Section: Scientific Foundations

Computer Graphics

The computer graphics in Digiplante inherits from the long tradition of AMAP works [54] , and is mostly conducted since 2004 in close partnership with the GreenLab team at LIAMA (Dr. ZHANG XiaoPeng and his students).

Simple plant LOD models and real time plant rendering

Principles and main research axis hold in the frame of the project :

Plant simulated visualization in DigiPlante project is still based on the AMAP prinicples [54] , characterized by a high level, device independant simulation output, called the line-tree. Geometrical ouput is a final process in the simulation stage, that can be processed or not. The Linetree structure describes a plant by a list of organs, refered by basic indetification information (organ type, age) and organ positioning in space 3D origin relative to seed position, and 3D orientation. Orientations (main direction, normal and secondary) are scalled, story thus internode diameter/length and other organ sizes. These geometrical informations are stored in a 3 by 4 mattrix, similar to a classical OpenGl Mattrix Model Viewer. This data structure shows several interests and drawbacks:

• The Line tree structure does not store an exhaustive geometry, but just the list of each plant componant with its orientation

• The structure is flat, no hierarchy or topology information is given. However, extension were developped, using a separated structure storing the componant father-son relations. From these, pruning operatin are available as well as branch per branch output or other hierarchical tree based data exploring schemes.

• The structure cannot be displayed directly; each componant must be given an exhaustive geometrical description, build from instance from CAD-CAM tools. On the other side, the tree skeletton is obvious to be extracted and displayed; each componant can be simply represented by a simple line linking its origin to the end point of its main direction (which is scalled). This approach allows thus a wide range of representations, open to many graphical systems. It can be compared to a classical Display List, in which primitive objects are to be defined according to the viewing system for each kind of organ (usually a simple cilinder for branch, and specific shapes for the other organs). Organ representation must be normalized; i.e. each organ must be defined using the same main axis and normal axis and normalized in size: unit volume or length for branch/fruits, unit area for leaves.

• Graphical rendering aspects, including illumination properties, textures are thus independant for the line tree, since define at the organ geometry descrition level.

• The structure allows Level of Details approaches and simplifications. According to componant sizes (length / diameter ) selection can be done in order to extract componants of interest (trunk and main axis). Coupled to statistical selection of leaves/flower, fruits, lower graphical model can be generated, without losing the global sahpe of the simulated plant. On branches, successive componants can be concataneted, remplacing list of internodes by a straith branch piece.

• The Line tree structure has shown a large interest for easy visualisation of complex trees on a wide range of systems, but is poorly adapted to advanced LOD models (such as hybrid polygonal/billboards models)and fast huge scene representation.

Scenes such as crops, forest, up to landscape visualisation was, up to now, mainly developped on a plant basis (not on a landscape use basis, as it is more and more the case in commercial products); and also without advanced dynamics, especially without plant/environment dynamics.

In the frame of the digiplante project, computer graphics development axis aims at developing new methods and approaches aiming to visualize large vegetation scenes in acceptable time (close to real time), in respect to plant structure and functioning. Prior is thus given to approaches keeping biological consistency (for instense leaf area) to appraoches keeping aesthetic aspects. Research axis do concern single plant LOD model definition, and new landscape simulation/visualisation approaches.

Single Plant LOD Models. :

Concerning single plant representation. In cooperation with LIAMA, we developped a set of algorithms dedicated to generate continuous LOD geometrical model from a Line tree and its associated hierarchical topology. The principles of the approach are the following:

• The tree structure (trunk and branch) is converted to a Frenet Diagram. This diagram can be compressed, merging consecutive elements together, under control (error controlled). A branch part of the diagramm is then described by an equation. At the rendering stage, according to the viewing parameter, the appropriate Diagram is retrieved, and the discretisation of branch in polygons is performed, also according to viewing parameters. If branch diameter gets lower than a pixel, transparent lines are used for rendering.

• For foliage, the proposed approach is to merge leaves two by two until reaching a simple quad for the whole plant crown [76] . Various methods were explored to obtain an efficient merging process. The main idea is to collapse leaf candidates within specific clusters, faithful to botanical hierarchical structures. Hierarchical Leaf Union keeps more leaf surface area, keeping functional consistency of the DigiPlant model. In the various collapsing models, principles of the LOD schemes are unchanged, while preprocessing stages are revisited, making the approach operative for huge trees and heavy forest scenes .

• in order to avoid multiple exchanges between GPU and CPU, scenes are rendered according to tree species and individuals (to optimise instanciations). The full foliage lod models are stored in a simple table, compressed and send to the GPU. wWile rendering, CPU computes the adequate LOD model, sending then the corresponding indexes of the table to the GPU. GPU draws then all items lying in the table between these indexes.

Specific LOD shemes were also defined for coniferes, replacing graphical primitives (cylinders) by lines, and defining line set replacement patterns by simple lines on far trees.

An extensive summary paper covering many aspects developped in the past four years has been published by Wesley in Computer Anination and Virtual Words journal CAVW08