Contents of Interactive Terrain Visualization

Interactive Terrain Visualization

Realistic visualization of terrain is very important in flight simulation and environmental assessment. Skewed views of terrain may easily span a huge domain: for instance, the field of view from an airplane may extend on a range of several hundreds kilometers. If a terrain model at very high resolution were used to visualize such kind of views, the number of graphical primitives to be rendered would overwhelm even the performance of most powerful graphical workstation. Moreover, resolution would be unnecessarily high on a large portion of terrain, since the size of each graphical primitive far from viewpoint would become smaller than a pixel. On the other hand, if a generalized model were used, at a resolution sufficiently low to be managed by the rendering engine, the visual quality of terrain close to the viewpoint would be highly degraded. Some improvement may be obtained with the latter approach by a clever use of texture mapping [UNA CITAZIONE, FORSE LAVORO PISANI]. However, texture mapping cannot help with the shape degradation of silhouettes (e.g., ridges) and other important geometric features (e.g., cliffs, roads, rivers, etc.).

The performance of a visualization system can be improved enormously if a representation at variable level of detail is used. In fact, this kind of solution is the subject of a number of recent proposals in the literature [Cignoni et al.1997,de Berg and Dobrindt1995,FlyAway????,Hoppe1998,Lindstrom et al.1996,Topovista????], and iit s also adopted in the context of commercial products and videogames.

[MANCANO RIFERIMENTI PER QUESTO. SICURAMENTE CI SONO VISUALIZZATORI COMMERCIALI CHE USANO MULTIRISOLUZIONE VARIABILE, MA NON SO QUALI. LE PM SONO STATE INCLUSE NELLA VERSIONE PIU' RECENTE DI DIRECTX, LA LIBRERIA GRAFICA DELLA MICROSOFT. NON SO SE LASCIARE L'AFFERMAZIONE SENZA RIFERIMENTI O TAGLIARE LA FRASE.]

The general idea is to visualize a terrain model whose resolution is high close to the viewpoint, while it progressively decreases with distance from it. Note that a different model must be provided at each frame, since the viewpoint changes its position continuously. Therefore, TIN extraction at variable resolution must be performed in real time.

VARIANT includes a prototype visualization module which permits the interactive visualization (i.e., over ten frames per second) of a terrain at variable resolution on low cost platforms, such as PCs, already. This module allows the user to drive its viewpoint through space to fly over terrain. COSE GIA' DETTE SOPRA. In order to obtain an interactive frame rate, it is necessary to keep the overall number of triangles sent to a rendering device below a certain bound. At the same time, the visual appearance of the mesh that is rendered must not be perceptually dissimilar from the mesh at the maximum resolution. The basic idea is using a low accuracy for terrain portions which are either distant or outside the field of view.

At each frame rate, we apply the dynamic algorithm for global queries, and extract a TIN covering the whole domain and obeying to a LOD threshold defined in the following way:

For all points lying outside the view frustum, resolution can be arbitrarily coarse, since they do not contribute to the visible scene.
For all points lying inside the view frustum, resolution is maximum at the viewpoint, and decreasing linearly with distance from it. SOLITA CONFUSIONE TRA ACCURACY E RESOLUTION - MEGLIO SOPRASSEDERE this corresponds to the idea that accuracy of triangles must be higher near the viewpoint and decrease progressively with distance.

In this way, the LOD threshold varies at each frame, since it depends on the position of the viewpoint. Therefore, the terrain model visualized is adaptively changed at each viewpoint position. At each frame, the dynamic algorithm for TIN extraction starts from the model corresponding to the previous frame, increasing detail where necessary, and decreasing it where it is unnecessarily high. Triangles of the extracted TIN lying outside the view frustum are culled out prior to rendering.

VERSIONE VECCHIA CON ALGORITMO LOCALE The basic idea is that a low accuracy is sufficient for distant terrain portions, and that triangles outside the field of view do not need to be generated, since they do not contribute to the visible scene. At each frame rate, we apply a local query, where:

The region of interest I is the view frustum.
For any poiny p, the threshold value τ(p) is an increasing function of the distance of p from the viewpoint; this corresponds to the idea that accuracy of triangles must be higher near the viewpoint and decrease progressively with distance.

The region of interest and the threshold function vary at each instant in time, while moving the viewpoint. The implementation running on a PC can extract nearly 100,000 triangles per second from the terrain models seen in Section $[*]$ .

Note that we could use alternatively a local algorithm for TIN extraction by providing the view frustum as a ROI, since this automatically culls triangles outside the view frustum. However, we found that the dynamic approach is faster, due to the computational advantage of starting from a TIN that is very similar to that in output. Figure $[*]$ shows the portion of TIN extracted at a given frame rate, bounded to the view frustum, and a corresponding perspective visualization. Note how the actual size of triangles increases with distance from the viewpoint, while their apparent size on the screen is almost uniform.

**Figure:** (a) A TIN used for interactive terrain visualization, clipped at the view frustum, and (b) the corresponding view of terrain.
$\begin{figure}\centerline{ \psfig{file=variab2DL.ps,width=5cm} \hspace{1cm} \psfig{file=variab.ps,width=5cm} } \centerline{ (a) \hspace{5cm} (b) }\end{figure}$