1. Stipple Placement using Distance in a Weighted Graph David Mould University of Saskatchewan

2. Stippling Sole primitive is a dot of ink [Wall Street Journal]

3. Automatic Stippling essentially halftoning: effort to match image intensity Weighted Voronoi diagrams often employed (introduced by Secord) [Secord 2002]

4. Importance of Edges [child's drawing] [Façade]

5. Areas and Lines Area-based methods inherently underemphasize linear features (edges) Paths travel across linear features, and the point of crossing can be detected Idea: multisource distance calculation (breadth-first search in graph) to find feature points

6. Graph Traversal Starting point Structure (union of all visited nodes) Frontier (all unvisited sites adjacent to at least one visited site)

7. Graph Traversal 4-connected isocontours path cost isocontours "edge" – high cost feature

8. Progressive Stippling Method

9. Algorithm Overview Construct graph Traverse graph –start at arbitrary point, distance zero –expand cheapest node –if cheapest node exceeds threshold: place stipple on frontier set cost of stipple location to zero –terminate when no nodes remain

10. Graph Construction Regular 4-connected lattice: one node per image pixel Weighted edges: a*I + b*G

11. Distance in Graph Distance between two nodes is cost of shortest path Cost of a path is the sum of all edge costs Dijkstra's algorithm: finds costs from single source to all other nodes –source: added to frontier with cost 0 –"expansion step": convert cheapest frontier node to known node, add neighbours to frontier

12. Stipple Placement Expansion step: choose cheapest node from frontier, update neighbour costs If expanded node cost exceeds threshold, trigger a stipple placement operation –choose stipple location (highest gradient location, in our implementation) –set cost to zero –add to frontier

13. Progressive Stipple Placement

14. Result

16. Blue Noise Distribution Blue noise: enforces minimum distance between any two samples Progressive algorithm: path cost between sites is at least the threshold (may be more) –at time of placing stipple, cost to frontier exceeds threshold –stipples never placed behind frontier

17. "handmade" distribution Voronoi Progressive

18. Edge Emphasis

19. progressive Secord -- halftoning

20. Progressive Secord – weight is half intensity, half gradient

21. Summary of Stippling Results High-quality stipple placement feature emphasis "handmade" stipple distribution "progressive" non-iterative algorithm have not investigated stipple size changes

23. Irregular Mosaics Regions naturally defined by distance+sites Regions irregular in shape, but tile boundaries conform to image edges Akin to historical "chip" and "pebble" mosaics

24. Mosaic Results