1. Volume Ray-Casting on Graphics Hardware
CDA 6938 : Spring 2009
Mangesh Nijasure

2. Algorithm
Ray-Casting
Sampling
Shading
Compositing

3. Ray-Casting

4. Setup in hardware

5. Ray-Plane intersection
t = -eye_pos.x / ray_dir.x
curr_point = eye_pos + (t * ray_dir)
if(curr_point.y>=0.0f && curr_point.y<=1.0f &&
curr_point.z>=0.0f && curr_point.z<=1.0f) {
if(t<tmin) tmin = t
if(t>tmax) tmax = t }

6. Visualizing tmin and tmax

7. Sampling At each sample interval, fetch from the 3D volume texture
Using this value fetch the color from the 1D transfer function

8. Tri-Linear interpolation

9. Shading
Use the Center-Differences method for gradient computation (pre-computation step)
Fetch from the 3D gradient texture
Compute light direction
Evaluate the N.L dot product to get the lighting factor

10. Compositing alpha = 1 – old_alpha
new_color = (shaded_col*alpha) + old_color
old_color = new_color

11. Demo

12. Results Software Solution GPU Hardware Solution Intel Pentium IV
3.8 GHz
1 GB Ram
GPU Hardware Solution
AMD Radeon HD mid range part (~ $100)
Core Clock 750MHz
320 Stream Processors
512 MB On board memory
DirectX 10 Graphics API implementation

13. Performance No Lighting at 512 x 512
Data Set
Software
Milisec/Frame
Hardware
Hydrogen Atom
1,375 11
Bucky Ball
1,394
Bonsai
1,451 12
CT Head
1,426
64 samples

14. Performance With Lighting at 512 x 512
Data Set
Software
Milisec/Frame
Hardware
Hydrogen Atom
6,346 16
Bucky Ball
6,322
Bonsai
7,002 28
CT Head
6,953 31
64 samples
* Results are not visually accurate, but timings are exact

15. Performance with Hydrogen Data Set Samples & Resolution
No Lighting
Times in miliseconds / frame
Samples
512x512
1024x1024
S/W
H/W 32
777 6
3,129 22 64
1,375 11
5,796 36
128
2,614
10,516 77
256
5,034
20,220
114

16. Performance with Hydrogen Data Set Interpolation Effect
No Lighting 512 x 512
Times in miliseconds / frame
Samples
Point Sampled
Tri-Linear
S/W
H/W 32
426 6
777 64
740 11
1,375
128
1340 22
2,614
256
2545 42
5,034 36

17. Keys to performance gain
Data level parallelism (embarrassingly parallel)
Ray coherence (texture cache efficiency)
Specialized interpolation hardware (texture filtering)
Minimized bus traffic for data sets (large on-chip memory)

18. Hydrogen Data Set

19. Bonsai Data Set

20. BuckyBall Data Set

21. Head Data Set

22. Questions ?