Visualization Techniques: Volume Rendering Basics

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Visualization TechniquesVolume Rendering
(Basics)
Raphael Grasset
Day 3
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Topics

• Volume Rendering Applications
• Volume Rendering Overview
• Direct Volume Rendering Techniques
• Transfer Functions and Classification
• Illuminations

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Medical
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Visible Human Project

• http//www.nlm.nih.gov/research/visible/visible_hu
  man.html
• VOXEL-MAN Gallery

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Biology
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Astronomy
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Applications
Engineering

• Engineering/Industrial

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Geophysics
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Whatever
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Context

• There are different techniques for different
  attributes
• Scalar Visualization Techniques
• Colour Mapping
• Contouring
• Isosurfaces
• Direct Volume Rendering
• Vector Visualization Techniques
• Tensor Visualization Techniques
• Modelling Visualization Techniques

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Context

• All the techniques we have seen so far are
  rendering a surface (surface rendering)

Surface as Representation of the Data
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Surface Rendering

• Use Standard 3D Computer Graphics Model

Intermediate Surface Representation Generally
Triangles
Dataset(ex scalar, vector,grid/unstructured,
etc)
Pixels on your Screen
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Direct Volume Rendering

• Use a Volume Graphics Model

Use a different Optical Model
Intermediate Surface Representation Generally
Triangles
Dataset(ex scalar, vector,grid/unstructured,
etc)
Pixels on your Screen
Same data voxels (general case)
For a certain type of data 3D Dense
Volume structured 3D grid of voxels regular
geometry and topology
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Volumetric Data

• Direct Volume Rendering and Volume Rendering
• use Volumetric/Volume Data
• Voxels (type of topology)
• Regular 3D geometry and topology
• The most used structure 3D regular grid

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Collecting Volumetric Data

• From sampling, simulation, or modelling
• Medical (assembling a sequence of 2D slices)
• Magnetic Resonance Imaging (MRI)
• Computed Tomography (CT)
• Functional MRI (fMRI)
• Positron Emission Tomography (PET)

reconstruction
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With the Viz Pipeline
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Types of Volumetric Data

• Engineering
• CT Scan mechanical piece
• Biology
• Confocal Microscope
• Computational Fluid Dynamics (CFD)
• Simulation from Supercomputer (3D Scalar Grid)

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Terminology

• Generally the term Volume Rendering includes
• Isosurface Rendering
• Direct Volume Rendering (DVR)
• Sometimes there is misuse of the terms
  (DVRVolume RenderingVolume Viz, etc)

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Volume Visualization

• Volume Visualization encompasses all techniques
  to visualize volume (2D or 3D)
• Isosurface technique is one of them
• Direct Volume Rendering is another
• What are the others?

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Volume Visualization Technique
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Volume Visualization Technique
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Volume Visualization Technique
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Volume Visualization Technique
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Not lost with all this terminology?
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Terminology Summary
SciViz
Scalar Field Techniques Color Mapping,
Contouring
Direct Volume Rendering
3D Standard Computer Graphics (Surface
Rendering)
Isosurfaces
Vector Field Techniques Hedgehogs, Glyphs,
Warping, Animation, Streamlines,
Streamribbons, Texture Advection
Tensor Field Techniques Hedgehogs, Multiple
Vector Fields, HyperStreamlines, Ellispoid

Modelling Techniques Polygonal surface (e.g. 3D
Reconstruciton)Implicit Modelling

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Terminology Summary
SciViz
Scalar Field Techniques Color Mapping,
Contouring
Volume Rendering
Direct Volume Rendering
3D Standard Computer Graphics (Surface
Rendering)
Isosurfaces
Vector Field Techniques Hedgehogs, Glyphs,
Warping, Animation, Streamlines,
Streamribbons, Texture Advection
Tensor Field Techniques Hedgehogs, Multiple
Vector Fields, HyperStreamlines, Ellispoid

Modelling Techniques Polygonal surface (e.g. 3D
Reconstruciton)Implicit Modelling

