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

2. Shading in Art Non-Linear Models Non-Linear Models Subtle Effects Subtle Effects Designed for Beauty Designed for Beauty

3. Goals Develop algorithms to map shading. Develop algorithms to map shading. Maintain global consistency. Maintain global consistency. Extract shading information from a Extract shading information from a complex scene. complex scene. Transfer that information to a new object Transfer that information to a new object or scene. or scene.

4. Background No NPR papers have tried to capture No NPR papers have tried to capture lighting effects from one medium and lighting effects from one medium and apply them to another. apply them to another. This has been done in the photorealistic This has been done in the photorealistic space for video augmentation. space for video augmentation. All boils down to reflectance. All boils down to reflectance. Determine amount of light reflected in Determine amount of light reflected in various directions various directions Sphere is a good object to use. Covers all Sphere is a good object to use. Covers all normals. normals.

5. “Paint By Normals” Use an object with similar material and Use an object with similar material and scale as sphere. scale as sphere. Lighting conditions will be similar! Lighting conditions will be similar! Replace sphere with surface in scene. Replace sphere with surface in scene. Light it according to sphere normals. Light it according to sphere normals. This is the Lit Sphere Model. This is the Lit Sphere Model.

6. Any Source! Spheres are too restrictive! Spheres are too restrictive! Extract spheres from any art source. Extract spheres from any art source. Locally spherical surfaces. Locally spherical surfaces. Approximate lighting by projecting Approximate lighting by projecting patches onto a lit sphere. patches onto a lit sphere.

7. Control  Spherical Triangles User has texture and lighting control. User has texture and lighting control. User defines distribution of normals. User defines distribution of normals. Midpoints defined for texture triangles. Midpoints defined for texture triangles. User can adjust midpoint. User can adjust midpoint. Midpoints imply a 4-1 subdivision. Midpoints imply a 4-1 subdivision. Subdivision is mapped to sphere. Subdivision is mapped to sphere. Result is spherical triangles. Result is spherical triangles.

8. Tweak & Apply Artifacts may exist along boundaries. Artifacts may exist along boundaries. Due to discontinuity between triangles. Due to discontinuity between triangles. Gaussian blur smoothes sphere map. Gaussian blur smoothes sphere map. Map is applied through environment Map is applied through environment mapping. mapping. Specialized SGI hardware enabled real- Specialized SGI hardware enabled real- time editing. time editing. Users direct lighting interactively. Users direct lighting interactively.

9. Strengths & Weaknesses User can work with multiple objects. User can work with multiple objects. Add, edit & del. spherical / planar triangles. Add, edit & del. spherical / planar triangles. User loads source images. User loads source images. User uses spheres to shade objects. User uses spheres to shade objects. User’s Power Limitations Destination geometry may self-shadow. Destination geometry may self-shadow. Additionally, objects may self-reflect. Additionally, objects may self-reflect. Proximity to light in source art may differ Proximity to light in source art may differ from final geometry. from final geometry. Lighting may be inconsistent for multiple Lighting may be inconsistent for multiple sources. sources.

10. Results Novices can achieve success. Novices can achieve success. Quality results darkening silhouettes. Quality results darkening silhouettes. Believable specular highlighting. Believable specular highlighting. Training on the system takes 5 minutes! Training on the system takes 5 minutes! Great results from novices in ten minutes. Great results from novices in ten minutes. View art from a different perspective! View art from a different perspective! Material effects can be simulated! Material effects can be simulated!

11. Results Continued

12. Future Work Animation: Animation: – Local surface features work for single view. – Reproject lit sphere according to viewpoint? For example if the doll were to move, the highlights would always follow the “center” of the legs. Local Texture Features: Local Texture Features: – When animated may appear to “swim”. Solution is to separate texture and lighting information! Band-Pass filtering. Band-Pass filtering. Virtual Phong subtraction. Virtual Phong subtraction.

13. Future Work Continued Implement shading in a 3D paint program. Implement shading in a 3D paint program. Automate the extraction of shading Automate the extraction of shading information from source art. information from source art.

14. Thank You! Jared Brown February 5 th, 2004