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. Siggraph 99.

video

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

A new framework Drawing primitives –triangle strips (or fans) –Strokes Graftals

Tufts A tuft is a hierarchical collection of graftals

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

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

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) 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 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  = 1  =.7  = 1.4

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

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

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

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   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. = 

Demo: truffula scene

Demo: night scene

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 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 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