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Terminology Summary
SciViz
Scalar Field Techniques Color Mapping,
Contouring
Volume Graphics
Volume Rendering
Direct Volume Rendering
3D Standard Computer Graphics (Surface
Rendering)
Isosurfaces
Vector Field Techniques Hedgehogs, Glyphs,
Warping, Animation, Streamlines,
Streamribbons, Texture Advection
Tensor Field Techniques Hedgehogs, Multiple
Vector Fields, HyperStreamlines, Ellispoid

Modelling Techniques Polygonal surface (e.g. 3D
Reconstruciton)Implicit Modelling

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Terminology Summary
SciViz
Volume Visualization Slice by
Slice Multiplanar Reconstruction MIP
Scalar Field Techniques Color Mapping,
Contouring
Volume Graphics
Volume Rendering
Direct Volume Rendering
3D Standard Computer Graphics (Surface
Rendering)
Isosurfaces
2D Standard Computer Graphics
Vector Field Techniques Hedgehogs, Glyphs,
Warping, Animation, Streamlines,
Streamribbons, Texture Advection


Tensor Field Techniques Hedgehogs, Multiple
Vector Fields, HyperStreamlines, Ellispoid

Modelling Techniques Polygonal surface (e.g. 3D
Reconstruciton)Implicit Modelling

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Motivations

• What is so great about direct volume rendering?
• You can see-through any model
• Overcome limitations of surface rendering
• Efficient use of transparency (other cues for HVS)

Volume Rendering
Surface Rendering (e.g Isosurface)
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Motivations

• Peeling, cutting, etc, are easy with a volume
  model
• Difficult with Surface model (involves
  complicated processing)

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Issues with DVR

• More CPU intensive and uses more memory than
  surface rendering
• The concept of voxel is not so intuitive
• The mapping process (for DVR, assigning colour
  and transparency to volume regions) is difficult

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Direct Volume Rendering Principle Classification
and Illuminations
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Direct Volume Rendering

• For surface rendering, there is an
• optical model between the camera, lights,
• and surface.
• For Direct Volume Rendering
• We also use a camera
• We also use optical model
• BUT we have volumetric data,
• ?Behaviour of light passing through a voxel
• ?Different (and more) complex lighting model

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Direct Volume Rendering

• 3 General techniques
• Object-Order
• Forward mapping (volume data mapped onto the
  image plane)
• Image-Order
• Backward mapping (raycasting from image plane to
  the volume)
• Domain-Based
• Transformation to another domain (e.g. wavelet),
  projection
• generated

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4.
3.
1.ThresholdValue
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Option 1 Threshold Value

• Stop above a certain scalar value (threshold)
• Intensity distance ray traversed

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Option 2 Iso Value

• Stop at a certain value (iso-value)
• Isosurface projection (we are in DVR)

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- Transparency projection
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Classification

• Mapping Process (so far) Intensity Transfer
  Function
• Mapping linearly scalar value to the voxel
  opacity
• As previously, we can use a transfer function for
  the mapping opacity transfer function
• We can also combine it with colour mapping,
  colour transfer function
• OpacityColour transfer function Classification
• Scalar Value density

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Classification/Transfer Function
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Classification

• Visualization software offers an interface to
  configure this transfer function

(R,G,B)
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Density (Scalar value)
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VTK Demo

• VTK supports Ray Casting Demo
• Composite projection
• MIP
• Isosurface projection
• VTK supports also
• Texture 2D (Object Order)
• Texture 3D (Object Order)

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ParaView demo

• Paraview uses (only) RayCasting Method
• composite intensity projection
• Linear transfer function scalar ??opacity

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Why is it named Classification?

• In general you want to identify some areas of
  interest in your volume data
• Space-Based Segmentation
• Density-Based Classification

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Combination of Techniques

• The Ray-Cast technique supports intermixing of
  volume and surface
• Combination of DVR
• Possible with contouring,
• clipping, cutting,
• isosurface, etc.

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The General Approach in Practice
slice

• Slicing display the volume data, mapped to
  colours, along a slice plane
• Iso-surfacing generate opaque and semi-opaque
  surfaces on the fly
• Transparency effects volume material attenuates
  reflected or emitted light

Semi-transparentmaterial
Iso-surface
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Example
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Summary

• Volume Rendering Terminology
• DVR, Volume Viz, Volume Graphics
• Intensity Transfer Function and Optical Model for
  Volume Data
• Classification
• Combination of Techniques