Use of Silhouette Edges and Ambient Occlusion in Particle

Visualization

James L. Bigler

School of ComputingAugust 16, 2004

Oral defense of

Outline

• Motivation and Introduction

• Ambient Occlusion Shading

• Silhouette Edges

• Conclusions and Future Work

Phong Shaded

With Silhouettes

With Ambient Occlusion

With Ambient Occlusion and SilhouettesSilhouettes

Why Particle Visualization?

Macro Micro Crop by value

Material Point MethodA B

C D

How are Particles Visualized?

Local Lighting Models

Good for local (micro) structure, bad for global (macro) structure.

Shadows

Global Illumination

• Variation in ambient regions

• Soft shadows

• Interreflection of light between surfaces

Acceleration Schemes

Ward et al.

Greger et al.

Wyman Global Illumination for Interactive Isosurfaces

• Wyman et al. cached global illumination values on a grid. Goal was to maintain interactivity during rendering.

Ambient Occlusion or Obscurances

• Zhukov et al. Iones et al.

• Precomputed

• Stored as textures

• Geometric property

Vicinity Shading

• James Stewart• Similar to Wyman, precomputes and stores

in a texture volume for later use in interactive applications.

Silhouette Edges

• Gooch et al. (“Interactive technical illustration) – OpenGL based method

(polygonal based)– Environment map (angle

between normal and eye)– Polygon by polygon software

method

• Object based methods not appropriate for particles

Silhouette Edges from Depth Buffer

• Usually black, emphasizes view dependent hull of objects

• Saito and Takahashi (“Comprehensible Rendering of 3-D Shapes”)– Cache various aspects of the

rendered image – Use depth and convolution

to find silhouette edges

Particle Ray Tracing

• Parker et al. show in “Interactive ray tracing” that large numbers of particles can interactively be rendered using a parallel ray tracer.

Outline

• Motivation and Introduction

• Ambient Occlusion Shading

• Silhouette Edges

• Conclusions and Future Work

Ambient Occlusion

Texture Mapping

• Common globe uv mapping

Texture Generation

cosine distribution

Texture Resolution

• 16x16 provides a nice compromise– Fidelity– Memory– Computation time

Dilation

How Does This Happen?

Linear interpolation is the culprit!

Inside Outside

How Is This Fixed?Inside Outside

Dilation based on value only would results in lightening all dark areas.

Inside or OutsideInside Outside

Only the “inside” texels should be changed. A way to determine if a texel is “inside” is needed.

ba

Inside Texel Detection

Proper DilationInside Outside

Now only “inside” texels are dilated.

Dilation Performed

Dilation Performed

Render Phase• Texture and sphere data loaded in

• Sphere ID used to lookup corresponding texture

• Removing textures seams

Precomputation Time and Memory

• Using 20 R14K processors on an SGI Origin 3800 (muse.sci.utah.edu). Textures were 16x16 with 49 samples per texel.

955,000

66 min.

233 MB

952,755

261 min.

232 MB

543,088

33 min.

132 MB

7,157,720

12 hours

1,747 MB

Fireball Bullet Foam

Impact on Performance• 10% slower than direct lighting alone.

• However, using only the ambient occlusion values can yield as good as or better performance than direct lighting alone.

Direct lighting

DL with Textures

Textures w/o DL

Fireball 6 16.43 f/s 14.97 f/s 16.75 f/s

Fireball 11 10.55 f/s 9.59 f/s 10.16 f/s

Cylinder 6 13.32 f/s 12.15 f/s 13.37 f/s

Cylinder 22

11.71 f/s 10.94 f/s 11.75 f/s

Bullet 2 28.17 f/s 25.59 f/s 28.79 f/s

Bullet 12 28.76 f/s 25.71 f/s 28.41 f/s

ImagesDirect Lighting

onlyDirect lighting with ambient occlusion

textures

Ambient occlusion textures only

Cylinder 22 Bullet 6 Fireball 11

Impact on Performance• 10% slower than direct lighting alone.

• However, using only the ambient occlusion values can yield as good as or better performance than direct lighting alone.

Direct lighting

DL with Textures

Textures w/o DL

Fireball 6 16.43 f/s 14.97 f/s 16.75 f/s

Fireball 11 10.55 f/s 9.59 f/s 10.16 f/s

Cylinder 6 13.32 f/s 12.15 f/s 13.37 f/s

Cylinder 22

11.71 f/s 10.94 f/s 11.75 f/s

Bullet 2 28.17 f/s 25.59 f/s 28.79 f/s

Bullet 12 28.76 f/s 25.71 f/s 28.41 f/s

Results

• Movie 1 (show off some data sets)

• Movie 2 (use with direct lighting and shadows)

Outline

• Motivation and Introduction

• Ambient Occlusion Shading

• Silhouette Edges

• Conclusions and Future Work

Silhouette Edges

• Two options– Precomputation

(object based)– Run time

• Object based

• Image based

Ingredients for Edges

• Image buffer • Depth buffer• Edge detection kernel• Threshold for zero

crossings

-1 -1 -1

-1 8 -1

-1 -1 -1Laplacian kernel

Threshold Edge Response

• Anatomy of a ray

• If a and |b| are the same for each pixel we can use the collection of t as a depth buffer.

Depth Buffer

p(t) = a + tb

t

Movies

• Movie 1 (Varying the threshold and changing the view point and field of view)

• Movie 2 (Time varying data)

Performance

Without With

A 17.064 f/s 16.056 f/s

B 2.220 f/s 2.179 f/s

C 2.220 f/s 2.197 f/s

D 1.155 f/s 1.162 f/s

E 2.683 f/s 2.632 f/s

A

B C

D E

Outline

• Motivation and Introduction

• Ambient Occlusion Shading

• Silhouette Edges

• Conclusions and Future Work

Ambient Occlusion

• Shows macroscopic structure well

• Renderings are still interactive

• Precomputation time is reasonable, but still expensive.

• Issues with Time-Dependent Visualization

Future Work for Ambient Occlusion

• Compress the textures to save memory• Reduce the texture generation time

– Smaller textures

– Better acceleration structures for ray intersections

– Look for occlusions in a predefined radius, rather than the whole volume

– View dependent texture generation

• Update textures during cropping

Silhouette Edges

• No precomputation time required

• Image based method developed has little impact on rendering time

• Intuitive user control for selection of how many silhouettes to view

• Improved visualization of structure

Future Work for Silhouette Edges

• Edges of silhouettes are aliased.

• Gray levels or varying thickness to indicate degrees of discontinuities in depth.

• How to appropriately apply silhouette edges to multi-sampled renderings.

Thanks

• Laura

• My family

• My committee: Chuck, Steve, & Pete

• The folks at SCI

• DOE, C-SAFE

• Friends along the way

Questions?