0. Acceleration Techniques for GPU-based Volume Rendering

1. Motivation Properties of real data sets Surface like structures
Surface illumination
Images courtesy of Joe Kniss and volren.org
Occlusions

2. Hardware accelerated rendering of totally invisible structures
Motivation
Texture based volume rendering today:
Hardware accelerated rendering of totally invisible structures
only 2% of all fragments visible

3. Our Contribution We use new GPU Features to:
Discard invisible fragments
early ray termination
empty space skipping
Implement a high quality GPU Raycaster
Accelerate the Raycaster

4. Our Toolkit Programmable graphics hardware
“Arbitrarily” programmable shaders
Assembly language
High level languages
32 Bit-Floating-point processing
Input: fp32 textures
Arithmetic: fp24-fp32
Output: fp32 render targets
Early Z-Test
Multiple dependent texture fetches

5. Our Toolkit Stream Architecture Rasterizer User Textures …
Geometry
Rasterizer
Fragments
User Textures …
Early Z-Test …
Fragment Shader …
Render Texture

6. Raycaster Slicing (SBVR) vs. GPU-Raycaster (RC)
SBVR  render proxy geometry (slices)
RC  render proxy geometry (faces)

7. Raycasting
Using only one main pass for Raymarching

8. Basic Idea Multi-pass approach Generate entry point
first hit in local texture coordinates
Compute ray direction
ray direction in local texture coordinates
Ray Marcher (simplified)
Main pass (Ray traversal)
Intermediate pass (Stopping criterion)

9. Entry Point Render into 2D texture
Front faces of the volume bounding box
Texture coordinates as color components
(0,1,0)
(1,1,1)
(0,0,1)
(1,0,0)
(1,0,1)

10. Ray Direction render into second texture
back faces of the bounding box
subtract from entry-point texture to get ray direction
(1,1,1)
(0,1,0)
(0,0,1)
(1,0,1)
(1,0,0)
subtract
(0,1,0)
(0,0,0)
(0,0,1)
(1,0,0)
(1,0,1)

11. Basic Idea Multi-pass approach Generate entry point
first hit in local texture coordinates
Compute ray direction
ray direction in local texture coordinates
Ray Marcher
Main passes (Ray traversal)
Intermediate pass (Stopping criterion)

12. Raycasting
Using multiple Raymarching passes

13. Ray Marcher (Main passes)
We know: Entry point and ray direction
Render front-faces
set Color like before C
set Texture coords to NDC (x,y)
activate direction texture DIR
a global counter t
compute ray as: r(t)=C+t*DIR(x,y)
float3 fDirection = tex2D(sDirection, v.TexCoords.rg).rgb;
float3 fFrontVal = v.Color.rgb;
float3 fVolCoord = fFrontVal+fDepth*fDirection;

14. Ray Marcher (Main passes)
Integration along the ray
use r(t) to access the volume
integrate over multiple steps (N)
combine with frame buffer contents
[..]
for (int i=0;i<N;i++) {
fResultVal = under(tex3D(sVolume, fVolCoord),fResultVal);
fVolCoord += fStepSize*fDirection; }
return fResultVal;

15. Raycasting Acceleration techniques Raycasting Early Ray Termination
Empty Space Skipping

16. Acceleration Techniques
Terminate Rays on one of the following conditions:
it has left the volume
it has reached a certain opacity
Suspend a ray if:
it‘s traversing an empty region
this needs to be done on a per-fragment basis
 early Z-Test

17. Acceleration techniques
Early-Z Restrictions:
on current GPUs early Z-Test only works if
no clip or texkill operation is executed
the Z-value is not changed in the shader
the raymarching shader can not terminate himself
Solution: use intermediate pass to do the ray-suspending/terminating

18. Acceleration techniques
Execute intermediate pass after every main pass (= N volume samples)
Access z-value only in this pass
If ray is terminated or suspended set z-value before main-pass-geometry
Reset depth behind it if ray is to be resumed

19. Ray Marcher (intermediate pass)
for early ray termination : check accumulated alpha
for empty space skipping : check “skip-volume”
Skip volume:
volume sampled down to 1/8 in every dimension
every sample contains the minimum and maximum of the 512 corresponding entries
Check Texture:
2D lookup texture
lookup(x,y)=1 iff all values between x and y are 0 under the current transfer function

20. Ray Marcher (intermediate pass)
for early ray termination : check accumulated alpha
for empty space skipping : check “skip-volume”
float4 fLastVal = tex2D(sLastFrame, v.TexCoords.rg);
float3 fDirection = tex2D(sDirection, v.TexCoords.rg).rgb;
float3 fFrontVal = v.Color.rgb;
float3 fVolCoord = fFrontVal+fDepth*fDirection;
float2 fMaxMin = tex3D(sEmptySpace, fVolCoord).rg;
float fEmpty = tex2D(sEmptyLookUp, fMaxMin);
OUT.depth = ((fLastVal.a < ) && (fEmpty<1)) ? 0 : 100;
OUT.col = fLastVal;
return OUT;

21. DEMOS Run on Intel Pentium IV 2.4GHz ATI Radeon 9800 Pro.
Microsoft Windows XP
DirectX 9
Pixel/Vertex Shader 2.0

22. Iso-Surface Raycasting
first 2 passes and intermediate passes remain unchanged
global traversal order still front to back
order within the main shader is back to front
keep only last isovalue
if isovalue was found compute illumination with 3D gradient texture and write it to the final image

23. Conclusion Using current GPU features we showed how to
Discard invisible fragments
early ray termination
empty space skipping
Implement a high quality GPU Raycaster
Accelerate that Raycaster

24. The End Thank you! Questions?
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