4. Connectome Team
• Harvard Center for Brain Science
– Jeff Lichtman & Clay Reid
• Microsoft Research / UW
– Michael Cohen
• Kitware Inc.
– Will Schroeder, Charles Law, Rusty Blue
• VRVis Vienna
– Markus Hadwiger, Johanna Beyer
• IIC
– Amelio Vazquez, Eric Miller (Tufts)
– Won-Ki Seung, Hanspeter Pfister
18. Ray Casting
• Image-order ray shooting
• Interpolate
• Assign color & opacity
• Composite
• Simple to implement
• Very flexible
(adaptive sampling, …)
• Correct perspective
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
19. Transfer Functions
• Mapping of density to optical properties
• Simplest: color table with opacity over density
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
21. Single-Pass Ray Casting
• Enabled by conditional loops
• Substitute multiple passes with single loop and early
loop exit
• Volume rendering example
in NVIDIA CUDA SDK
(procedural ray setup)
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
22. Basic Ray Setup / Termination
• Two main approaches:
• Procedural ray/box intersection
[Röttger et al., 2003], [Green, 2004]
• Rasterize bounding box
[Krüger and Westermann, 2003]
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
23. Procedural Ray Setup / Term.
• Procedural ray / box intersection
• Everything handled in
fragment shader
• Ray given by camera position
and volume entry position
• Exit criterion needed
• Pro: simple and self-contained
• Con: full load on fragment shader
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
24. quot;Image-Basedquot; Ray Setup / Term.
• Rasterize bounding box
front faces and back faces
• Ray start positions:
front faces
• Direction vectors:
back faces − front faces
- =
• Independent of projection (orthogonal/perspective)
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
25. Kernel
• Image-based
ray setup
• Ray start image
• Direction image
• Ray-cast loop
• Sample volume
• Accumulate
color and opacity
• Terminate
• Store output
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
26. Standard Ray Casting Optim. (1)
Early ray termination
• Isosurfaces:
stop when surface hit
• Direct volume rendering:
stop when opacity >= threshold
• Several possibilities
• Current GPUs: early loop exit works well
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
27. Standard Ray Casting Optim. (2)
Empty space skipping
• Skip transparent samples
• Depends on transfer function
• Start casting close to first hit
• Several possibilities
• Per-sample check of opacity (expensive)
• Hierarchical data store (e.g., octree with stack-less
traversal [Gobbetti et al., 2008] )
• These are image-order:
what about object-order?
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
28. Object-Order Empty Space Skip. (1)
• Modify initial rasterization step
rasterize bounding box rasterize “tightquot; bounding geometry
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
29. Object-Order Empty Space Skip. (2)
• Store min-max values of volume blocks
• Cull blocks against transfer function or isovalue
• Rasterize front and back faces of active blocks
MARKUS HADWIGER, VRVIS RESEARCH CENTER, VIENNA, AUSTRIA
32. Addressing the Data
Challenge
• Multi-Scale Imaging
• Hierarchical Data Representation
• Distributed Heterogeneous Computing
• Visualization
• Segmentation
• Analysis
33. Active Ribbons
Active Ribbon:
A set of two non-intersecting and
coupled Active Contours
Active Contour:
Deformable closed curve that can be
used to segment objects in an image
Outer Active Inner Active
Contour
Contour
Active Ribbon
37. How did the Universe
start?
The MWA Project
Kevin Dale, Richard Edgar, Daniel Mitchell, Randall Wayth,
Lincoln Greenhill, and Hanspeter Pfister
38. MWA CfA / IIC Team
• Harvard Center for Astrophysics /
Smithsonian Astrophysical
Observatory
– Lincoln Greenhill
– Daniel Mitchell
– Randall Wayth
– Stephen Ord
• IIC / SEAS
– Richard Edgar
– Kevin Dale, Hanspeter Pfister
39. The Scientific Goals
• Epoch of Re- ionized
Inonisation (EOR) neutral
• Heliospheric and
(H)
The “Gap”
Ionospheric
• Transient detection
• Pulsars, Surveys,
Interstellar Medium, ionized
Galactic Magnetic
Field, …
40. The Murchison Widefield Array (MWA)
• Located in the remote Australian
outback
• Extremely wide fields of view for radio
astronomy in the 80-300 MHz band
• 512 tiles, each a 4x4 array of dipoles,
scattered over ~ 1.5 km
• Data center for real-time processing
co-located with the array
http://www.haystack.mit.edu/ast/arrays/mwa/index.html
45. The Data Rate
Challenge
ent
v. p ang
arall led Calibration Loop
el c
om
put
atio
n
FFT
Averaging ( !)
Gridding
Vector Rotation Mapping
(1) GB/s
16 GB/s
Science
8s cadence
0.5s cadence
49. Scaling to a Cluster
• 1000 frequency channels, 65 sources every
8 seconds, and 16002 output image
• 20-40 frequencies / GPU
• 32-64 GPUs, i.e., 16 Tesla S1070s
• Need MPI for internal data transfer
50. Conclusions
• GPUs enable high-throughput scientific
computing
• Performance gains of 10-100x
• CUDA makes life easier (but not perfect)
• Rasterization / OpenGL still useful
• Need CUDA MPI for clusters