuated
Now: Attenuation of light as it travels through
the volume
Single scattering
scattering,with
no attenuation.
attenuation.
Light is
reaches
attenuated
everyalong
pointist
unimpededly
way
through the volume
(Volumetric shadows)
Light is scattered once
Light
before
is itscattered
reaches once
the eye
before it reaches the eye
Not physically plausible
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Shadows Maps
For Surfaces:
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Shadow Maps
For Volume Rendering:
Update image and shadow buffer slice-by-slice
Need proxy geometry, that can be rendered from two
different views.
Half Angle
Slicing
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Light Source Attenuation
1. Draw slice into light buffer
2. Draw slice into image buffer
3. Proceed with next slice
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Light Source Attenuation
Example:
Visible Human CT Head
Direct Light +
Attenuation
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Volume Illumination
Up until now: External light is attenuated by
volume
Now: Light is scattered inside the volume
Single scattering
Multiple
scatteringwith attenuation.
Light is scattered
attenuatedmultiple
along ist
times
way through
before
it reaches
the volume
the eye
(Volumetric
(Global
illumination)
shadows)
Light is scattered once
before it reaches the eye
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Scattering
Approximation:
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Volumetric Scattering
2. Draw slice into image buffer
Sample light buffer multiple times
jittered around the original position
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Volumetric Scattering
Scattering part:
Chromatic out-scattering term
can be
used to change color of light as it travels
throught the volume
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Volumetric Scattering
Photograph
of real
wax block
Volumetric
Scattering
+ Chromatic
Attenuation
Bright blue
reflective
color
Direct
attenuation
only
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Examples Phase Function
HenyeyGreenstein
Lit from
behind
HenyeyGreenstein
Lit from front
HenyeyGreenstein
HenyeyGreenstein
Lit 45°
degrees
from above
+ Mie Phase
function
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Examples
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Examples
Direct light only
Achromatic indirect light
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Chromatic indirect light
Eurographics 2006
Examples
Approximation of opalescent
materials (lit from above)
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006
Examples
Surface
Shading
Direct Light
+ Shadows
Direct Light+
Indirect Light
Direct Light+
Indirect Light
Surface
shading on
the leaves
only
REAL-TIME VOLUME GRAPHICS
Christof Rezk Salama
Computer Graphics and Multimedia Group, University of Siegen, Germany
Eurographics 2006