1. Interactive High-Quality Volume Rendering on Flexible Consumer Graphics Hardware Klaus Engel, Martin Kraus, Thomas Ertl Visualization and Interactive Systems Group University of Stuttgart, Germany Graphiktag 2001 - Tübingen

2. Visualization and Interactive Systems Group,University of Stuttgart Volume Data Sources Measurements (CT,MRI) Synthetic data (radial distance volume, + Perlin Noise) Simulations (convection flow)

3. Visualization and Interactive Systems Group,University of Stuttgart Volume Rendering - Physical Model Physics of light transport Simplified to Volume Rendering Integral Transfer Function: Assigns opacity and color Discretization

4. Visualization and Interactive Systems Group,University of Stuttgart Volume Rendering - Methods indirect methods isosurface reconstruction: huge amount of polygons direct methods ray-casting: huge amount of trilinear interpolations

5. Visualization and Interactive Systems Group,University of Stuttgart Texture-based Volume Rendering 2D textures (axis-aligned slices) 3D textures (view-aligned Slices) texturing (trilinear interpolation) texturing (trilinear interpolation) compositing (blending) compositing (blending) texturing (bilinear interpolation) texturing (bilinear interpolation) compositing (blending) compositing (blending)

6. Visualization and Interactive Systems Group,University of Stuttgart Problems - Axis Aligned Slices Missing trilinearly interpolated slices visual artifacts due to fixed number of slices higher number of slices Undersampling Artifacts

7. Visualization and Interactive Systems Group,University of Stuttgart Trilinear Interpolation using 2D Multi-Textures Idea: Compute intermediate slices SiSi S i+1 bilinear interpolation by texture environment S i+α S i+α = (1-α) S i + α S i+1 blending of two textures Real trilinear interpolation Efficient implementation using multitextures

8. Visualization and Interactive Systems Group,University of Stuttgart Flexible PC Graphics Hardware – Multi-Textures  (modulate) = lightmaps only decal only combined scene Light maps in Quake2

9. Visualization and Interactive Systems Group,University of Stuttgart Flexible PC Graphics Hardware – Register Combiners rasterization primitive assembly texture fetching texture environment application color sum fog coverage application texture unit 0 texture unit 1 register combiners final cmb general cmb1 general cmb0

10. Visualization and Interactive Systems Group,University of Stuttgart Flexible PC Graphics Hardware – Result Cryoelectron-microscopic Volume Isosurface of Escherichia Coli Ribosome at 18 Ångström All data slices 10 times more slices

11. Visualization and Interactive Systems Group,University of Stuttgart Higher Sampling Rates – Problem Discrete Approximation of Volume Rendering Integral will converge to correct result for d  –According to Sampling Theorem sampling rate must be greater than the Nyquist frequency –But: High frequencies in the Transfer Function may considerably increase the required sampling rate Pre-Integrated Volume Rendering –Idea: Split numerical integration into one pre-integration for the transfer function one integration for the scalar field –Pre-Integrate Ray-Segments in a pre-processing step

12. Visualization and Interactive Systems Group,University of Stuttgart Volume Rendering - Classification voxels Post- classification interpolation Pre- classification transfer functions classification

13. Visualization and Interactive Systems Group,University of Stuttgart Pre-Integrated Volume Rendering slice-by-slice slab-by-slab sbsb sfsf sfsf sbsb fetch integral from dependent texture sbsb sfsf pre-integrate all possible combinations hardware-accelerated implementation on NVidia GeForce3 chip project slice sfsf sbsb front slice back slice texture polygon

14. Visualization and Interactive Systems Group,University of Stuttgart Flexible Rasterization Hardware - Texture Shaders rasterization primitive assembly texture fetching texture environment color sumfog coverage application texture unit 0 texture unit 1 register combiners final cmb general cmb1 general cmb0 texture shaders

15. Visualization and Interactive Systems Group,University of Stuttgart Flexible Rasterization Hardware - Texture Shaders Texture Shaders (when enabled) replace the standard OpenGL texture fetch mechanism ARB_multitexture with NV_texture_shader ARB_multitexture only (s,t,r,q) 0 (s,t,r,q) 1 (s,t,r,q) 2 (s,t,r,q) 0 (s,t,r,q) 1 (s,t,r,q) 2 stage 0 math & fetch stage 1 math & fetch stage 2 math & fetch unit 0 fetch unit 1 fetch unit 2 fetch With texture shaders, results from previous stages can affect the lookup of a subsequent stage

16. Visualization and Interactive Systems Group,University of Stuttgart Pre-Integrated Volume Rendering RGB 1 (s 2,t 2,r 2 )  (1,0,0) stage 2 DOT_PRODUCT_NV (1,0,0) RGB 1 =s b stage 2 DOT_PRODUCT_NV (1,0,0) RGB 1 =s b RGB 0 (s 3,t 3,r 3 )  (1,0,0) stage 3 DOT_PRODUCT_ TEXTURE2D_NV (1,0,0) RGB 0 =s f stage 3 DOT_PRODUCT_ TEXTURE2D_NV (1,0,0) RGB 0 =s f RGBA result on to register combiners front slice front slice back slice back slice sbsb sbsb stage 1 TEXTURE_2D stage 1 TEXTURE_2D (s 1,t 1 ) RGBA result sfsf sfsf stage 0 TEXTURE_2D stage 0 TEXTURE_2D (s 0,t 0 ) RGBA result sbsb sfsf

17. Visualization and Interactive Systems Group,University of Stuttgart Pre-Integrated Volume Rendering - Isosurfaces Isosurfaces: particular dependent texture 3296 sfsf sbsb 32 96 front slice back slice 1. 1 front slice back slice 4. 4 front slice back slice 2. 2 front slice back slice 3. 3 3a

18. Visualization and Interactive Systems Group,University of Stuttgart Results – Direct Volume Rendering 128 slices pre- classification 128 slices pre-integrated 284 slices post- classification 128 slices post- classification

19. Visualization and Interactive Systems Group,University of Stuttgart Results - Direct Volume Rendering – Random TF

20. Visualization and Interactive Systems Group,University of Stuttgart Results - Isosurfaces

21. Visualization and Interactive Systems Group,University of Stuttgart Results - Tooth data set

22. Visualization and Interactive Systems Group,University of Stuttgart Conclusions Interactive High-Quality Volume Rendering on Consumer Graphics Hardware –Higher Sampling Rates  Trilinear Interpolation using 2D Textures  But: high rasterization requirements –Better: Pre-Integration of Ray-Segments –Pre-Integration as general as post- and pre- classification already applied to cell projection (Roettger et al., Vis‘00) can be applied to ray-casting as well

23. Visualization and Interactive Systems Group,University of Stuttgart Pre-Integrated Volume Rendering Demo Demo and Data Download: http://wwwvis.informatik.uni-stuttgart.de/~engel/pre-integrated/