demviewer

The main goal has been the creation of a 3D display of terrain with reasonable interactive rendering performance. We have concentrated on providing a vehicle view of the terrain, i.e., a view from a specific 2D position at just above the elevation at that position. This requires the capability to obtain the elevation value at any point within the 2D extent of the elevation model, which will also be useful for displaying features from a 2D vector map database within the 3D display, for example, to display roads on the 3D surface.

The Dem and VtkUnidraw libraries currently provides the following steps for terrain visualization:

• Access to elevation data

  The Elevation class is an abstract base class that defines a common structure for elevation models. Derived classes like UsgsDem know how to read the specific file formats for particular elevation databases. The usgsdemdump is an example of this capability: it reads a USGS 1 degree DEM file and prints it out in text form.

• Processing elevation data into displayable form

  The density of 100-meter and 30-meter elevation datasets is so high that some form of data reduction must be done in order to obtain good display performance on today's computers. We have experimented with a technique called decimation (see chapter 9 in the vtk book.) Decimation is an algorithm to reduce the number of triangles in a triangle mesh, preserving the original topology and forming a good approximation to the original geometry. The advantage of decimation, as compared to subsampling techniques, is that the mesh is adaptively modified to retain more detail in areas of high curvature.
  We have built the following pipeline into the usgsdem2vtk program:

  • Create a vtkStructuredGrid dataset for each tile of elevation data. This starts us out with a data structure into which we insert UTM-projected x,y,z points.
  • Extract polygonal cells from the structured grid. This is done by the vtkStructuredGridGeometryFilter object. The output consists of quadrilaterals outlining each group of adjacent four points.
  • Triangulate the polygonal cells. This is needed because decimation only works with triangle lists. Quadrilaterals are converted to triangles by the vtkTriangleFilter object.
  • Decimate the triangle mesh. Performed by the vtkDecimate filter. There are various parameters to control the decimation process. In practice we have found that the feature angle parameters provide one way to obtain gradually varying output sizes.
  • Write the decimated triangle mesh to a file. This is performed by the vtkPolyWriter object. The file format is native to vtk and can be read in by the vtkDataSetReader object.
  usgsdem2vtk currently reads in a 1 degree USGS DEM (1201x1201 elevation values), divides it into 6x6 tiles of 201x201 values (one overlapping edge for each tile) and runs each tile through the pipeline just described, resulting in 36 decimated tiles ready for rendering. Decimation can provide order-of-magnitude reduction in dataset size; a sample tile went from about 2.7 Mb in raw form to about 160 Kb after decimation.

• 3D rendering of processed elevation data

  The demviewer program provides a 3D perspective view of the elevation data and allows for probing to obtain the elevation value at any x,y position within the data's extent. It is structured as an OverlayUnidraw derived editor with a viewer (vtkViewer) that contains a vtk renderer and custom command menus, buttons, etc. In addition it receives updates to the camera position which has been added as CameraPos and CameraPath tools to the ivmaps sample programs vpfviewer and dlgviewer.
  demviewer provides the following capabilities:

  • Loading of processed elevation datasets. The vtkDataSetReader object reads in files in vtk format created by usgsdem2vtk. These are piped into vtkPolyMapper and vtkActor to be added to the vtkRenderer within a VtkViewer, in a structure similar to other Unidraw derived programs. (The loading is done manually by selecting an input file; eventually it should have a positional index and automatically load the tile at a given position.) At this point we can set up rendering related attributes; we currently just use two colors (land and sea) but this can be extended to provide better visual cues, perhaps with more colors, texture mapping or other tricks. Also we set up some lights in the scene; more experimentation with lights could be done to try to obtain a better picture.
  • Vehicular camera positional update. We implement this by providing camera positioning tools in the ivmaps 2D map display programs. The user can draw a rubberband line on the 2D map; the start point of the line determines the x,y position of the camera and the end point determines the x,y position of the camera's focal point. The coordinates of this line are sent from the 2D map viewer program to the demviewer program. demviewer receives the camera update command with the two x,y points, obtains the z elevation value at the camera x,y position, adjusts the camera to be positioned slightly above the z value, pointing straight at the focal point x,y with the same z elevation value, and redraws the 3D view. Elevation at an x,y point is obtained using the vtkProbeFilter object which performs interpolation to obtain the result. The camera is updated via the vtkCamera member functions.
  • Other camera controls. We have currently implemented buttons to move the camera straight up or down and to yaw (rotate around current position to look around) left or right. Many other camera controls are usefule and will be added in the future.

Possible future improvements for demviewer:

• Display map features and other objects in the 3D view.
• Implement an effective LOD (level of detail.)
• Improved camera controls, ability to walk or fly around, etc.
• Improve rendering appearance with colors, texture mapping, lights.

Last modified: Tue Aug 5 15:32:49 PDT 1997