Hardware-Accelerated Parallel Non-Photorealistic Volume Rendering Eric B.Lum Kwan-Liu Ma University of California at Davis

Outline Volume Rendering Related Work Introduction Paletted Texture Detail –Tone Shading –Silhouette Illustration –Color Based on Position –Gradient Based Enhancement –Multiple Transfer Functions Results Conclusion

Volume Rendering Volume (a 3D image) –It is a 3D array of point samples, called voxels(volume elements) –Each voxel has color(RGBA) or density or… A 2D Image (Pixels) A 3D Image (Voxels)

Volume Acquisition Aqusition Techniques –CT, MRI, Ultra sound… –Physical simulation… –Converting 3D surface models…

Preliminaries Volume Rendering –The process of generating a 2D image from 3D volume

Preliminaries Volume Rendering –Visible Human

Preliminaries Volume Rendering –Computational Fluid Dynamics

Preliminaries Volume Rendering –Non-destructive testing

Volume Rendering Pipeline Simplified version Gradient Computation Classification Shading Compositing

Gradient Computation The gradient –A normal vector – : acquired value in data set Finding edges or boundaries between different materials Shading

Classification assign opacity to the voxels – assign color to the voxels –

Classification Face Brain Density Opacity Density Opacity

Shading Using gradient, assign color to the voxels Non -shaded Engine Shaded Engine

Accumulating –accumulate the hundreds of values sampled along a ray into one pixel –compositing function “over-operator” back-to-front front-to-back

Over Operator Fundamental operator in rendering –Binary operator on pairs of colors and pairs of opacity values (C, ) C b =(1,0,0) a b =1.0 C f =(0,1,0) a f =0.4

Ray Casting Basic Comlexity –Depth*ImageSize

Plane Compositing Divides the volume into slices. The slices are composes one over the other Basic Complexity = VolumeSize

Related Work NPR –A non-photorealistic lighting model… SIGGRAPH 98 Tone shading

Related Work VR [Direct volume rendering, ACM volvis. 96] NPR + VR [Volume illustration, IEEE vis. 2000]

Introduction Non-photo realistic rendering High performance volume rendering –Parallel processing –3D textures, Multi-texturing, Paletted textures…

Introduction Interactive Rendering –What to preprocess? Gradient Computation Classification Shading Compositing Density Normal Opacity Color Rendered Image

Introduction Preprocessed input (texture) –Density –Gradient Direction –Gradient Magnitude Output –Non-photo realistically rendered image

VR using paletted textures [Van Gelder, A., Hoffman] –ACM volvis. 96 Paletted textures –Indexed Color (at most 256 entry) Paletted TexturePalette Volume data (large) Transfer Function, Shading Function…

VR using paletted textures [Van Gelder, A., Hoffman] –ACM volvis. 96 Paletted textures –Indexed Color Paletted TexturePalette Volume data (large) Transfer Function, Shading Function… Density Opacity

VR using paletted textures [Van Gelder, A., Hoffman] –ACM volvis. 96 Paletted textures –Indexed Color Paletted TexturePalette Volume data (large) Transfer Function, Shading Function…

VR using paletted textures

Tone Shading Lighting through the variation of color temperature –Ambient lighting: cooler color –Direct lighting: warmer color [gooch98]

Tone Shading Using pre-computed table (palette)

Silhouette Illustration Effective in showing an objects structure Without silhouetteWith silhouette

Silhouette Illustration Silhouette Use palette – 시점이 바뀌면 다시 계산 N V Opacity,Color=Black

Silhouette Illustration For each slice polygon 1 st Pass: Tone shading 2 nd Pass: Blend silhouette term

Color Based on Position To improve depth perception –Warmer hues for foreground Without depth based tone manipulation With depth based tone manipulation

Color Based on Position Depth based tone manipulation –Assign different vertex color to each slice

Color Based on Position Arbitrary direction –Use additional 1D texture, and texture coordinate transform matrix

Gradient Based Enhancement The transition between features in a volume tend to have the highest gradient magnitude –Enhance the opacity in these regions Gradient magnitude Opacity Skin rendering

Multiple Transfer Functions For each slice polygon –1-pass for each transfer function

Combining Techniques Multi-pass, multi-stage rendering Classification Shading Surface enhancing Color based on position RGBA

Combining Techniques Multi-pass, multi-stage rendering Compute opacity again Silhouette & Specular lighting Compute again

A Complete Example

Parallel Rendering Binary-swap[13] Divide and conquer method –Subdivide and distribute the volume using k-d tree subdivision –Composite the rendered image of the subvolume

Results PC cluster with 9 computers Each –AMD Athlon 1.3 Ghz –1GB ram –Geforce3 with 64mb video memory –100Base-T fast ethernet 512*512*512 volume to 512*512 image –2.0 FPS Two sets of rendering parameter –4Path rendering –1.2 FPS Interactive control of –Transfer function, View direction, Rendering parameters

Conclusions Perceptually effective Interactive Future Work –Extracting more information –Rendering wider variety of styles