1. Siggraph/Eurographics Workshop on Graphics Hardware 2001
High-Quality Pre-Integrated Volume Rendering Using Hardware Accelerated Pixel Shading
Klaus Engel, Martin Kraus, Thomas Ertl
Visualization and Interactive Systems Group
University of Stuttgart, Germany
Siggraph/Eurographics Workshop on Graphics Hardware 2001

2. Volume Rendering Quality Improvement
Higher sampling rates: additional tri-linearly interpolated slices
2D textures:
render intermediate slices using multi-texture approach (Rezk-Salama,Engel et al., GH2000)
- 3D textures:
render additional slice polygons
But:
decreases rendering speed due to additional rasterization
many additional slices required even for low-resolution volumes
non-linear transfer functions: not sufficient to sample volume with the Nyquist frequency of scalar data

3. Volume Classification
transfer functions
Pre-
classification
interpolation
classification
interpolation
voxels
classification
Post-
classification

4. Pre-Integrated Volume Rendering
The approach:
texture-based (2D/3D)
ray-segments computed in a pre-processing step
pre-computed ray-segment lookup (dependent texture)
small number of slices => fast
Especially suited for:
low resolution volume data
non-linear transfer functions
Capable of:
high-quality rendering
direct volume rendering, volume shading, isosurfaces

5. Pre-integrated Volume Rendering - Idea
slice-by-slice
slab-by-slab sf sb
front
slice
back
project slice
texture polygon sb sf
fetch integral from
dependent texture
pre-integrate all
possible combinations
hardware-accelerated implementation
on NVidia GeForce3 chip

6. DOT_PRODUCT_ TEXTURE2D_NV
Pre-integrated Volume Rendering - Texel Fetch sf
stage 0
TEXTURE_2D
(s0,t0)
RGBA
result sb
stage 1
TEXTURE_2D
(s1,t1)
RGBA
result
RGB0
(s3,t3,r3) 
(1,0,0)
stage 3
DOT_PRODUCT_ TEXTURE2D_NV
(1,0,0) • RGB0=sf
front
slice
back
slice
RGB1
(s2,t2,r2)  (1,0,0)
stage 2
DOT_PRODUCT_NV
(1,0,0) • RGB1=sb sb sf
RGBA result
on to register combiners

7. Results – Direct Volume Rendering
128 slices
pre- classification
128 slices
post- classification
284 slices
post- classification
128 slices
pre-integrated

8. Isosurfaces Isosurfaces: particular dependent texture 1. 2. sb 4. 3.
front
slice
back 1. 1
front
slice
back 2. 2 sb
front
slice
back 4. 4 96
front
slice
back 3. 3 3a 32 sf 32 96

9. Isosurfaces – Gradient Interpolation
g = IP gb + (1-IP) gf interpolation weight: IP = (siso – sf)/(sb – sf)
store gradient with scalar data in RGBA texture
store IP in dependent texture
implement interpolation in reg. combiners sb gb gf g
front
slice
back
slice

10. Pre-integrated Volume Rendering – Register Combiners
Combiner 0+1: rebuild gradients (front + back) R B G A R B G A s
gy gz gx
gx gy gz s
Combiner 2: interpolate gradients
IP = (siso – sf)/(sb – sf) g = IP gb + (1-IP) gf
Combiner 3-7: shading (isosurface or volume)
I = Ia + Id ( n . l ) + Is ( n . h )p
pmax = 256

12. Results – Direct Volume Rendering, Random TF

13. Results - Isosurfaces