Acceleration Techniques for GPUbased Volume Rendering

1
Acceleration Techniques for GPU-based Volume
Rendering
2
Motivation

• Properties of real data sets

Surface like structures
Surface illumination
Images courtesy of Joe Kniss and volren.org
Occlusions
3
Motivation

• Texture based volume rendering today

Hardware accelerated renderingof totally
invisible structures
4
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

5
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

6
Our Toolkit

• Stream Architecture

Geometry
Rasterizer
Fragments
User Textures

Early Z-Test

Fragment Shader

Render Texture
7
Raycaster

• Slicing (SBVR) vs. GPU-Raycaster (RC)
• SBVR ? render proxy geometry (slices)
• RC ? render proxy geometry (faces)

8
Raycasting

• Using only one main pass for Raymarching

9
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)

10
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)
11
Ray Direction

• render into second texture
• back faces of the bounding box
• subtract from entry-point texture to get ray
  direction

(0,1,0)
(0,0,0)
(0,0,1)
(1,0,0)
(1,0,1)
12
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)

13
Raycasting

• Using multiple Raymarching passes

14
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)CtDIR(x,y)

float3 fDirection tex2D(sDirection,
v.TexCoords.rg).rgb float3 fFrontVal
v.Color.rgb float3 fVolCoord
fFrontValfDepthfDirection
15
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 i0iltNi) fResultVal
under(tex3D(sVolume, fVolCoord),fResultVal) fVol
Coord fStepSizefDirection return
fResultVal
16
Raycasting

• Acceleration techniques

Raycasting
Early Ray Termination
Empty Space Skipping
17
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
• its traversing an empty region
• this needs to be done on a per-fragment basis
• ? early Z-Test

18
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

19
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

20
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

21
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
fFrontValfDepthfDirection float2 fMaxMin
tex3D(sEmptySpace, fVolCoord).rg float
fEmpty tex2D(sEmptyLookUp, fMaxMin) OUT.depth
((fLastVal.a lt 0.9999) (fEmptylt1)) ? 0
100 OUT.col fLastVal return OUT
22
DEMOS

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

23
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 computeillumination with
  3D gradienttexture and write it to thefinal
  image

24
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

25
The End

• Thank you!
• Questions?
• Download Slides at http//wwwcg.in.tum.de/Researc
  h/Publications/Raycast/