Caves, Overhangs, Non-heightfield Elevation

• there is relatively little work yet on modelling caves and overhangs in 3D
• there are a handful of formats for cave data, but no standard file format
• Carl Anderson [email] writes:
  • "Subterranean cave data is [currently] collected as a series of vectors with a box for volume at each vertex. Height above point, floor below point, left wall, and right wall, all collected in the aspect of the flow on the survey. Using civil engineering techniques a volume can be generated from the base data. Preserving the base data is very important as a cave survey project may take years, the longest that I have worked on lasted from 1966 to 1994. Error detection, Loop closure, and error reduction through least squares regression take place after EACH and every trip.
  • Some experiments are ongoing to collect volumetric data directly using laser distance tools to measure the volume in less discrete slices.  This technology is too expensive and too delicate for most uses.  Cave survey may include underwater portions, portions with passage heights less than 1 foot, extremely muddy, and extremely vertical passages.
  • Caves can be vertical mazes, horizontal mazes, have deep shafts (300 - 1000 feet), have distinct passages overlaying each other vertically, but separated by tens to 100s of feet.
    What [cavers] want to do is to lay geological structures (faults, dikes, ...), geological separations (named rock units) and second order derivates of LandSat 7 ETM+ data on top of hillsides to try to visualize the underlying structures. Caves are voids that have exploited the underlying geological structures (regional to very local).
  • As far as raw cave data goes there is a project RosettaStal to pull from the most common cave data formats."
• WinKarst Cave Mapping Software
  • shareware for Windows, supports the whole process of surveying caves, from sketches and notes, through digitizing maps, all the way to simple 3D visualization of cave passages
  • can import geodata from DEM, DLG, SDTS and GeoTIFF, survey data from many survey formats and GPS
  • can export cave data to 3D DXF
• The Survex Project
  • open software for surveying and visualizing cave systems, but the caves are modelled as vectors, not as volumes
• HadesGL
  • I'm not certain, as the site is in French, but it appears to be a viewer for cave data which uses the .top files generated by related software.  As Jean-Pierre Cassou described it to me, "HadesGL is a simple viewer for GHTopo software.  GHTopo is a powerful software for cave surveying, using TOPOROBOT methodology and full compatibles file formats."
  • source and binaries for Windows
  • site and software are in French

Non-heightfield Terrain Engines

• Voxlap Cave Demo
  • a project of Ken Silverman and Tom Dobrowolski, demo released August 2003
  • fast, impressive demo (Win32) with attractive procedurally generated non-heightfield scenery, fully destructible, with a high voxel resolution
  • the demo is fixed at a relatively low screen resolution, and the surfaces do pixelate up close, though that's likely unavoidable with a voxel approach
• Geek [appears to be offline as of 040908]
  • a terrain engine for Linux by Jack Strohm
  • supports arbitrary topology (including any sort of caves) using volume data as input (latest version uses metaballs)
  • Linux demo executable (was) available
  • was an IOTD in March 2001
• Sven Forstmann's HVox engine
  • "Uses large voxel volumes to display the landscape. The 3D-Space in the engine consists currently of 5 Boxes (64x64x64 Pixels), which are placed around the observer. The size of the first box is 64 pixels, the next one twice (128) and so on. So i can display a Volume of 2048x2048x2048 at about 12 fps unoptimized."
  • has some precompiled example executables for Linux and Windows
  • adds details to close-up geometry by using Perlin noise
• Iehovah Terrain Library
  • closed-source demo that does LOD on arbitrary manifolds, although it has no connection with actual data

Pseudo-Voxels

• Although a real voxel [wikipedia] is a volume element, there was a convention in the game-developer community in the 1990s to use the term 'voxel' to refer to heightfield rendering, generally using approximate raycasting for a non-polygonal render.  These were not true voxels, as they were incapable of representing or rendering actual volumes such as caves or overhangs.  Some references to this mis-named 'voxel' can still be found:
  • Realtime Voxel Landscape Engines, flipcode tutorial by Alex Champandard, 2000
  • Voxel terrain engine by Greg Massal, who claims that a pseudo-voxel is "a more restrictive definition of a voxel."