Real-Time Volume Graphics [06] Local Volume Illumination

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Real-Time Volume Graphics06 Local Volume
Illumination
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Volume Illumination

• Up until now Light was emitted by the volume
• Now Illumination from external light sources

Types of Volume Illumination

• 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)

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

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Single Scattering

• Local illumination, similar to surface lighting
• Lambertian reflection(light is reflected equally
  in all directions)
• Perfect mirror reflection(light is reflected in
  exactly one direction)
• Specular reflection(light is reflected scattered
  around the direction of perfect reflection)

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Blinn/Phong Illumination

• Diffuse Term (Lambertian reflection)

n
l
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Phong Illumination

• Specular Term (view-dependent)

n
r
l
v
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Blinn/Phong Illumination
Phong Illumination

• Specular Term (view-dependent)

n
h
l
v
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Blinn/Phong Illumination

• Ambient Term (constant illumination)lightens
  up the shadows, also decreases the contrast!

Example images taken from Jeremy Birn Digital
Lighting Rendering,New Riders Publishing, 2000
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Local Illumination

• Surface lighting Light is reflected at surfaces
• Volume lighting Light is scattered at isosurfaces

• Isosurface extraction not required!
• We only need the normal vector
• Normal vector of isosurface is equal to
  (normalized) gradient vector

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Gradient Estimation

• The gradient vector is the first-order derivative
  of the scalar field
• We can estimate the gradient vector using finite
  differencing schemes

partial derivativein x-direction
partial derivativein y-direction
partial derivativein z-direction
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Finite Differences

• Taylor expansion

Forward Difference
Backward Difference
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Finite Differences
Central Difference
Gradient Approximation using Central Differences
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Pre-computed Gradients

• Calculate the gradient for each voxel
• Store the normalized gradient in an additional
  texture.

Example Use an RGBA texture
Example Use an RGB texture
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Basic Idea of Ray-casting Pipeline

• Data are defined at the corners
• of each cell (voxel)
• The data value inside the
• voxel is determined using
• interpolation (e.g. tri-linear)
• Composite colors and opacities
• along the ray path
• Can use other ray-traversal schemes as well

c1
c2
c3
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Ray Traversal Schemes
Intensity
Max
Average
Accumulate
First
Depth
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Ray Traversal - First
Intensity
First
Depth

• First extracts iso-surfaces (again!)done by
  TuyTuy 84

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Ray Traversal - Average
Intensity
Average
Depth

• Average produces basically an X-ray picture

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Ray Traversal - MIP
Intensity
Max
Depth

• Max Maximum Intensity Projectionused for
  Magnetic Resonance Angiogram

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Ray Traversal - Accumulate
Intensity
Accumulate
Depth

• Accumulate opacity while compositing colors make
  transparent layers visible!Levoy 88

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Raycasting
volumetric compositing
color
opacity
object (color, opacity)
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Raycasting
Interpolationkernel
volumetric compositing
color
opacity
1.0
object (color, opacity)
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Raycasting
Interpolationkernel
volumetric compositing
color c c s ?s(1 - ?) c
opacity ? ? s (1 - ?) ?
object (color, opacity)
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Raycasting
volumetric compositing
color
opacity
1.0
object (color, opacity)
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Raycasting
volumetric compositing
color
opacity
1.0
object (color, opacity)
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Raycasting
volumetric compositing
color
opacity
1.0
object (color, opacity)
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Raycasting
volumetric compositing
color
opacity
1.0
object (color, opacity)
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Raycasting
volumetric compositing
color
opacity
object (color, opacity)
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Brute-Force Algorithm
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Volume Rendering Pipeline
Acquired values
Data preparation
Prepared values
classification
shading
Voxel colors
Voxel opacities
Ray-tracing / resampling
Ray-tracing / resampling
Sample colors
Sample opacities
compositing
Image Pixels