Rapid Construction of Virtual Worlds Quarterly Report #1 SRI Project ECU-7058

Rapid Construction of Virtual Worlds Quarterly Report #1
SRI Project ECU-7058 August 1995
Pascal V. Fua Lynn H. Quam Arti cial Intelligence Center SRI International 333 Ravenswood Avenue Menlo Park, California 94025 Prepared for: U.S. Army Topographic Engineering Center ATTN: CETEC-RI-T Fort Belvoir, Virginia 22060-5546
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Contents
1 Introduction 2 Terrain Modeling: Integration into RCDE of Existing Software 3 Consistent Virtual Worlds: Constrained Optimization 4 Interoperability: Loading ITD data 5 Summary 3 3 4 4 6
These objects classes constitute the building blocks that will be used to optimize terrain, roads, buildings and other features. Both of these tasks are complete and the system's capabilities will be demonstrated at the SE #2 meeting.
2 Terrain Modeling: Integration into RCDE of Existing Software
The goal of this subtask is to create an integrated environment in which we can both develop our algorithms and demonstrate their performance. We have begun by incorporating our existing surface reconstruction method into RCDE. This method uses 3{D triangulations not only as a representational tool but also as a computational one. As a result, terrain and 3{D features can be computed using similar representations that will interact naturally when we endeavor to impose consistency constraints. Our technique is described in more details in the appendix and we refer the interested reader to our earlier publications 6, 7]. This has involved Implementing data structures that allows our software to access RCDE images. Developing a simple user interface that allows the user to select the portion of the DEM to be re ned. We have also reorganized the \snake" package developed under the RADIUS BAA for use in this project 5]. Our goal is to develop uniform methods to optimize both linear features and terrain features so that they can be re ned simultaneously while enforcing consistency constraints. For this purpose. we have introduced three new generic object classes.

3 Consistent Virtual Worlds: Constrained Optimization
To be able to express the consistency constraints in a computationally e ective way, we have begun developing a technique to impose generic hard constraints on \snake-like" deformable models while both preserving the good convergence properties of snakes and avoiding having to solve large and ill-conditioned linear systems of equations. A traditional way to enforce such constraints is to add a penalty term to the model's energy function for each new constraint. However, it is well known that minimizing an objective function that includes such penalty terms constitutes an ill-behaved optimization problem with poor convergence properties 9]: In essence the optimizer is likely to minimize the constraint terms while ignoring the remaining terms of the objective function. In addition one then has to face the di cult problem of adequately weighing the various constraints. Constrained optimization is a much better approach to enforcing such constraints. However, while there are powerful techniques for constrained optimization, few are tailored to preserve the convergence properties of snake-like approaches to feature delineation and surface modeling. Our method relies on projecting the current state onto the constraint surface and then taking an optimization step in a direction that is tangent to the constraint surface and is an extension of an algorithm rst proposed in 2, 3]. This can be achieved by solving mxm linear systems|where m is the number of constraints and|that are well behaved as long as the constraints are independent. We have tested this method to enforce constraints on 2{D snakes as shown in Figure 1 and will expand its scope to allow the simultaneous optimization of terrain, roads and rivers under consistency constraints.
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1 Introduction
As additional and more accurate sensors become available, high-resolution cartographic modeling from mapping, reconnaissance, and earth-resource resources imagery becomes technically feasible, but at the cost of an enormous amount of manual labor. As applications for virtual world models grow increasingly demanding and more numerous, the need for higher degrees of automation in model construction becomes critical. Today's technology for model construction fails to enforce domain constraints and requires too much manual interaction to produce highly detailed, realistic models. In response to this need, our ultimate goal is to provide a user with a tool that would allow him to generate a realistic, high-resolution and consistent model of a geographic site using as wide a range of sources of information as possible but with minimal intervention on his part. To achieve this goal, we have begun work on three tasks the rst related to terrain modeling, the second to the generation of consistent virtual worlds and the last to interoperability issues.