Graftal textures

1. Art-based Rendering with Continuous Levels of DetailLee Markosian, Barb Meier, Michael Kowalski, Loring Holden, J. D. Northrup, and John Hughes.

2. Graftal textures “Art-based Rendering of Fur, Grass and Trees,” by Kowalski, Markosian, Northrup, Bourdev, Barzel, Holden, and Hughes. Siggraph 99.

3. video

4. Problems • Coherence • excessive introduction/elimination of graftals • popping • Graftal textures defined in code • hard to edit • how to extend with UI?

5. A new framework • Drawing primitives • triangle strips (or fans) • Strokes • Graftals

6. Tufts A tuft is a hierarchical collection of graftals

7. Basic graftals • Collection of drawing primitives • Canonical vertices • Local coordinate frame • Affine map transforms canonical vertices to the local frame

8. y´ x´ local frame x canonical space The local frame • Base position (e.g. on surface) • y´(e.g. surface normal) • x´(e.g. cross product of y´ and view vector) M y

9. Placement and duplication • Designer creates a few “example graftals” • Duplicates of these are distributed over surfaces (“static” placement) • explicit distribution • procedural distribution • In duplication, graftal parameters can be varied randomly within specified range of values

11. Level of detail (LOD) • Each graftal computes a desired LOD • Then draws its primitives accordingly • each primitive has an associated threshold value • it’s drawn if the computed LOD exceeds the threshold

12. Computing LOD • Desired LOD is quantified by value   0 •  computed from 3 values: •  (depends on apparent size) •  (depends on orientation) •  (depends on elapsed time since graftal’s introduction)

13. is the ratio of the graftal’s current screen size to its “expected” screen size  = .7  = 1.4  = 1

14. Computing  •  lies in the range [0, 1] • We use  to suppress the final LOD value  in some regions • E.g.,  = 1 - |v · n|

15. Tufts Graftals in a tuft are grouped into levels level 2 level 1

16. Tufts, cont’d • Each level i has an associated value i • Graftals at level i are drawn if   i • Actually, we use hysterisis to choose the current active level • discourages level transitions

17. Computing  •  is used to smoothly introduce graftals when a given level becomes active • Each level has an associated “transition time,” e.g. 0.8 seconds • Say the level became active 0.6 seconds ago • Then  = 0.6 / 0.8 = 0.75

18. Using  •  can be used to animate or morph a graftal’s shape • we’ve done this by scaling and rotating graftals • It can also affect the computed LOD  • e.g.  = 

19. Demo: truffula scene

20. Demo: night scene

21. Conclusions • New framework provides more flexibility • range of graftal looks / behaviors • editing text files easier than writing code • Much better temporal coherence

22. Conclusions, cont’d • New approach is slower for complex scenes • night scene takes about 1 fps • work is expended on off-screen graftals • should use culling • Handling of LOD is too inflexible • levels have pre-assigned order

23. Future work • Generalize handling of LOD • UI for directly sketching graftals • UI for sketching other stroke-based textures by example • UI for sketching free-form shapes • continuing work on “skin” (Siggraph 99) • Integrating these into a single system