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Art-based Rendering with Continuous Levels of Detail Lee Markosian, Barb Meier, Michael Kowalski, Loring Holden, J. D. Northrup, and John Hughes. Graftal textures. “Art-based Rendering of Fur, Grass and Trees,”. by Kowalski, Markosian, Northrup, Bourdev, Barzel, Holden, and Hughes.

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


Graftal textures
Graftal textures

“Art-based Rendering of Fur, Grass and Trees,”

by Kowalski, Markosian, Northrup, Bourdev, Barzel, Holden, and Hughes.

Siggraph 99.



Problems
Problems

  • Coherence

    • excessive introduction/elimination of graftals

    • popping

  • Graftal textures defined in code

    • hard to edit

    • how to extend with UI?


A new framework
A new framework

  • Drawing primitives

    • triangle strips (or fans)

    • Strokes

  • Graftals


Tufts
Tufts

A tuft is a hierarchical collection of graftals


Basic graftals
Basic graftals

  • Collection of drawing primitives

  • Canonical vertices

  • Local coordinate frame

    • Affine map transforms canonical vertices to the local frame


The local frame

y´

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


Placement and duplication
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


Level of detail lod
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


Computing lod
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)


is the ratio of the graftal’s current screen size to its “expected” screen size

 = .7

 = 1.4

 = 1


Computing
Computing

  •  lies in the range [0, 1]

  • We use  to suppress the final LOD value  in some regions

  • E.g.,  = 1 - |v · n|


Tufts1
Tufts

Graftals in a tuft are grouped into levels

level 2

level 1


Tufts cont d
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


Computing1
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


Using
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.  = 




Conclusions
Conclusions

  • New framework provides more flexibility

    • range of graftal looks / behaviors

    • editing text files easier than writing code

  • Much better temporal coherence


Conclusions cont d
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


Future work
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


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