1. A Model for Capturing NPR Shading from Art
The Lit Sphere:
A Model for Capturing NPR Shading from Art
Peter-Pike J. Sloan
William Martin
Amy Gooch
Bruce Gooch

2. Motivation Interactive shading of 3D characters
Artists shading study on the sphere
The original goal in this project was to shade 3D models of character in a style reminiscent of “cheesecake” photos – a NPR shading model for skin.
Through several iterations it turned into something else, a simple way to shade a model using environment maps created from scanned in source artwork.
There are two separate motivations/justifications for this model. One, this is somewhat similar to how artist sometimes start a picture by designing a shady study on a sphere to guide them as they are creating the image. The other is the notion of a “light probe” from the literature, where a sphere with a chosen material is placed in a real world scene – this can then be used to either place synthetic objects with similar materials in the scene, or to illuminate synthetic objects with light that is similar to that of the scene being filmed. We essentially want to place a light probe in a painting.

3. Motivation Acquire reflectance from real world
Use to illuminate and place synthetic objects in scene

4. Previous Work NPR Acquiring Shading Models Lake2000 – Cartoon Shading
Gooch98 – Technical Illustration
Praun2001 – Real time hatching
Acquiring Shading Models
Environment Maps
Blinn76,Miller84,Green86
More general models
Yu98, Yu99, Sato99
NPR
Generally focuses on specific style. Prauns work is also interactive and presents a novel way to represent texture as a function of lighting.
Acquiring the “lighting environment” of a scene, or material parameters from objects has been done extensively in the past – for photorealistic techniques.

5. Simple Idea “Paint by normals” Surface parameterized by normal field
Store shading model in a environment mapped indexed by surface normal (not reflection vector)
The basic idea is very simple – we are just going to “Paint by normals”, given any surface with associated normals, we are going to simply texture the surface by looking the normal up in an specially created environment map. This clearly means all shading is purely a function of orientation – there is no notion of variations of material properties over a surface except those induced by the change in surface normal.

6. Simple Examples

7. Simple Demo

8. Creating the Environment Map
Could just paint it
requires artistic ability

9. Extracting EMap

10. Creating the Environment Map
“Extract” from source artwork
Map triangles from source onto sphere
Simple control over parameterization

11. 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

12. Simple Demo 2

13. More Examples
Demo

14. 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

15. 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

16. 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…

17. 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