1. Computer-Generated Pen-and-Ink Illustration of Trees Oliver Deussen Thomas Strothotte University of Magdeburg

2. Why pen, ink, and trees Drawings convey visual information Illustrations in books Architecture and landscaping

3. Main ideas Using existing models Drawing the foliage Large data Abstract representation Drawing styles

4. Related work Leaves Alvy Smith Fractals Small disks Reeves and Blau Particle system to represent leaves Sasada Tree sketch in architecture environments View facing polygons Strokes Simulate brush Tone and texture

5. Interactive Pen and Ink Illustration 2D drawing Tone and texture High level cumulative effect Randomness to the strokes Stored stroke textures

7. Traditional methods Trunk Silhouette Shading Leaves Primitive shape Foliage Well lit Half shadow Deep shadow

8. Image from xfrog

9. Input model xfrog system Tree skeleton 2 levels of branches Foliage Particles vs. polygons 183,000 to 8,800

10. Tree skeleton Silhouette Mokosian Rakar Shading Floyd Steinberg method Difference image algorithm Stroke area and direction

11. Foliage First attempt Merging shapes into outlines Abstractions Depth difference Saito and Takahashi Enhance outlines

12. Depth differences Zero order difference determines outlines z-buffer Difference with neighbor Threshold

13. Using z-buffer Parameters: Depth value from the camera: z0, z1 z-buffer value: d0=0, d1=65535 Threshold: 10% of the depth of the tree

14. Disk size and threshold a) size: 0.15, threshold: 1000b) size: 0.7, threshold: 2000

15. Abstract primitives 3D impression

16. Level of abstraction Using z-buffer value Primitive size

17. Software framework Interactive vs. high quality Software shadows Vectoriation of primitive polygons

18. Results: Interactive: 3 trees of 20,000 particles each and 25,000 ground particles at 3 fps. 13,200 elliptic primitives - 10 seconds (SGI Octane)

19. Results: 200,000 leaves reduce to 16,200 particles 90,000 tree particles, 23,000 grass particles - 1 minute

21. Conclusion Pro: Using existing models Primitive shape Level of abstraction Hand drawn style Con: Interactive speed Dealing with large models

22. Extensions Trees only? Depth difference extends to other models.