Hardware-Accelerated Adaptive
EWA Volume Splatting
Wei Chen
Liu Ren
Matthias Zwicker
Hanspeter Pfister
ZJU
CMU
MIT
MERL
Volume Splatting
Object-order method
 3D reconstruction kernel centered at each voxel
(elliptical Gaussian)
 Voxel contribution = 2D footprint (color, opacity)
 Weighted footprints accumulated into image

2D footprints = splats
Voxel kernels
Screen
2
Related Work
Quality
Westover1989
Crawfis 1993
EWA
Swan 1997
Mueller 1999
Imagealigned
Huang 2000
Our work
Zwicker 2001
Swan
Xue 2003
Axis-
aligned
Software
Texture splats
Fast splats
OpenGL ex
Speed
3
Outline
EWA volume splatting
 Adaptive EWA splatting
 GPU implementation
 Results and conclusions

4
EWA Volume Splatting


Compensate aliasing artifacts due to perspective projection
EWA Filter = low-pass filter warped reconstruction filter
Low-Pass
Filter
Projection
W
Convolution
r1
xk
r0
Volume
EWA volume resampling filter
k
5
EWA Volume Splatting
(512x512x3)
Reconstruction filter only: 6.25 fps
Low-pass filter only: 6.14 fps
EWA filter: 4.97 fps
EWA filter: 3.79 fps
6
Analysis of EWA Filter
Warped reconstruction kernel
Low-pass
filter
Resampling
filter
Minification
Magnification
7
Analysis of EWA Filter

Shape of EWA Splat is dependent on
distance from the view plane
r1
x
r0
EWA splat
rk
2
r0 

r
h
2
u2


rk2 1 x02  x12
2
r1 

r
h
2
u2
rh Low-pass filter radius
rk Reconstruction filter radius
u2 Distance to the view plane
2
2
Note that 1 x0  x1  1.0 , 1.0  Cons tan t 
8
Adaptive EWA Filtering
Warped reconstruction kernel
Low-pass
filter
Resampling
filter
if u2 > A
use low-pass filter
if A<u2< B
use EWA filter
if u2< B
use reconstruction filter
9
Patch Processing
Process a 8 x 8 patch of voxels at a time
 Filter selection based on four corners of each
patch (choose smallest)

Traversal order
Patch
Distance
10
Adaptive EWA Volume Splatting
(512x512x3)
Adaptive EWA filter: 6.88 fps
Adaptive EWA filter: 1.84 fps
EWA filter: 4.83 fps
EWA filter: 1.75 fps
11
Outline
EWA volume splatting
 Adaptive EWA splatting
 GPU implementation
 Results and conclusions

12
Object-Space EWA Splatting

Object-space EWA splatting with texture mapping
[Ren et al. Eurographics 2002]
EWA Splat (elliptical Gaussian)
Texture (unit Gaussian)
(0,1)
(0,0)
(1,1)
(1,0)
Projection
Unit quad
Textured quad
13
Proxy Geometry Template


Rectilinear volumes: use one proxy geometry
template for all slices in each direction
Store vertex indices in AGP memory
..
.
Regularity
Voxel geometry
Proxy geometry
template
Quad
geometry
14
Vertex Compression
Compress each vertex to 32 bits
 Decompression on-the-fly in programmable
hardware
 To store vertex information of 256x256x256
volume in video memory
• Without compression 2,048 MBytes 
• With compression 12 MBytes 
 Retained-mode hardware acceleration feasible

15
Retained vs. Immediate Mode
Data
#Total
Splats
#Rendered
Splats
Immediate
Mode
Retained
Mode
Bonsai
8388608
274866
0.53 fps
7.53
Engine
7208960
247577
1.40 fps
10.28 fps
Lobster
5461344
555976
1.19 fps
10.60 fps
Head
13631488
2955242
0.12 fps
2.86
fps
fps
Factor of ~10 improvement
16
Interactive Classification:
Opacity Culling

Hardware-accelerated list-based traversal

For each slice
• For each 32 x 32 patch of voxels (smaller indices)
• Indices of proxy geometry organized into iso-value lists using
bucket sort; CPU merges lists on-line
• Render only iso-value lists with visible voxels
0
128
Patch
256
17
Interactive Classification:
Opacity Culling
Includes changes to TF every frame
Data
List-based
opacity culling
Standard
opacity culling
Head
2.80
fps
0.3 fps
Engine
10.18 fps
0.8 fps
Bonsai
7.23
fps
0.8 fps
Lobster
10.30 fps
1.1 fps
Factor of ~10 improvement
18
Deferred Shading
Volume texture access is only possible in
fragment programs*
 However, per fragment shading is expensive
 Solution: deferred shading in two passes

*
Newer GPUs allow texture access in vertex
programs
19
Deferred Shading



Pass one: 3D texture access, classification and
illumination in vertex shader, render one pixel
per voxel
Pass two: reuse the pixel data from the first
pass to shade the 2D footprint
Performance gain: 5%-10% speedup
Pass one
Pass two
Final result
20
Experiments



P4 2.4 GHz
ATI 9800 Pro with 256 MB RAM
Direct3D 9.0b with VS 2.0 and PS 2.0
Type
EWA
Reconstruction Only
Low-pass Only
Regular
70
45
26
Rectilinear
74
45
26
Vertex shader instructions
21
Sheet-buffer Composition
0.80 fps
3.00 fps
3.45 fps
Axis-aligned traversal, addition in sheet buffers,
then blending front-to-back
22
UNC Head: 208x256x225
#Rendered splats:
2,955,242
2.86 fps
8.5M splats / sec
23
Bonsai: 256x256x128
#Rendered splats:
274,866
7.53 fps
2M splats / sec
24
Engine: 256x256x110
#Rendered splats:
247,577
10.28 fps
2.5M splats / sec
25
Lobster: 301x324x56
#Rendered splats:
555,976
10.60 fps
5.9M splats / sec
26
Video
Our Contributions





Adaptive EWA computation
Volume data compression
Retained-mode hardware acceleration
Interactive opacity culling
Deferred two-pass shading
28
Future Work





Image-aligned EWA volume splatting
Irregular volume splatting
Pointsprites in OpenGL
Floating point textures
Vertex texture for classification
29
Acknowledgements
Jessica Hodgins (CMU)
 Markus Gross (ETH)

http://graphics.cs.cmu.edu/projects/adpewa/index.html
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