Upload
aaron-gunn
View
213
Download
0
Tags:
Embed Size (px)
Citation preview
Feature Aligned Volume Manipulation for Illustration and Visualization
Carlos D. Correa, Deborah Silver
Rutgers, The State University of New Jersey
Min Chen
University of Wales, Swansea, UK
Motivation
Nucleus Inc Antonio Scrantoni and Paolo Mascagni, 1833. U.S. National Library of Medicine
Hand-drawn illustrations often include manipulating parts of an object:• They often contain cuts• They allow feature sensitive operations• They often represent virtual operations (do not necessarily conform to
reality)
Motivation (cont.)
We refer to such manipulation as lllustrative Deformation• Priority to interactivity, operatability and quality• As opposed to physically-based deformation, this can be thought of
as a top-down approach This type of deformations provides an intuitive depiction of internal
structure. It serves as an abstraction of different stages of a procedure, e.g. a
surgical operation. It is useful in surgery illustration/planning, education, and as a
visualization tool in general.
Feature Alignment
Traditional volume deformations are continuous and treat volumes as an homogeneous collection of points [Westermann et al. 2001, Rezk-Salama et al. 2001]
McGuffin [2003] introduced 3D widgets with pre-computed segmented data to allow feature sensitive manipulation of volumes. Can this approach be extended to direct volume rendering?
Recent approaches allow the definition of cuts [Correa,2006]. However, cuts appear flat as no semantics are introduced axis alignment
Cuts in general are difficult to model in computer graphics. Require costly re-tessellations. This is further complicated when cuts have to be aligned with certain features.
Axis Alignment
Treating volumes as homogeneous collections of voxels leads to axis alignment of cuts.
Difficult to see features of interest
Goal
To render deformations while preserving features of interest, by aligning cuts to a given:• Distance from surface surface alignment• Feature based on segmentation segment alignment
CT Dataset Illustrative Deformation
Illustration
Rendering Pipeline (axis aligned cuts)
Select operator
Sample and Deform
OPERATORS TRANSFORMATION
Adapted from Correa et al. 2006
via inverse space warping
Feature-Aligned Rendering Pipeline
Select operator
Apply mask
Sample and Deform
OPERATORS
MASKS
TRANSFORMATION
Adjust opacity/lighting according to alignment
Definition of features using a volumetric mask
Operators
Inspired by surgical tools and procedures Generic: they can be applied to any dataset Defined as a 3D texture. Iconic representations
are obtained when applied to a volumetric cube (or cylinder)OPERATORS
Volume Transformation
Sample Apply transformation Estimate normal Compute lighting
Subject to Alignment Mask
Sample and Deform
TRANSFORMATION
Modeling Deformation and Cuts
Forward transformation is simple but limited for volumes undersampling unless space between points is interpolated (for cuts, it requires re-tessellation)
Inverse transformation. Solves sampling problem, but discontinuous deformations are not a 1:1 mapping.
Forward Transformation Backward Transformation. Note introduction of special value to model discontinuity
Modeling Feature Alignment
Define a smooth mask M • Binary masks may cause aliasing• M(p) >= 0.5 p is non operatable• M(p) < 0.5 p is operatable
Three cases for inverse transformation
Not affected by mask: apply inverse mapping
Point inside mask: it is not transformed
Point outside mask but maps back into mask: empty space left by feature
Definition of Features
Distance-based vs. Segmentation based (1) Surface Alignment: Features are defined with the shape. Distance
field of surface of object defines a series of “shells”, which define features. Useful when no segmentation is available.
MASKS
M = 0.0
M = 0.5
M = 1.0
Outer shell ()
Feature’s outer surface (DT = )
Feature’s interior
Mask Definition
(2) Segment Alignment: Mask is already defined as a segment Usually segmentations are discrete (binary for 1 segment), For
proper rendering without aliasing, a smooth definition is required: Using a smoothing operator Using Distance Field of segmented part
M = 1M = 0.5
M = 0.0
Rendering and Lighting
Cuts are now along a certain feature. For surface alignment, a new surface appears. Pre-computed normals
are not necessarily perpendicular to that surface For segment alignment, gradient already defines “almost” correctly one
surface. However, it cannot define properly the “underside” of the cut
Normal Adjustment: Surface Alignment
Normals are oriented depending on density, not necessarily aligned with surface “shell”. This can be fixed by blending of normals
T
*T
DT
Normal Adjustment: Segment Alignment
Normals on the underside of a cut point in the opposite direction
T
*T
-T
Results (1) Peeling of Skin
AXIS SURFACE SEGMENTOriginal Dataset
Results (2) Frog Dissection
AXIS SURFACE SEGMENT
Original Dataset
Results (3) Hand Surgery
AXIS SURFACE SEGMENT
Original Dataset
Results (4) Forefoot Retractor
AXIS SURFACE SEGMENT
Original Dataset
Implementation Details
Based on discontinuous displacement mapping [Correa et al. 2006], using texture based volume rendering
Operators are stored as 3D textures (size is much smaller than size of dataset). Feature Mask is also stored as a 3D texture
Interactive results (Pentium XEON 2.8GHz Quadro FX 4400 (512 MB): d =1
Video
Applications
Medical and Biological Illustration.• Operators are metaphors of the tools used in dissection
Surgical Planning• Manipulation of operators allows the generation of deformations and
cuts in various stages of a procedure Improved Visualization
• Cutaway views with arbitrary cut geometry• Focus+Context, Distortion Lens
Conclusions
Volume deformation techniques often treat volumes as homogeneous collection of voxels. When modeling cuts and breaks, they appear to be axis aligned, which results in decreased realism and limited use.
It is possible to extend volume deformation to align cuts with certain features of interest. These can be defined as “shells” of the surface of the object using the distance transform, or as the product of segmentation
Feature alignment can be implemented efficiently on commodity hardware. Proper handling of cut information to reduce aliasing, and adjustment of normals near the surfaces of cuts are necessary to produce high quality rendering of cuts.
Future Work
Merge illustrative deformation with illustrative rendering• NPR techniques can be used to emphasize new surfaces due to
cuts or to exaggerate deformation (e.g. rendering of stress lines) Inclusion of rigid constraints for enhanced deformations, collision
avoidance. More intuitive user interface and manipulation widgets to create and
place operators.
Thanks
Acknowledgements• Volumetric datasets are courtesy of Lawrence Berkeley Laboratory,
UNC Chapel Hill, University of Iowa, U.S. National Library of Medicine, Viatronix Inc. and Vienna University of Technology. The illustrations are courtesy of U.S. National Library of Medicine and Nucleus Medical Art, Inc.
• We want to thank Dr. Stanley Trooskin, Dr. Sid Roychowdhury and Dr. Marsha Jessup for valuable input on surgical and medical illustration.
Further Informationhttp://www.caip.rutgers.edu/~cdcorrea
Thanks