Post on 07-Apr-2018
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Medical Visualization:Concepts & Overview
Abhishek MitraPh.D in Medical Imaging
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Visualization:
Surface & Volume Rendering
2D screen
3D volume dataset
2D display of
3D dataset
Volume Rendering: The term volume renderingis used to describetechniques which allow the visualization of three-dimensional
data.
Volume rendering is a technique for visualizing sampledfunctions of three spatial dimensions by computing 2-D
projections of a colored semitransparent volume.
Volume Rendering: The term volume renderingis used to describetechniques which allow the visualization of three-dimensional
data.
Volume rendering is a technique for visualizing sampledfunctions of three spatial dimensions by computing 2-D
projections of a colored semitransparent volume.
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Iso value = 80
Iso value =
200
Visualization: Surface Rendering
Isosurface :
A closed surface separates outside and inside For volume data, define isovalue:All voxels with values bigger than isovalue are inside
and others outside
Then, an surface is defined by this isovalue, theisosurface
Isosurface :
A closed surface separates outside and inside For volume data, define isovalue:All voxels with values bigger than isovalue are inside
and others outside
Then, an surface is defined by this isovalue, theisosurface
3D volume
dataset
Isosurface
50
77
87
95
5
55
7
Isocontour (of value 5)
extraction in 2D
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Visualization: Volume Rendering
Classification
Classification
Gradient Computation
Compositing
Shading
Shading
Preparing
3D Volume data
prepared from stack of
2D images
Volume rendered image
77
50
87
95
5
55
7Stack of 2D images
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Classification
Classification
Gradient Computation
Gradient Computation
Compositing
Shading
Shading
Preparing
Visualization: Gradient Computation
Why Compute Gradient :
Gradient of densities provides info on surface:- Density changes at surface boundaries- Change also appears in their gradient
- Presence of surface is qualified by magnitude
of gradient
Gradient vector will be used as surface normal in theshading process
Gradient vectors norm is used in classification
Why Compute Gradient :
Gradient of densities provides info on surface:- Density changes at surface boundaries- Change also appears in their gradient
- Presence of surface is qualified by magnitude
of gradient
Gradient vector will be used as surface normal in theshading process
Gradient vectors norm is used in classification
LR
EyeN
Shading
Isosurface
3D volume datasetStack of 2D images
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Visualization: Classification
Classification
Classification
Gradient Computation
Compositing
Shading
Shading
PreparingClassification Goals :
Make sense of a given data set Group voxels of same substanceAssign color/opacity value that guides visualization Color Transfer Function-> Maps data to a given color Opacity Trans. Fn.-> Maps data to a given opacity
Classification Goals :
Make sense of a given data set Group voxels of same substanceAssign color/opacity value that guides visualization Color Transfer Function-> Maps data to a given color Opacity Trans. Fn.-> Maps data to a given opacity
Color Transfer
Function
77
50
87
95
555
7
C(xi)xi
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Visualization: Classification Opacity TransferFunction
Opacity
Volume
Data1.00.1 0.4
Bone:
Opaque
1.0
Volume
Data0.1 0.4
1.0
Tissue:
Opaque
Opacity
Skull + Skin
Opacity
0.1
Tissue:
Semi transparent
0.4
1.0
Bone:
Opaque
Volume
Data
C(xi)xi C(xi), O(xi)
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Visualization: Shading
Stack of2D images 50
77
87
95
555
7
xi
Data
preparation
3D volume
dataset
C(xi)
Color
Transfer
Function
3D colored dataset
C(xi), O(xi)OpacityTransfer
Function
3D colored
semi-transperent dataset C(xi), O(xi), S(xi)
Shading
3D colored
semi-transperent
shaded dataset
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Eye
Visualization: Compositing or Ray-casting
For every pixel in output image:
Treat volume as a translucent object shoot ray into volume at least one per pixel at evenly spaced ray locations, obtain color and opacity
by interpolation
Accumulate color and opacities of voxels from back to
front Stop when ray exits grid or reaches full opacity
For every pixel in output image:
Treat volume as a translucent object shoot ray into volume at least one per pixel at evenly spaced ray locations, obtain color and opacity
by interpolation
Accumulate color and opacities of voxels from back to
front Stop when ray exits grid or reaches full opacity
Classification
Gradient Computation
CompositingCompositing
ShadingShading
Preparing
C(xi), O(xi), S(xi)
2D screen