The Lit Sphere:

A Model for Capturing NPR Shading from Art

Peter-Pike J. Sloan

William Martin

Amy Gooch

Bruce Gooch

Motivation

• Interactive shading of 3D characters

• Artists shading study on the sphere

Motivation

• Acquire reflectance from real world– Use to illuminate and

place synthetic objects in scene

Previous Work

• NPR– Lake2000 – Cartoon Shading– Gooch98 – Technical Illustration– Praun2001 – Real time hatching

• Acquiring Shading Models– Environment Maps

• Blinn76,Miller84,Green86

– More general models• Yu98, Yu99, Sato99

Simple Idea

• “Paint by normals”– Surface parameterized by normal field– Store shading model in a environment

mapped indexed by surface normal (not reflection vector)

Simple Examples

Simple Demo

Creating the Environment Map

• Could just paint it– requires artistic ability

Extracting EMap

Creating the Environment Map

• “Extract” from source artwork– Map triangles from

source onto sphere– Simple control over

parameterization

Creating The Environment Map

• Fill in gaps– Render at multiple

scales– Blend into final image

• (1-dest,1)

– Could solve for colors by minimizing some high pass filter (ala Hakura99 and McCool01)

– Texture synthesis

Simple Demo 2

More Examples

Issues

• Texture “crawls” when view changes• Texture as a function of curvature – seems odd

but works better then one would expect, flat regions have no variation

• Could attach texture to the surface, or at least separate it from shading– Probably want texture anchored on the surface, but

able to respond to shading changes – kind of like Praun2001 but not just for hatching

Simple Hacks

• Have separate “texture sphere” that rotates with the object– Really should just live on the surface, bad

parameterization for something very coherent– Just use cube maps

• Modulate final results with “canvas” image– Kind of like detail textures, sometimes can

make the “swimming” less objectionable

Conclusions

• Simple way to interactively shade objects based on source artwork

• Works ok if you aren’t trying to transfer something that has strong texture

• Probably should factor the model into more then 1 term…

Acknowledgments

• University of Utah Computer Graphics groups

• Louise Bell, Grue, Susan Ashurst

• Michael Cohen, Adam Finkelstein, James Mahoney, Wolfgang Heidrich

• DARPA, NSF STC for CG&SV