This document proposes a method for drawing in 3D using view-dependent silhouette strokes that can represent uncertainty and occlusion. It discusses previous work on 3D drawing that was limited to specific interfaces or primitives. The proposed approach models silhouette strokes as 3D curves with associated surfaces, and renders them from different viewpoints while managing occlusion. An interface allows drawing lines and silhouettes in empty space or on objects. Applications include illustration, annotation, and guided 3D design. Future work includes handling tubular objects and real-world testing in teaching anatomy.

1. Drawing for Illustration and Annotation in 3D David Bourguignon*, Marie-Paule Cani* and George Drettakis** *i MAGIS, INRIA Rhône-Alpes, France **REVES, INRIA Sophia-Antipolis, France

2. Motivation Drawing Scene complexity in an effective manner Indication of uncertainty Limitation to single viewpoint Applications Fine arts Teaching Early stages of design Human heart

3. Motivation Goal: drawing in 3D Augment strokes to true 3D entities Free-form drawing Annotation of existing 3D models Many viewpoints Problems to be solved Silhouette stroke modeling Silhouette stroke rendering from different viewpoints Eye Back

4. Overview Previous work Contributions Silhouette stroke modeling Silhouette stroke rendering Interface for drawing Results

5. Previous Work [Akeo et al., 1994] [Pugh, 1992] 2D drawing is converted to 3D Specify hidden parts by hand

6. Previous Work [Lipson and Shpitalni, 1996] [Eggli et al., 1997] No free-form drawing Limited number of primitives

7. Previous Work [Cohen et al., 1999] [Tolba et al., 1999] 3D curves design, no drawing 2D drawings reprojected, no visibility changes

8. Previous Work [Igarashi et al., 1999] [Zeleznik et al., 1996] Limited to a given gestural interface Closed strokes only

9. Previous Work [Cohen et al., 2000] Drawing modes adapted to landscaping only, no 3D model reconstruction in billboard mode

10. Overview Previous work Contributions Silhouette stroke modeling Silhouette stroke rendering Interface for drawing Results

11. Our Approach Strokes 2D drawing on the screen plane Line stroke for 1D details Silhouette stroke for view-dependent information Problems to be solved Inferring local surface from silhouette Representing uncertainty Managing occlusions Silhouette stroke Line stroke (3D spline)

12. Overview Previous work Contributions Silhouette stroke modeling Silhouette stroke rendering Interface for drawing Results

13. Silhouette Stroke – Modeling Infer local surface Fit Bézier curve to 2D user input Evaluate local curvature Process curvature vectors Clamp relatively to inflexion points Set consistent in/out orientation

14. Silhouette Stroke – Modeling Infer local surface Fit Bézier curve to 2D user input Evaluate local curvature Process curvature vectors Clamp relatively to inflexion points Set consistent in/out orientation

15. Silhouette Stroke – Modeling Infer local surface Fit Bézier curve to 2D user input Evaluate local curvature Process curvature vectors Clamp relatively to inflexion points Set consistent in/out orientation

16. Silhouette Stroke – Modeling Infer local surface 3D circles from vectors of curvature Local Bézier surface

17. Overview Previous work Contributions Silhouette stroke modeling Silhouette stroke rendering Interface for drawing Results

18. Silhouette Stroke – Rendering New silhouette from new viewpoint Efficient silhouette approximation using clipping planes Good for local surfaces associated with strokes

19. Silhouette Stroke – Rendering Represent uncertainty Use stroke texture Color stroke as desired Front (original view) Side (30°) Side (90°) Stroke alpha texture

20. Silhouette Stroke – Rendering Manage occlusion Use occluder texture for soft occlusion Various drawing styles Occluder alpha texture

21. Silhouette Stroke – Rendering Multipass algorithm (Previously render scene and line strokes) First pass: Rendering silhouette strokes With stroke texture With stroke color Second pass: Soft occlusion by local surfaces (set depth) With occluder texture Third pass: Soft occlusion by local surfaces (achieve blend) With occluder texture With occluder color

22. Overview Previous work Contributions Silhouette stroke modeling Silhouette stroke rendering Interface for drawing Results

23. Interface for Drawing Two types of strokes Line stroke Silhouette stroke Two drawing modes In empty space Relatively to other objects On one object Between two objects

24. Overview Previous work Contributions Silhouette stroke modeling Silhouette stroke rendering Interface for drawing Results

25. Applications Illustration in 3D

26. Applications Annotation of a 3D scene

27. Applications "Guided design"

28. Video

29. Conclusion System for drawing in 3D View-dependent strokes with occlusion Useful for drawing simple scenes in 3D Useful for annotations Future work Handling tubular objects Real world test: using it to teach anatomy

30. Acknowledgements Eric Ferley for feedback throughout the project Laurence Boissieux for creating some of the drawings, Marc Pont for help with models Frédo Durand for advice on the paper iMAGIS is a joint project of CNRS, INPG, INRIA and UJF