- commonly used for years in the Visual Simulation field, a picture of the plant is applied to either 1 or 2 flat polygons
- the most common case is a 2-polygon "X" configuration (4 triangles)
- alternately, you can use a single square polygon (2 triangles) and use
code to make it turn toward the viewpoint
- for a technical explanation, see Billboards from SGI's Programming with OpenGL: Advanced Rendering
- a shadow on the ground under a tree helps a lot in "grounding" it -
making it appear to actually be on the ground, not floating above it
- shadow can be implemented as a black polygon with alpha = 0.5
- for more realistic shadows, see Creating Shadows (SGI again)
- creating or acquiring high-quality plant textures can be difficult
- the idea is to use the simple geometry of a billboard, but switch between one of several possible texture maps depending on the direction of view
- the images can be photographic, pre-rendered, or dynamically created on the fly from a full polygonal model
- this only works if the plant will not be viewed close up, from above, or below
-
3D
Pipeline TreeFX [http://www.3dpipelinesim.com/tree_fx.html seems to be
gone as of late 2005]- real-time engine bundled with a library of trees and ground cover
- claims "fully 3-dimensional branches" - which is actually a fuzzy blend between a set of directional billboard images
- apparently intended to license to game/vis-sim developers, and required a high-end card for the time (GeForce 2 Ultra)
- the tutorial lighting_flat_objects by Olivier Renouard discussed how to use Maya with custom plugins to pre-compute the normals for each texel of a billboard, allowing for dynamic lighting without the polygon count
-
Image-Based
Vegetation Rendering- demo and academic paper by Aleks Jakulin, University of Ljubljana, Slovenia
- paper: Interactive Vegetation Rendering with Slicing and Blending (pdf), Eurographics 2000
- going beyond a single billboard polygon, the "fuzzy" parts of the tree are rendered a series of "slices" which are stationary billboards, each containing part of the foliage to provide parallax
- this still only works if the plant will not be viewed close up, from above, or below
- an intermediate state between a 1-2 polygon billboard and the fully geometry of a plant
- requires a 3D artist to create each tree individually
- usually a tree will have a tubular trunk (e.g. 5-6 sides) and each branch will composed of very few polygons, often one billboard for a whole branch
- currently popular with MMORPGs (massively multiplayer online role-playing games)
- leverage an large amount of research in the area.
- provide realistic polygonal models based on real plant data.
- use stochastic models to provide variety among different instances of the same plant species.
- the most well-known researcher in plant generation research
- Dr. P belongs to the Biological Modeling and Visualization department at the University of Calgary
- developed by Aristid Lindenmayer in the late 1960's
- L-systems are rule-based productions based on Chomsky string replacing grammars where the productions are are applied in parallel. The grammar used in L-systems are typically interpreted as Turtle Graphics commands. The resulting images are fractal-like expanding on the concept that simple rules when recursively applied can create complex structures (ala Chaos/Complexity theory).
- systematically extended to provide greater flexibility and more accurate
plant models.
- Context-free vs. Context-sensitive L-systems to model signals in plants and handle more sophisticated branching structures.)
- Open L-systems to model external forces (light, gravity, resources, etc.).
- Stochastic L-systems to model different instances of a plant species by allowing controllably random application of productions.
- dL-systems are used to model growth through application of PODE's (partial ordinary differential equations) to productions.
- Spiral Phyllotaxis is used to model complex plant organs such as as seeds and flower petals that correspond to a regular arrangement of lateral organization (related to the packing problem). Examples: Sunflowers - Roses - Tulips.
- the challenge is finding L-systems that correspond to higher plants
- this is known as the inference problem
- there has even been one experiment in using an L-system in voxel space to model plants.
- many plant software tools e.g.. Xfrog
- see other systems such as Texture-Lobes on Plant Papers
